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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 > > Date: Wed, 4 Feb 2026 08:46:08 -0800 > > Subject: [PATCH] vmstat: use preempt disable instead of try_cmpxchg > > > > Signed-off-by: Shakeel Butt > > --- [...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, ...) > 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. -- Cheers, Harry / Hyeonggon