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 11:59 PM Andrew Morton
<akpm@linux-foundation.org> wrote:
>
> On Thu, 26 Dec 2024 16:56:00 -0800 Suren Baghdasaryan <surenb@google.com> wrote:
>
> > 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%.
>
> Well, a 50% reduction in a 0.0000000001% overhead ain't much.

I wish the overhead was that low :)

I ran more comprehensive testing on Pixel 6 on Big, Medium and Little cores:

                 Overhead before fixes            Overhead after fixes
                 slab alloc      page alloc          slab alloc      page alloc
Big               6.21%           5.32%                3.31%          4.93%
Medium       4.51%           5.05%                3.79%          4.39%
Little            7.62%           1.82%                6.68%          1.02%


> But I
> added the final sentence to the changelog.
>
> It still doesn't tell us the very simple thing which we're all eager to
> know: how much faster did the kernel get??