[PATCH] alloc_tag: introduce Kconfig option for default compressed profiling

[PATCH] alloc_tag: introduce Kconfig option for default compressed profiling

[Usama Arif]

With this Kconfig option enabled, the kernel stores allocation tag references
in the page flags by default.

There are 2 reasons to introduce this:
- As mentioned in [1], compressed tags dont have system memory overhead
and much lower performance overhead. It would be preferrable to have this as
the default option, and to be able to switch it at compile time. Another
option is to just declare the static key as true by default?
- As compressed option is the best one, it doesn't make sense to have to
change both defconfig and command line options to enable memory
allocation profiling. Changing commandline across a large number of services
can result in signifcant work, which shouldn't be needed if the kernel
defconfig needs to be changed anyways.

[1] https://lore.kernel.org/all/20241023170759.999909-7-surenb@google.com/T/#m0da08879435f7673eaa10871a6e9d1be4f605ac8

Signed-off-by: Usama Arif <usamaarif642@gmail.com>
---
 include/linux/pgalloc_tag.h | 4 ++++
 lib/Kconfig.debug           | 5 +++++
 lib/alloc_tag.c             | 4 ++++
 3 files changed, 13 insertions(+)

diff --git a/include/linux/pgalloc_tag.h b/include/linux/pgalloc_tag.h
index c74077977830..0226059bcf00 100644
--- a/include/linux/pgalloc_tag.h
+++ b/include/linux/pgalloc_tag.h
@@ -16,7 +16,11 @@ extern unsigned long alloc_tag_ref_mask;
 extern int alloc_tag_ref_offs;
 extern struct alloc_tag_kernel_section kernel_tags;
 
+#ifdef CONFIG_MEM_ALLOC_PROFILING_COMPRESSED_ENABLED_BY_DEFAULT
+DECLARE_STATIC_KEY_TRUE(mem_profiling_compressed);
+#else
 DECLARE_STATIC_KEY_FALSE(mem_profiling_compressed);
+#endif
 
 typedef u16	pgalloc_tag_idx;
 
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 9fe4d8dfe578..66d8995f3514 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1028,6 +1028,11 @@ config MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT
 	default y
 	depends on MEM_ALLOC_PROFILING
 
+config MEM_ALLOC_PROFILING_COMPRESSED_ENABLED_BY_DEFAULT
+	bool "store page allocation tag references in the page flags by default"
+	default y
+	depends on MEM_ALLOC_PROFILING
+
 config MEM_ALLOC_PROFILING_DEBUG
 	bool "Memory allocation profiler debugging"
 	default n
diff --git a/lib/alloc_tag.c b/lib/alloc_tag.c
index 25ecc1334b67..30adad5630dd 100644
--- a/lib/alloc_tag.c
+++ b/lib/alloc_tag.c
@@ -31,7 +31,11 @@ DEFINE_STATIC_KEY_MAYBE(CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT,
 			mem_alloc_profiling_key);
 EXPORT_SYMBOL(mem_alloc_profiling_key);
 
+#ifdef CONFIG_MEM_ALLOC_PROFILING_COMPRESSED_ENABLED_BY_DEFAULT
+DEFINE_STATIC_KEY_TRUE(mem_profiling_compressed);
+#else
 DEFINE_STATIC_KEY_FALSE(mem_profiling_compressed);
+#endif
 
 struct alloc_tag_kernel_section kernel_tags = { NULL, 0 };
 unsigned long alloc_tag_ref_mask;
-- 
2.47.1

Re: [PATCH] alloc_tag: introduce Kconfig option for default compressed profiling

[Suren Baghdasaryan]

On Wed, Apr 16, 2025 at 11:06 AM Usama Arif <usamaarif642@gmail.com> wrote:
>
> With this Kconfig option enabled, the kernel stores allocation tag references
> in the page flags by default.
>
> There are 2 reasons to introduce this:
> - As mentioned in [1], compressed tags dont have system memory overhead
> and much lower performance overhead. It would be preferrable to have this as
> the default option, and to be able to switch it at compile time. Another
> option is to just declare the static key as true by default?
> - As compressed option is the best one, it doesn't make sense to have to
> change both defconfig and command line options to enable memory
> allocation profiling. Changing commandline across a large number of services
> can result in signifcant work, which shouldn't be needed if the kernel
> defconfig needs to be changed anyways.

The reason tag compression is not the default option is because it
works only if there are enough free bits in the page flags to store a
tag index. If you configure it to use page flags and your build does
not have enough free bits, the profiling will be disabled (see
alloc_tag_sec_init()). IOW there is no graceful fallback to use page
extensions. Therefore, the current default option is not the most
performant but the one which works on all builds. Instead of this just
set sysctl.vm.mem_profiling boot parameter in your config file.

Your change effectively changes the default value of
mem_profiling_compressed and I don't see why you need to introduce a
new config option for that. But that really does not matter because
changing default to compressed tags is not the right choice IMO.

>
> [1] https://lore.kernel.org/all/20241023170759.999909-7-surenb@google.com/T/#m0da08879435f7673eaa10871a6e9d1be4f605ac8
>
> Signed-off-by: Usama Arif <usamaarif642@gmail.com>
> ---
>  include/linux/pgalloc_tag.h | 4 ++++
>  lib/Kconfig.debug           | 5 +++++
>  lib/alloc_tag.c             | 4 ++++
>  3 files changed, 13 insertions(+)
>
> diff --git a/include/linux/pgalloc_tag.h b/include/linux/pgalloc_tag.h
> index c74077977830..0226059bcf00 100644
> --- a/include/linux/pgalloc_tag.h
> +++ b/include/linux/pgalloc_tag.h
> @@ -16,7 +16,11 @@ extern unsigned long alloc_tag_ref_mask;
>  extern int alloc_tag_ref_offs;
>  extern struct alloc_tag_kernel_section kernel_tags;
>
> +#ifdef CONFIG_MEM_ALLOC_PROFILING_COMPRESSED_ENABLED_BY_DEFAULT
> +DECLARE_STATIC_KEY_TRUE(mem_profiling_compressed);
> +#else
>  DECLARE_STATIC_KEY_FALSE(mem_profiling_compressed);
> +#endif
>
>  typedef u16    pgalloc_tag_idx;
>
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 9fe4d8dfe578..66d8995f3514 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -1028,6 +1028,11 @@ config MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT
>         default y
>         depends on MEM_ALLOC_PROFILING
>
> +config MEM_ALLOC_PROFILING_COMPRESSED_ENABLED_BY_DEFAULT
> +       bool "store page allocation tag references in the page flags by default"
> +       default y
> +       depends on MEM_ALLOC_PROFILING
> +
>  config MEM_ALLOC_PROFILING_DEBUG
>         bool "Memory allocation profiler debugging"
>         default n
> diff --git a/lib/alloc_tag.c b/lib/alloc_tag.c
> index 25ecc1334b67..30adad5630dd 100644
> --- a/lib/alloc_tag.c
> +++ b/lib/alloc_tag.c
> @@ -31,7 +31,11 @@ DEFINE_STATIC_KEY_MAYBE(CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT,
>                         mem_alloc_profiling_key);
>  EXPORT_SYMBOL(mem_alloc_profiling_key);
>
> +#ifdef CONFIG_MEM_ALLOC_PROFILING_COMPRESSED_ENABLED_BY_DEFAULT
> +DEFINE_STATIC_KEY_TRUE(mem_profiling_compressed);
> +#else
>  DEFINE_STATIC_KEY_FALSE(mem_profiling_compressed);
> +#endif
>
>  struct alloc_tag_kernel_section kernel_tags = { NULL, 0 };
>  unsigned long alloc_tag_ref_mask;
> --
> 2.47.1
>

Re: [PATCH] alloc_tag: introduce Kconfig option for default compressed profiling

[Shakeel Butt]

On Wed, Apr 16, 2025 at 02:08:31PM -0700, Suren Baghdasaryan wrote:
> On Wed, Apr 16, 2025 at 11:06 AM Usama Arif <usamaarif642@gmail.com> wrote:
> >
> > With this Kconfig option enabled, the kernel stores allocation tag references
> > in the page flags by default.
> >
> > There are 2 reasons to introduce this:
> > - As mentioned in [1], compressed tags dont have system memory overhead
> > and much lower performance overhead. It would be preferrable to have this as
> > the default option, and to be able to switch it at compile time. Another
> > option is to just declare the static key as true by default?
> > - As compressed option is the best one, it doesn't make sense to have to
> > change both defconfig and command line options to enable memory
> > allocation profiling. Changing commandline across a large number of services
> > can result in signifcant work, which shouldn't be needed if the kernel
> > defconfig needs to be changed anyways.
> 
> The reason tag compression is not the default option is because it
> works only if there are enough free bits in the page flags to store a
> tag index. If you configure it to use page flags and your build does
> not have enough free bits, the profiling will be disabled (see
> alloc_tag_sec_init()).

Is it possible to fail the build in that case i.e. check the page flags
availability at build time?

Re: [PATCH] alloc_tag: introduce Kconfig option for default compressed profiling

[Suren Baghdasaryan]

On Wed, Apr 16, 2025 at 2:41 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
>
> On Wed, Apr 16, 2025 at 02:08:31PM -0700, Suren Baghdasaryan wrote:
> > On Wed, Apr 16, 2025 at 11:06 AM Usama Arif <usamaarif642@gmail.com> wrote:
> > >
> > > With this Kconfig option enabled, the kernel stores allocation tag references
> > > in the page flags by default.
> > >
> > > There are 2 reasons to introduce this:
> > > - As mentioned in [1], compressed tags dont have system memory overhead
> > > and much lower performance overhead. It would be preferrable to have this as
> > > the default option, and to be able to switch it at compile time. Another
> > > option is to just declare the static key as true by default?
> > > - As compressed option is the best one, it doesn't make sense to have to
> > > change both defconfig and command line options to enable memory
> > > allocation profiling. Changing commandline across a large number of services
> > > can result in signifcant work, which shouldn't be needed if the kernel
> > > defconfig needs to be changed anyways.
> >
> > The reason tag compression is not the default option is because it
> > works only if there are enough free bits in the page flags to store a
> > tag index. If you configure it to use page flags and your build does
> > not have enough free bits, the profiling will be disabled (see
> > alloc_tag_sec_init()).
>
> Is it possible to fail the build in that case i.e. check the page flags
> availability at build time?

The difficulty is finding out the number of allocation tags in the
kernel before it gets built. Maybe there is a way to add an additional
post-build stage to run that check. But even then making this option
default and causing build failures does not seem like a good idea to
me but maybe I'm being too cautious?

Re: [PATCH] alloc_tag: introduce Kconfig option for default compressed profiling

[Shakeel Butt]

On Wed, Apr 16, 2025 at 05:11:11PM -0700, Suren Baghdasaryan wrote:
> On Wed, Apr 16, 2025 at 2:41 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> >
> > On Wed, Apr 16, 2025 at 02:08:31PM -0700, Suren Baghdasaryan wrote:
> > > On Wed, Apr 16, 2025 at 11:06 AM Usama Arif <usamaarif642@gmail.com> wrote:
> > > >
> > > > With this Kconfig option enabled, the kernel stores allocation tag references
> > > > in the page flags by default.
> > > >
> > > > There are 2 reasons to introduce this:
> > > > - As mentioned in [1], compressed tags dont have system memory overhead
> > > > and much lower performance overhead. It would be preferrable to have this as
> > > > the default option, and to be able to switch it at compile time. Another
> > > > option is to just declare the static key as true by default?
> > > > - As compressed option is the best one, it doesn't make sense to have to
> > > > change both defconfig and command line options to enable memory
> > > > allocation profiling. Changing commandline across a large number of services
> > > > can result in signifcant work, which shouldn't be needed if the kernel
> > > > defconfig needs to be changed anyways.
> > >
> > > The reason tag compression is not the default option is because it
> > > works only if there are enough free bits in the page flags to store a
> > > tag index. If you configure it to use page flags and your build does
> > > not have enough free bits, the profiling will be disabled (see
> > > alloc_tag_sec_init()).
> >
> > Is it possible to fail the build in that case i.e. check the page flags
> > availability at build time?
> 
> The difficulty is finding out the number of allocation tags in the
> kernel before it gets built. Maybe there is a way to add an additional
> post-build stage to run that check.

Yeah that would be good to have.

> But even then making this option
> default and causing build failures does not seem like a good idea to
> me but maybe I'm being too cautious?

Oh my question was orthogonal to the patch. Basically some users may
want build time guarantee for this and they can enable such
build-failing opt-in config/check.

Re: [PATCH] alloc_tag: introduce Kconfig option for default compressed profiling

[Suren Baghdasaryan]

On Thu, Apr 17, 2025 at 8:47 AM Shakeel Butt <shakeel.butt@linux.dev> wrote:
>
> On Wed, Apr 16, 2025 at 05:11:11PM -0700, Suren Baghdasaryan wrote:
> > On Wed, Apr 16, 2025 at 2:41 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> > >
> > > On Wed, Apr 16, 2025 at 02:08:31PM -0700, Suren Baghdasaryan wrote:
> > > > On Wed, Apr 16, 2025 at 11:06 AM Usama Arif <usamaarif642@gmail.com> wrote:
> > > > >
> > > > > With this Kconfig option enabled, the kernel stores allocation tag references
> > > > > in the page flags by default.
> > > > >
> > > > > There are 2 reasons to introduce this:
> > > > > - As mentioned in [1], compressed tags dont have system memory overhead
> > > > > and much lower performance overhead. It would be preferrable to have this as
> > > > > the default option, and to be able to switch it at compile time. Another
> > > > > option is to just declare the static key as true by default?
> > > > > - As compressed option is the best one, it doesn't make sense to have to
> > > > > change both defconfig and command line options to enable memory
> > > > > allocation profiling. Changing commandline across a large number of services
> > > > > can result in signifcant work, which shouldn't be needed if the kernel
> > > > > defconfig needs to be changed anyways.
> > > >
> > > > The reason tag compression is not the default option is because it
> > > > works only if there are enough free bits in the page flags to store a
> > > > tag index. If you configure it to use page flags and your build does
> > > > not have enough free bits, the profiling will be disabled (see
> > > > alloc_tag_sec_init()).
> > >
> > > Is it possible to fail the build in that case i.e. check the page flags
> > > availability at build time?
> >
> > The difficulty is finding out the number of allocation tags in the
> > kernel before it gets built. Maybe there is a way to add an additional
> > post-build stage to run that check.
>
> Yeah that would be good to have.
>
> > But even then making this option
> > default and causing build failures does not seem like a good idea to
> > me but maybe I'm being too cautious?
>
> Oh my question was orthogonal to the patch. Basically some users may
> want build time guarantee for this and they can enable such
> build-failing opt-in config/check.

Yes, that would require the post-build step to check the number of
tags vs the number of available page flag bits. I'll add it to my TODO
list but it won't be at the top, sorry :) Volunteers to help with that
would be highly appreciated.

Re: [PATCH] alloc_tag: introduce Kconfig option for default compressed profiling

[Usama Arif]

On 17/04/2025 17:00, Suren Baghdasaryan wrote:
> On Thu, Apr 17, 2025 at 8:47 AM Shakeel Butt <shakeel.butt@linux.dev> wrote:
>>
>> On Wed, Apr 16, 2025 at 05:11:11PM -0700, Suren Baghdasaryan wrote:
>>> On Wed, Apr 16, 2025 at 2:41 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
>>>>
>>>> On Wed, Apr 16, 2025 at 02:08:31PM -0700, Suren Baghdasaryan wrote:
>>>>> On Wed, Apr 16, 2025 at 11:06 AM Usama Arif <usamaarif642@gmail.com> wrote:
>>>>>>
>>>>>> With this Kconfig option enabled, the kernel stores allocation tag references
>>>>>> in the page flags by default.
>>>>>>
>>>>>> There are 2 reasons to introduce this:
>>>>>> - As mentioned in [1], compressed tags dont have system memory overhead
>>>>>> and much lower performance overhead. It would be preferrable to have this as
>>>>>> the default option, and to be able to switch it at compile time. Another
>>>>>> option is to just declare the static key as true by default?
>>>>>> - As compressed option is the best one, it doesn't make sense to have to
>>>>>> change both defconfig and command line options to enable memory
>>>>>> allocation profiling. Changing commandline across a large number of services
>>>>>> can result in signifcant work, which shouldn't be needed if the kernel
>>>>>> defconfig needs to be changed anyways.
>>>>>
>>>>> The reason tag compression is not the default option is because it
>>>>> works only if there are enough free bits in the page flags to store a
>>>>> tag index. If you configure it to use page flags and your build does
>>>>> not have enough free bits, the profiling will be disabled (see
>>>>> alloc_tag_sec_init()).
>>>>
>>>> Is it possible to fail the build in that case i.e. check the page flags
>>>> availability at build time?
>>>
>>> The difficulty is finding out the number of allocation tags in the
>>> kernel before it gets built. Maybe there is a way to add an additional
>>> post-build stage to run that check.
>>
>> Yeah that would be good to have.
>>
>>> But even then making this option
>>> default and causing build failures does not seem like a good idea to
>>> me but maybe I'm being too cautious?
>>
>> Oh my question was orthogonal to the patch. Basically some users may
>> want build time guarantee for this and they can enable such
>> build-failing opt-in config/check.
> 
> Yes, that would require the post-build step to check the number of
> tags vs the number of available page flag bits. I'll add it to my TODO
> list but it won't be at the top, sorry :) Volunteers to help with that
> would be highly appreciated.

Hi Suren,

A question orthogonal to the patch, the defconfig entry is defined as below:

config MEM_ALLOC_PROFILING
	bool "Enable memory allocation profiling"
	default n
	depends on MMU
	depends on PROC_FS
	depends on !DEBUG_FORCE_WEAK_PER_CPU
	select CODE_TAGGING
	select PAGE_EXTENSION
	select SLAB_OBJ_EXT

i.e. we select PAGE_EXTENSION even if we use compressed profiling and use page flags
instead of page extension. Which means the 0.2% (8 bytes per struct page) memory overhead
will still exist even when we dont need it?

Should we have some defconfig option (happy with any other way) that only allows compressed
profiling (otherwise nothing), so that we don't have the dependency on page extension
and thus not have the overhead if we only plan to use compressed profiling?

Thanks

Re: [PATCH] alloc_tag: introduce Kconfig option for default compressed profiling

[Usama Arif]

On 17/04/2025 18:50, Usama Arif wrote:
> 
> 
> On 17/04/2025 17:00, Suren Baghdasaryan wrote:
>> On Thu, Apr 17, 2025 at 8:47 AM Shakeel Butt <shakeel.butt@linux.dev> wrote:
>>>
>>> On Wed, Apr 16, 2025 at 05:11:11PM -0700, Suren Baghdasaryan wrote:
>>>> On Wed, Apr 16, 2025 at 2:41 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
>>>>>
>>>>> On Wed, Apr 16, 2025 at 02:08:31PM -0700, Suren Baghdasaryan wrote:
>>>>>> On Wed, Apr 16, 2025 at 11:06 AM Usama Arif <usamaarif642@gmail.com> wrote:
>>>>>>>
>>>>>>> With this Kconfig option enabled, the kernel stores allocation tag references
>>>>>>> in the page flags by default.
>>>>>>>
>>>>>>> There are 2 reasons to introduce this:
>>>>>>> - As mentioned in [1], compressed tags dont have system memory overhead
>>>>>>> and much lower performance overhead. It would be preferrable to have this as
>>>>>>> the default option, and to be able to switch it at compile time. Another
>>>>>>> option is to just declare the static key as true by default?
>>>>>>> - As compressed option is the best one, it doesn't make sense to have to
>>>>>>> change both defconfig and command line options to enable memory
>>>>>>> allocation profiling. Changing commandline across a large number of services
>>>>>>> can result in signifcant work, which shouldn't be needed if the kernel
>>>>>>> defconfig needs to be changed anyways.
>>>>>>
>>>>>> The reason tag compression is not the default option is because it
>>>>>> works only if there are enough free bits in the page flags to store a
>>>>>> tag index. If you configure it to use page flags and your build does
>>>>>> not have enough free bits, the profiling will be disabled (see
>>>>>> alloc_tag_sec_init()).
>>>>>
>>>>> Is it possible to fail the build in that case i.e. check the page flags
>>>>> availability at build time?
>>>>
>>>> The difficulty is finding out the number of allocation tags in the
>>>> kernel before it gets built. Maybe there is a way to add an additional
>>>> post-build stage to run that check.
>>>
>>> Yeah that would be good to have.
>>>
>>>> But even then making this option
>>>> default and causing build failures does not seem like a good idea to
>>>> me but maybe I'm being too cautious?
>>>
>>> Oh my question was orthogonal to the patch. Basically some users may
>>> want build time guarantee for this and they can enable such
>>> build-failing opt-in config/check.
>>
>> Yes, that would require the post-build step to check the number of
>> tags vs the number of available page flag bits. I'll add it to my TODO
>> list but it won't be at the top, sorry :) Volunteers to help with that
>> would be highly appreciated.
> 
> Hi Suren,
> 
> A question orthogonal to the patch, the defconfig entry is defined as below:
> 
> config MEM_ALLOC_PROFILING
> 	bool "Enable memory allocation profiling"
> 	default n
> 	depends on MMU
> 	depends on PROC_FS
> 	depends on !DEBUG_FORCE_WEAK_PER_CPU
> 	select CODE_TAGGING
> 	select PAGE_EXTENSION
> 	select SLAB_OBJ_EXT
> 
> i.e. we select PAGE_EXTENSION even if we use compressed profiling and use page flags
> instead of page extension. Which means the 0.2% (8 bytes per struct page) memory overhead
> will still exist even when we dont need it?
> 
> Should we have some defconfig option (happy with any other way) that only allows compressed
> profiling (otherwise nothing), so that we don't have the dependency on page extension
> and thus not have the overhead if we only plan to use compressed profiling?
> 
> Thanks


Johannes pointed out the .need function of page_alloc_tagging_ops, i.e. need_page_alloc_tagging,
so page extensions wouldn't be enabled and hopefully there is no memory overhead when
mem_profiling_compressed is true. Let me know if my understanding is wrong.
And sorry for the noise!

Re: [PATCH] alloc_tag: introduce Kconfig option for default compressed profiling

[Suren Baghdasaryan]

On Thu, Apr 17, 2025 at 11:35 AM Usama Arif <usamaarif642@gmail.com> wrote:
>
>
>
> On 17/04/2025 18:50, Usama Arif wrote:
> >
> >
> > On 17/04/2025 17:00, Suren Baghdasaryan wrote:
> >> On Thu, Apr 17, 2025 at 8:47 AM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> >>>
> >>> On Wed, Apr 16, 2025 at 05:11:11PM -0700, Suren Baghdasaryan wrote:
> >>>> On Wed, Apr 16, 2025 at 2:41 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> >>>>>
> >>>>> On Wed, Apr 16, 2025 at 02:08:31PM -0700, Suren Baghdasaryan wrote:
> >>>>>> On Wed, Apr 16, 2025 at 11:06 AM Usama Arif <usamaarif642@gmail.com> wrote:
> >>>>>>>
> >>>>>>> With this Kconfig option enabled, the kernel stores allocation tag references
> >>>>>>> in the page flags by default.
> >>>>>>>
> >>>>>>> There are 2 reasons to introduce this:
> >>>>>>> - As mentioned in [1], compressed tags dont have system memory overhead
> >>>>>>> and much lower performance overhead. It would be preferrable to have this as
> >>>>>>> the default option, and to be able to switch it at compile time. Another
> >>>>>>> option is to just declare the static key as true by default?
> >>>>>>> - As compressed option is the best one, it doesn't make sense to have to
> >>>>>>> change both defconfig and command line options to enable memory
> >>>>>>> allocation profiling. Changing commandline across a large number of services
> >>>>>>> can result in signifcant work, which shouldn't be needed if the kernel
> >>>>>>> defconfig needs to be changed anyways.
> >>>>>>
> >>>>>> The reason tag compression is not the default option is because it
> >>>>>> works only if there are enough free bits in the page flags to store a
> >>>>>> tag index. If you configure it to use page flags and your build does
> >>>>>> not have enough free bits, the profiling will be disabled (see
> >>>>>> alloc_tag_sec_init()).
> >>>>>
> >>>>> Is it possible to fail the build in that case i.e. check the page flags
> >>>>> availability at build time?
> >>>>
> >>>> The difficulty is finding out the number of allocation tags in the
> >>>> kernel before it gets built. Maybe there is a way to add an additional
> >>>> post-build stage to run that check.
> >>>
> >>> Yeah that would be good to have.
> >>>
> >>>> But even then making this option
> >>>> default and causing build failures does not seem like a good idea to
> >>>> me but maybe I'm being too cautious?
> >>>
> >>> Oh my question was orthogonal to the patch. Basically some users may
> >>> want build time guarantee for this and they can enable such
> >>> build-failing opt-in config/check.
> >>
> >> Yes, that would require the post-build step to check the number of
> >> tags vs the number of available page flag bits. I'll add it to my TODO
> >> list but it won't be at the top, sorry :) Volunteers to help with that
> >> would be highly appreciated.
> >
> > Hi Suren,
> >
> > A question orthogonal to the patch, the defconfig entry is defined as below:
> >
> > config MEM_ALLOC_PROFILING
> >       bool "Enable memory allocation profiling"
> >       default n
> >       depends on MMU
> >       depends on PROC_FS
> >       depends on !DEBUG_FORCE_WEAK_PER_CPU
> >       select CODE_TAGGING
> >       select PAGE_EXTENSION
> >       select SLAB_OBJ_EXT
> >
> > i.e. we select PAGE_EXTENSION even if we use compressed profiling and use page flags
> > instead of page extension. Which means the 0.2% (8 bytes per struct page) memory overhead
> > will still exist even when we dont need it?
> >
> > Should we have some defconfig option (happy with any other way) that only allows compressed
> > profiling (otherwise nothing), so that we don't have the dependency on page extension
> > and thus not have the overhead if we only plan to use compressed profiling?
> >
> > Thanks
>
>
> Johannes pointed out the .need function of page_alloc_tagging_ops, i.e. need_page_alloc_tagging,
> so page extensions wouldn't be enabled and hopefully there is no memory overhead when
> mem_profiling_compressed is true. Let me know if my understanding is wrong.
> And sorry for the noise!

Sorry for the delay. Meetings...
Yep, Johannes is right, need_page_alloc_tagging() should disable that overhead.

Re: [PATCH] alloc_tag: introduce Kconfig option for default compressed profiling

[Usama Arif]

On 16/04/2025 22:08, Suren Baghdasaryan wrote:
> On Wed, Apr 16, 2025 at 11:06 AM Usama Arif <usamaarif642@gmail.com> wrote:
>>
>> With this Kconfig option enabled, the kernel stores allocation tag references
>> in the page flags by default.
>>
>> There are 2 reasons to introduce this:
>> - As mentioned in [1], compressed tags dont have system memory overhead
>> and much lower performance overhead. It would be preferrable to have this as
>> the default option, and to be able to switch it at compile time. Another
>> option is to just declare the static key as true by default?
>> - As compressed option is the best one, it doesn't make sense to have to
>> change both defconfig and command line options to enable memory
>> allocation profiling. Changing commandline across a large number of services
>> can result in signifcant work, which shouldn't be needed if the kernel
>> defconfig needs to be changed anyways.
> 
> The reason tag compression is not the default option is because it
> works only if there are enough free bits in the page flags to store a
> tag index. If you configure it to use page flags and your build does
> not have enough free bits, the profiling will be disabled (see
> alloc_tag_sec_init()). IOW there is no graceful fallback to use page
> extensions. Therefore, the current default option is not the most
> performant but the one which works on all builds. Instead of this just
> set sysctl.vm.mem_profiling boot parameter in your config file.

Hi Suren,

Thanks for the review! The main reason is to not have to make a change in
both defconfig and kernel command line while deploying it. We can ofcourse
set the commandline as well, but just makes deployment more tedious, and
adds an extra commandline parameter. In our case, we only want to deploy
compressed tags, and if there aren't enough free bits, we would prefer to
disable memory allocation profiling than to take the memory and performance
hit.

Would keeping the default value of this config disabled be an acceptable option?
i.e. the below diff on top of this patch?

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 66d8995f3514..163ffcece47a 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1030,7 +1030,7 @@ config MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT
 
 config MEM_ALLOC_PROFILING_COMPRESSED_ENABLED_BY_DEFAULT
        bool "store page allocation tag references in the page flags by default"
-       default y
+       default n
        depends on MEM_ALLOC_PROFILING
 
 config MEM_ALLOC_PROFILING_DEBUG


Thanks,
Usama
> 
> Your change effectively changes the default value of
> mem_profiling_compressed and I don't see why you need to introduce a
> new config option for that. But that really does not matter because
> changing default to compressed tags is not the right choice IMO.
> 
>>
>> [1] https://lore.kernel.org/all/20241023170759.999909-7-surenb@google.com/T/#m0da08879435f7673eaa10871a6e9d1be4f605ac8
>>
>> Signed-off-by: Usama Arif <usamaarif642@gmail.com>
>> ---
>>  include/linux/pgalloc_tag.h | 4 ++++
>>  lib/Kconfig.debug           | 5 +++++
>>  lib/alloc_tag.c             | 4 ++++
>>  3 files changed, 13 insertions(+)
>>
>> diff --git a/include/linux/pgalloc_tag.h b/include/linux/pgalloc_tag.h
>> index c74077977830..0226059bcf00 100644
>> --- a/include/linux/pgalloc_tag.h
>> +++ b/include/linux/pgalloc_tag.h
>> @@ -16,7 +16,11 @@ extern unsigned long alloc_tag_ref_mask;
>>  extern int alloc_tag_ref_offs;
>>  extern struct alloc_tag_kernel_section kernel_tags;
>>
>> +#ifdef CONFIG_MEM_ALLOC_PROFILING_COMPRESSED_ENABLED_BY_DEFAULT
>> +DECLARE_STATIC_KEY_TRUE(mem_profiling_compressed);
>> +#else
>>  DECLARE_STATIC_KEY_FALSE(mem_profiling_compressed);
>> +#endif
>>
>>  typedef u16    pgalloc_tag_idx;
>>
>> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
>> index 9fe4d8dfe578..66d8995f3514 100644
>> --- a/lib/Kconfig.debug
>> +++ b/lib/Kconfig.debug
>> @@ -1028,6 +1028,11 @@ config MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT
>>         default y
>>         depends on MEM_ALLOC_PROFILING
>>
>> +config MEM_ALLOC_PROFILING_COMPRESSED_ENABLED_BY_DEFAULT
>> +       bool "store page allocation tag references in the page flags by default"
>> +       default y
>> +       depends on MEM_ALLOC_PROFILING
>> +
>>  config MEM_ALLOC_PROFILING_DEBUG
>>         bool "Memory allocation profiler debugging"
>>         default n
>> diff --git a/lib/alloc_tag.c b/lib/alloc_tag.c
>> index 25ecc1334b67..30adad5630dd 100644
>> --- a/lib/alloc_tag.c
>> +++ b/lib/alloc_tag.c
>> @@ -31,7 +31,11 @@ DEFINE_STATIC_KEY_MAYBE(CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT,
>>                         mem_alloc_profiling_key);
>>  EXPORT_SYMBOL(mem_alloc_profiling_key);
>>
>> +#ifdef CONFIG_MEM_ALLOC_PROFILING_COMPRESSED_ENABLED_BY_DEFAULT
>> +DEFINE_STATIC_KEY_TRUE(mem_profiling_compressed);
>> +#else
>>  DEFINE_STATIC_KEY_FALSE(mem_profiling_compressed);
>> +#endif
>>
>>  struct alloc_tag_kernel_section kernel_tags = { NULL, 0 };
>>  unsigned long alloc_tag_ref_mask;
>> --
>> 2.47.1
>>

Re: [PATCH] alloc_tag: introduce Kconfig option for default compressed profiling

[Suren Baghdasaryan]

On Wed, Apr 16, 2025 at 2:52 PM Usama Arif <usamaarif642@gmail.com> wrote:
>
>
>
> On 16/04/2025 22:08, Suren Baghdasaryan wrote:
> > On Wed, Apr 16, 2025 at 11:06 AM Usama Arif <usamaarif642@gmail.com> wrote:
> >>
> >> With this Kconfig option enabled, the kernel stores allocation tag references
> >> in the page flags by default.
> >>
> >> There are 2 reasons to introduce this:
> >> - As mentioned in [1], compressed tags dont have system memory overhead
> >> and much lower performance overhead. It would be preferrable to have this as
> >> the default option, and to be able to switch it at compile time. Another
> >> option is to just declare the static key as true by default?
> >> - As compressed option is the best one, it doesn't make sense to have to
> >> change both defconfig and command line options to enable memory
> >> allocation profiling. Changing commandline across a large number of services
> >> can result in signifcant work, which shouldn't be needed if the kernel
> >> defconfig needs to be changed anyways.
> >
> > The reason tag compression is not the default option is because it
> > works only if there are enough free bits in the page flags to store a
> > tag index. If you configure it to use page flags and your build does
> > not have enough free bits, the profiling will be disabled (see
> > alloc_tag_sec_init()). IOW there is no graceful fallback to use page
> > extensions. Therefore, the current default option is not the most
> > performant but the one which works on all builds. Instead of this just
> > set sysctl.vm.mem_profiling boot parameter in your config file.
>
> Hi Suren,

Hi Usama,

>
> Thanks for the review! The main reason is to not have to make a change in
> both defconfig and kernel command line while deploying it. We can ofcourse
> set the commandline as well, but just makes deployment more tedious, and
> adds an extra commandline parameter. In our case, we only want to deploy
> compressed tags, and if there aren't enough free bits, we would prefer to
> disable memory allocation profiling than to take the memory and performance
> hit.
>
> Would keeping the default value of this config disabled be an acceptable option?
> i.e. the below diff on top of this patch?

Well, in that case I fail to see why
CONFIG_MEM_ALLOC_PROFILING_COMPRESSED_ENABLED_BY_DEFAULT=y is better
than CONFIG_CMDLINE="sysctl.vm.mem_profiling=1,compressed" ? Either
way you need to change the config file, no?

>
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 66d8995f3514..163ffcece47a 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -1030,7 +1030,7 @@ config MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT
>
>  config MEM_ALLOC_PROFILING_COMPRESSED_ENABLED_BY_DEFAULT
>         bool "store page allocation tag references in the page flags by default"
> -       default y
> +       default n
>         depends on MEM_ALLOC_PROFILING
>
>  config MEM_ALLOC_PROFILING_DEBUG
>
>
> Thanks,
> Usama
> >
> > Your change effectively changes the default value of
> > mem_profiling_compressed and I don't see why you need to introduce a
> > new config option for that. But that really does not matter because
> > changing default to compressed tags is not the right choice IMO.
> >
> >>
> >> [1] https://lore.kernel.org/all/20241023170759.999909-7-surenb@google.com/T/#m0da08879435f7673eaa10871a6e9d1be4f605ac8
> >>
> >> Signed-off-by: Usama Arif <usamaarif642@gmail.com>
> >> ---
> >>  include/linux/pgalloc_tag.h | 4 ++++
> >>  lib/Kconfig.debug           | 5 +++++
> >>  lib/alloc_tag.c             | 4 ++++
> >>  3 files changed, 13 insertions(+)
> >>
> >> diff --git a/include/linux/pgalloc_tag.h b/include/linux/pgalloc_tag.h
> >> index c74077977830..0226059bcf00 100644
> >> --- a/include/linux/pgalloc_tag.h
> >> +++ b/include/linux/pgalloc_tag.h
> >> @@ -16,7 +16,11 @@ extern unsigned long alloc_tag_ref_mask;
> >>  extern int alloc_tag_ref_offs;
> >>  extern struct alloc_tag_kernel_section kernel_tags;
> >>
> >> +#ifdef CONFIG_MEM_ALLOC_PROFILING_COMPRESSED_ENABLED_BY_DEFAULT
> >> +DECLARE_STATIC_KEY_TRUE(mem_profiling_compressed);
> >> +#else
> >>  DECLARE_STATIC_KEY_FALSE(mem_profiling_compressed);
> >> +#endif
> >>
> >>  typedef u16    pgalloc_tag_idx;
> >>
> >> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> >> index 9fe4d8dfe578..66d8995f3514 100644
> >> --- a/lib/Kconfig.debug
> >> +++ b/lib/Kconfig.debug
> >> @@ -1028,6 +1028,11 @@ config MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT
> >>         default y
> >>         depends on MEM_ALLOC_PROFILING
> >>
> >> +config MEM_ALLOC_PROFILING_COMPRESSED_ENABLED_BY_DEFAULT
> >> +       bool "store page allocation tag references in the page flags by default"
> >> +       default y
> >> +       depends on MEM_ALLOC_PROFILING
> >> +
> >>  config MEM_ALLOC_PROFILING_DEBUG
> >>         bool "Memory allocation profiler debugging"
> >>         default n
> >> diff --git a/lib/alloc_tag.c b/lib/alloc_tag.c
> >> index 25ecc1334b67..30adad5630dd 100644
> >> --- a/lib/alloc_tag.c
> >> +++ b/lib/alloc_tag.c
> >> @@ -31,7 +31,11 @@ DEFINE_STATIC_KEY_MAYBE(CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT,
> >>                         mem_alloc_profiling_key);
> >>  EXPORT_SYMBOL(mem_alloc_profiling_key);
> >>
> >> +#ifdef CONFIG_MEM_ALLOC_PROFILING_COMPRESSED_ENABLED_BY_DEFAULT
> >> +DEFINE_STATIC_KEY_TRUE(mem_profiling_compressed);
> >> +#else
> >>  DEFINE_STATIC_KEY_FALSE(mem_profiling_compressed);
> >> +#endif
> >>
> >>  struct alloc_tag_kernel_section kernel_tags = { NULL, 0 };
> >>  unsigned long alloc_tag_ref_mask;
> >> --
> >> 2.47.1
> >>
>

Re: [PATCH] alloc_tag: introduce Kconfig option for default compressed profiling

[Usama Arif]

On 17/04/2025 01:15, Suren Baghdasaryan wrote:
> On Wed, Apr 16, 2025 at 2:52 PM Usama Arif <usamaarif642@gmail.com> wrote:
>>
>>
>>
>> On 16/04/2025 22:08, Suren Baghdasaryan wrote:
>>> On Wed, Apr 16, 2025 at 11:06 AM Usama Arif <usamaarif642@gmail.com> wrote:
>>>>
>>>> With this Kconfig option enabled, the kernel stores allocation tag references
>>>> in the page flags by default.
>>>>
>>>> There are 2 reasons to introduce this:
>>>> - As mentioned in [1], compressed tags dont have system memory overhead
>>>> and much lower performance overhead. It would be preferrable to have this as
>>>> the default option, and to be able to switch it at compile time. Another
>>>> option is to just declare the static key as true by default?
>>>> - As compressed option is the best one, it doesn't make sense to have to
>>>> change both defconfig and command line options to enable memory
>>>> allocation profiling. Changing commandline across a large number of services
>>>> can result in signifcant work, which shouldn't be needed if the kernel
>>>> defconfig needs to be changed anyways.
>>>
>>> The reason tag compression is not the default option is because it
>>> works only if there are enough free bits in the page flags to store a
>>> tag index. If you configure it to use page flags and your build does
>>> not have enough free bits, the profiling will be disabled (see
>>> alloc_tag_sec_init()). IOW there is no graceful fallback to use page
>>> extensions. Therefore, the current default option is not the most
>>> performant but the one which works on all builds. Instead of this just
>>> set sysctl.vm.mem_profiling boot parameter in your config file.
>>
>> Hi Suren,
> 
> Hi Usama,
> 
>>
>> Thanks for the review! The main reason is to not have to make a change in
>> both defconfig and kernel command line while deploying it. We can ofcourse
>> set the commandline as well, but just makes deployment more tedious, and
>> adds an extra commandline parameter. In our case, we only want to deploy
>> compressed tags, and if there aren't enough free bits, we would prefer to
>> disable memory allocation profiling than to take the memory and performance
>> hit.
>>
>> Would keeping the default value of this config disabled be an acceptable option?
>> i.e. the below diff on top of this patch?
> 
> Well, in that case I fail to see why
> CONFIG_MEM_ALLOC_PROFILING_COMPRESSED_ENABLED_BY_DEFAULT=y is better
> than CONFIG_CMDLINE="sysctl.vm.mem_profiling=1,compressed" ? Either
> way you need to change the config file, no?
> 

Ah ok, I always thought CONFIG_CMDLINE overrides what the kernel gets from the bootloader,
but I just tried it and it appends, as long as CONFIG_CMDLINE_OVERRIDE is not set.
Your above suggestion works as well. Thanks!