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From: Shakeel Butt To: Harry Yoo , Dev Jain Cc: Andrew Morton , Johannes Weiner , Michal Hocko , Roman Gushchin , Muchun Song , Qi Zheng , Vlastimil Babka , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, Meta kernel team , shy828301@gmail.com, cl@gentwo.org Subject: Re: [PATCH 1/4] memcg: use mod_node_page_state to update stats Message-ID: References: <20251110232008.1352063-2-shakeel.butt@linux.dev> <1052a452-9ba3-4da7-be47-7d27d27b3d1d@arm.com> <2638bd96-d8cc-4733-a4ce-efdf8f223183@arm.com> <51819ca5a15d8928caac720426cd1ce82e89b429@linux.dev> <05aec69b-8e73-49ac-aa89-47b371fb6269@arm.com> <4847c300-c7bb-4259-867c-4bbf4d760576@arm.com> <7df681ae0f8254f09de0b8e258b909eaacafadf4@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <7df681ae0f8254f09de0b8e258b909eaacafadf4@linux.dev> X-Migadu-Flow: FLOW_OUT +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 > > > 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, ...) > > > > 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/