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

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

> 
> 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. 

> > 
> > Also can you confirm whether my analysis of the regression was correct?
> >  Because if it was, then this diff looks wrong - AFAIU preempt_disable()
> >  won't stop an irq handler from interrupting the execution, so this
> >  will introduce a bug for code paths running in irq context.
> > 
> I was worried about the correctness too, but this_cpu_add() is safe
> against IRQs and so the stat will be _eventually_ consistent?
> 
> Ofc it's so confusing! Maybe I'm the one confused.

Yeah there is no issue with proposed patch as it is making the function
re-entrant safe.