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

[Suren Baghdasaryan <surenb@google.com>]

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.