From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 749422C027A; Tue, 10 Feb 2026 07:38:56 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770709138; cv=none; b=eyOR0yz6wmjXhqLBtFm7o4Kdmc7vJ7FrDHx6xR44/mOJIa8PKgFT34Cms8bv/1ZuTQBQXIrhUfGPd/IwFoG2IuWu8maZJvbDrx71SA8VhRN3hFzdZOc5hIMy5QjYZRljEDA+zY8VeQEdOvOB35ka06Up1McLysn/Qmm7J9i3JZw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770709138; c=relaxed/simple; bh=KAnkHeUW/rhbCgg0wxStZiTN11d44jYJ3LRA79jcEOs=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=j4hRqIOALwcXxsjWkXB22i74JWwQMQuQiysSSO7aIbcq5XHQfytctXOejpV7H6agGaeaNZzeK7rXnfxis48sEBQ3JqIOProE4QhxeumjkDIqUB6xGuB1zwMfPIq/u3V+mGQozru8dIfYaxyBYX49bcnSiPAC073fgethDFsrYz0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 3F2F9339; Mon, 9 Feb 2026 23:38:49 -0800 (PST) Received: from [10.164.19.61] (unknown [10.164.19.61]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 46E0D3F740; Mon, 9 Feb 2026 23:38:52 -0800 (PST) Message-ID: Date: Tue, 10 Feb 2026 13:08:49 +0530 Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/4] memcg: use mod_node_page_state to update stats To: Shakeel Butt , Harry Yoo 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 References: <20251110232008.1352063-1-shakeel.butt@linux.dev> <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> Content-Language: en-US From: Dev Jain In-Reply-To: <7df681ae0f8254f09de0b8e258b909eaacafadf4@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit 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 >>> 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. > >>> 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.