Re: [PATCH 1/4] memcg: use mod_node_page_state to update stats

[Shakeel Butt <shakeel.butt@linux.dev>]

+Yang Shi and Christoph Lameter

On Thu, Feb 05, 2026 at 05:58:44AM +0000, Shakeel Butt wrote:
> > 
> > On Thu, Feb 05, 2026 at 10:50:06AM +0530, Dev Jain wrote:
> > 
> > > 
> > > On 05/02/26 2:08 am, Shakeel Butt wrote:
> > >  On Mon, Feb 02, 2026 at 02:23:54PM +0530, Dev Jain wrote:
> > >  On 02/02/26 10:24 am, Shakeel Butt wrote:
> > >  Hello Shakeel,
> > > 
> > >  We are seeing a regression in micromm/munmap benchmark with this patch, on arm64 -
> > >  the benchmark mmmaps a lot of memory, memsets it, and measures the time taken
> > >  to munmap. Please see below if my understanding of this patch is correct.
> > > 
> > >  Thanks for the report. Are you seeing regression in just the benchmark
> > >  or some real workload as well? Also how much regression are you seeing?
> > >  I have a kernel rebot regression report [1] for this patch as well which
> > >  says 2.6% regression and thus it was on the back-burner for now. I will
> > >  take look at this again soon.
> > > 
> > >  The munmap regression is ~24%. Haven't observed a regression in any other
> > >  benchmark yet.
> > >  Please share the code/benchmark which shows such regression, also if you can
> > >  share the perf profile, that would be awesome.
> > >  https://gitlab.arm.com/tooling/fastpath/-/blob/main/containers/microbench/micromm.c
> > >  You can run this with
> > >  ./micromm 0 munmap 10
> > > 
> > >  Don't have a perf profile, I measured the time taken by above command, with and
> > >  without the patch.
> > > 
> > >  Hi Dev, can you please try the following patch?
> > > 
> > >  From 40155feca7e7bc846800ab8449735bdb03164d6d Mon Sep 17 00:00:00 2001
> > >  From: Shakeel Butt <shakeel.butt@linux.dev>
> > >  Date: Wed, 4 Feb 2026 08:46:08 -0800
> > >  Subject: [PATCH] vmstat: use preempt disable instead of try_cmpxchg
> > > 
> > >  Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
> > >  ---
> > > 
> > [...snip...]
> > 
> > > 
> > > Thanks for looking into this.
> > >  
> > >  But this doesn't solve it :( preempt_disable() contains a compiler barrier,
> > >  probably that's why.
> > > 
> > I think the reason why it doesn't solve the regression is because of how
> > arm64 implements this_cpu_add_8() and this_cpu_try_cmpxchg_8().
> > 
> > On arm64, IIUC both this_cpu_try_cmpxchg_8() and this_cpu_add_8() are
> > implemented using LL/SC instructions or LSE atomics (if supported).
> > 
> > See:
> > - this_cpu_add_8()
> >  -> __percpu_add_case_64
> >  (which is generated from PERCPU_OP)
> > 
> > - this_cpu_try_cmpxchg_8()
> >  -> __cpu_fallback_try_cmpxchg(..., this_cpu_cmpxchg_8)
> >  -> this_cpu_cmpxchg_8()
> >  -> cmpxchg_relaxed()
> >  -> raw_cmpxchg_relaxed()
> >  -> arch_cmpxchg_relaxed()
> >  -> __cmpxchg_wrapper()
> >  -> __cmpxchg_case_64()
> >  -> __lse_ll_sc_body(_cmpxchg_case_64, ...)
> > 
> 
> Oh so it is arm64 specific issue. I tested on x86-64 machine and it solves
> the little regression it had before. So, on arm64 all this_cpu_ops i.e. without
> double underscore, uses LL/SC instructions. 
> 
> Need more thought on this. 
>

It seems like Yang Shi is looking into improving this_cpu_ops for arm64.

https://lore.kernel.org/CAHbLzkpcN-T8MH6=W3jCxcFj1gVZp8fRqe231yzZT-rV_E_org@mail.gmail.com/