Re: [PATCH] mm/page_alloc: simplify kmem cgroup charge/uncharge code

[Shakeel Butt]

+Michal Hocko

Message starts at https://lkml.kernel.org/r/20201207142204.GA18516@rlk

On Mon, Dec 7, 2020 at 10:08 PM Hui Su <sh_def@xxxxxxx> wrote:
>
> On Mon, Dec 07, 2020 at 09:28:46AM -0800, Shakeel Butt wrote:
> > On Mon, Dec 7, 2020 at 6:22 AM Hui Su <sh_def@xxxxxxx> wrote:
> >
> > The reason to keep __memcg_kmem_[un]charge_page functions is that they
> > were called in the very hot path. Can you please check the performance
> > impact of your change and if the generated code is actually same or
> > different.
>
> Hi, Shakeel:
>
> I objdump the mm/page_alloc.o and comapre them, it change the assemble code
> indeed. In fact, it change some code order, which i personally think won't have
> impact on performance. And i ran the ltp mm and conatiner test, it seems nothing
> abnormal.

Did you run the tests in a memcg? The change is behind a static key of
kmem accounting which is enabled for subcontainers.

>
> BUT i still want to check whether this change will have negative impact on
> perforance due to this change code was called in the very hot path like you
> said, AND saddly i did not find a way to quantify the impact on performance.
> Can you give me some suggestion about how to quantify the performance or some
> tool?
>

At least I think we can try with a simple page allocation in a loop
i.e. alloc_page(GFP_KERNEL_ACCOUNT). I will think of any existing
benchmark which exercises this code path.

Michal, do you have any suggestions?