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

[Dev Jain <dev.jain@arm.com>]

On 05/02/26 11:28 am, 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. 
>
>>> 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.

Ah yes, this_cpu_add() does the addition in one shot without read-modify-write.

I am still puzzled whether the original patch was a bug fix or an optimization.
The patch description says that node stat updation uses irq unsafe interface.
Therefore, we had foo() calling __foo() nested with local_irq_save/restore. But
there were code paths which directly called __foo() - so, your patch fixes a bug right
(in which case we should have a Fixes tag)? The patch ensures that mod_node_page_state
is used, and depending on HAVE_CMPXCHG_LOCAL, either uses irq disabling or
preempt_disable + cmpxchg - making the interface irq safe.