Re: [PATCH 2/2] alloc_tag: skip pgalloc_tag_swap if profiling is disabled

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Dec 26, 2024 at 4:23 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Thu, 26 Dec 2024 15:07:39 -0800 Suren Baghdasaryan <surenb@google.com> wrote:
>
> > On Thu, Dec 26, 2024 at 3:01 PM Andrew Morton <akpm@linux-foundation.org> wrote:
> > >
> > > On Thu, 26 Dec 2024 13:16:39 -0800 Suren Baghdasaryan <surenb@google.com> wrote:
> > >
> > > > When memory allocation profiling is disabled, there is no need to swap
> > > > allocation tags during migration. Skip it to avoid unnecessary overhead.
> > > >
> > > > Fixes: e0a955bf7f61 ("mm/codetag: add pgalloc_tag_copy()")
> > > > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > > > Cc: stable@vger.kernel.org
> > >
> > > Are these changes worth backporting?  Some indication of how much
> > > difference the patches make would help people understand why we're
> > > proposing a backport.
> >
> > The first patch ("alloc_tag: avoid current->alloc_tag manipulations
> > when profiling is disabled") I think is worth backporting. It
> > eliminates about half of the regression for slab allocations when
> > profiling is disabled.
>
> um, what regression?  The changelog makes no mention of this.  Please
> send along a suitable Reported-by: and Closes: and a summary of the
> benefits so that people can actually see what this patch does, and why.

Sorry, I should have used "overhead" instead of "regression".
When one sets CONFIG_MEM_ALLOC_PROFILING=y, the code gets instrumented
and even if profiling is turned off, it still has a small performance
cost minimized by the use of mem_alloc_profiling_key static key. I
found a couple of places which were not protected with
mem_alloc_profiling_key, which means that even when profiling is
turned off, the code is still executed. Once I added these checks, the
overhead of the mode when memory profiling is enabled but turned off
went down by about 50%.