From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pl1-f169.google.com (mail-pl1-f169.google.com [209.85.214.169]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 0B5FE330320 for ; Mon, 10 Nov 2025 19:24:09 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.214.169 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762802651; cv=none; b=Bo7IE5V9pqjEQnjQpMJSKrfk3zaaIPwxkP+x8lxgXyCVOT6hBoZUQDfF085MgxhIfOrRFoJz2FNYzybaXIokQrWvcn4U9iXRqfR0KvHwmahUPaSu8lvlVCBhp+jKlFGDvTwtR7NBwJkcll5EFxV/FOiMoK6xMUxA8PDRu2LzH8U= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762802651; c=relaxed/simple; bh=KtWEwtg7MDJfegORsZg1yms6PhzLTV6xv9D+IvLBI+M=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=AG+GID1dePBjOyWUhvIxDerjC3LBhy9tLB8IzbAJaUyYZHXwzFQiaLgL/tNrZozn7n0q/QMMFVG/QJSnpGgoZrXk2+c4+cKjNwcblfOOa04iw/HadqQTsFXJKoNEApUQN9MnMIGbul4I+UkDbf6apPGagPHPiEW0cORE3D5o6LU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com; spf=pass smtp.mailfrom=gmail.com; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b=j8l3KWm2; arc=none smtp.client-ip=209.85.214.169 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="j8l3KWm2" Received: by mail-pl1-f169.google.com with SMTP id d9443c01a7336-298144fb9bcso13504355ad.0 for ; Mon, 10 Nov 2025 11:24:09 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20230601; t=1762802649; x=1763407449; darn=vger.kernel.org; h=content-transfer-encoding:in-reply-to:from:content-language :references:cc:to:subject:user-agent:mime-version:date:message-id :from:to:cc:subject:date:message-id:reply-to; bh=6aZiAriaFpcSO4WwPdL0RMujLkTEysUvR5KxZIxL0hk=; b=j8l3KWm2pDo9Kw53yv6aMJaJeE9bxY/G3o1UAqtCsq+mp1S5jGbqDYyiM6DourmeJB We1G3j1ng+VpqDJhEZ7yE5+JWINO7ZU07M//FoBcHM1LXkLSorg/mgWFfMOmsf8eYqBb 1paJhfwIEevLkG9CyBv+G4BXBtKUxSaIeY/Y0kSXbVh4P480j7NiddRXH1ht7fGMIDOc +BSWzF4aCtQ8ZIsap36OhOefiI1bE5cBOzoGE1+W9yz6t46CQWSTpJ71zeg4uEHXpeGN ek2N2euvBhL2bQK0HlqZCK0pQqamrRyEnS2O7eXXKoAJatsBEidPGT5bmLRCUXBPjxIy 6JZg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1762802649; x=1763407449; h=content-transfer-encoding:in-reply-to:from:content-language :references:cc:to:subject:user-agent:mime-version:date:message-id :x-gm-gg:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=6aZiAriaFpcSO4WwPdL0RMujLkTEysUvR5KxZIxL0hk=; b=vNX8iJ9SNM05W/lK1KjOQ4DSDxgobC7SF9GyAIvYNPfpE2FAnngvGyg2Cig0tL7ADj Gn+fzHo7bjO9WSVgBC5/M/hIRNjq0qXTPe2FcVYZV3qfzgLIVAQLVLgQYI6TYFxLg55O f/+pQLcp68km5MkSP+GMpdAB+eD1VZF7Vm8z7oqIBJ4ptkGcvG/qvkg4J1L+zey54z9o xjpo9pggoFcagKa2H1JWBhWvm6+LdfocIGBexI/rnM9GGrujf1C3jgbD41Nptp4Wna5x WuISz0EhOF3HXdcFE4RzFrDhcvHLSuuqFmzbDRfhC/Z1J6zwAQ+xKnfDf8Poa/M+7wQK 49qQ== X-Forwarded-Encrypted: i=1; AJvYcCWkUsb39wPVmSHnpo3+hUwZlyNSOdoIU66HXIeBjzsm5tFACcFojN0BDzb0O+aNbGcxhtwxs67cz5s=@vger.kernel.org X-Gm-Message-State: AOJu0YwvOgXjSk8DIwhxUQYXCyLKg/TzklcuzWjg12lGP9Tpw6BFJ3uT /xZQYRTRc2tfvmGJZsrj281QCWdP8kULIGlkhhvyIXd0cX6ScN7XO35X X-Gm-Gg: ASbGnctmJ4gdqNsU4r2G1gPGdLjoupbU99YCktkTgH4XHjPpJDkXGG9OYO4Vk2re8bG ekntJ7TDCVth+mL+9885dYY6HDA8jIbmw+5YJrATmDyZPz+6vTwRbPdIj/uuLl2yc3z1QW0ymsW 3tvjgIKUkMpxAYrLKzh0Hx809PKKZBi+J6Hhqeki0T3jg2hNzZUnY4jamspeLVh0LMaCXxJP5ls yfVfIyltXdcOYsnxPYoaWCJIBXptdLX4nffMUAyQeJV+2jcuhAcauSbyFKPI/EIZzXM2KSEqqgx hmBlQp7Tz/zybTI95uJU4DwgxZMCcLijZ+o3UCqOvNlGMXENoF73iUJ35d/XVtVNpJCSDHMqJ+Y eQspYEzLb1W2pgrtM8c4EH2/B5rfewle1t0WXXLRWcLN7EzHeBZtvsrQqqFusitrsTkPEwMFN5N 7soWpyUwOX7sxmAukct6V0dh6G1kVxcOC3wvCAYTQw7xeI X-Google-Smtp-Source: AGHT+IEfAMgWyE3Ndc3ssUFwIgGkinkSWPsXOWikHUjdgiJLp2PdUMQ1cJ8E1GKkTroGDK1oj6VzdA== X-Received: by 2002:a17:903:fad:b0:295:9b9a:6a7f with SMTP id d9443c01a7336-297e571290cmr120832745ad.49.1762802649165; Mon, 10 Nov 2025 11:24:09 -0800 (PST) Received: from ?IPV6:2a03:83e0:1151:15:9ac6:a0c2:ad3d:6884? ([2620:10d:c090:500::6:8ce7]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-296509680e5sm159021765ad.1.2025.11.10.11.24.06 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 10 Nov 2025 11:24:08 -0800 (PST) Message-ID: <51f1f343-c29f-49b5-8016-bbda4bc778a2@gmail.com> Date: Mon, 10 Nov 2025 11:24:05 -0800 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-new v2] mm/memcontrol: Flush stats when write stat file To: Leon Huang Fu Cc: akpm@linux-foundation.org, cgroups@vger.kernel.org, corbet@lwn.net, hannes@cmpxchg.org, jack@suse.cz, joel.granados@kernel.org, kyle.meyer@hpe.com, lance.yang@linux.dev, laoar.shao@gmail.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, mclapinski@google.com, mhocko@kernel.org, muchun.song@linux.dev, roman.gushchin@linux.dev, shakeel.butt@linux.dev References: <37aa86c5-2659-4626-a80b-b3d07c2512c9@gmail.com> <20251110062053.83754-1-leon.huangfu@shopee.com> Content-Language: en-US From: JP Kobryn In-Reply-To: <20251110062053.83754-1-leon.huangfu@shopee.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 11/9/25 10:20 PM, Leon Huang Fu wrote: > On Fri, Nov 7, 2025 at 1:02 AM JP Kobryn wrote: >> >> On 11/4/25 11:49 PM, Leon Huang Fu wrote: >>> On high-core count systems, memory cgroup statistics can become stale >>> due to per-CPU caching and deferred aggregation. Monitoring tools and >>> management applications sometimes need guaranteed up-to-date statistics >>> at specific points in time to make accurate decisions. >>> >>> This patch adds write handlers to both memory.stat and memory.numa_stat >>> files to allow userspace to explicitly force an immediate flush of >>> memory statistics. When "1" is written to either file, it triggers >>> __mem_cgroup_flush_stats(memcg, true), which unconditionally flushes >>> all pending statistics for the cgroup and its descendants. >>> >>> The write operation validates the input and only accepts the value "1", >>> returning -EINVAL for any other input. >>> >>> Usage example: >>> # Force immediate flush before reading critical statistics >>> echo 1 > /sys/fs/cgroup/mygroup/memory.stat >>> cat /sys/fs/cgroup/mygroup/memory.stat >>> >>> This provides several benefits: >>> >>> 1. On-demand accuracy: Tools can flush only when needed, avoiding >>> continuous overhead >>> >>> 2. Targeted flushing: Allows flushing specific cgroups when precision >>> is required for particular workloads >> >> I'm curious about your use case. Since you mention required precision, >> are you planning on manually flushing before every read? >> > > Yes, for our use case, manual flushing before critical reads is necessary. > We're going to run on high-core count servers (224-256 cores), where the > per-CPU batching threshold (MEMCG_CHARGE_BATCH * num_online_cpus) can > accumulate up to 16,384 events (on 256 cores) before an automatic flush is > triggered. This means memory statistics can be likely stale, often exceeding > acceptable tolerance for critical memory management decisions. > > Our monitoring tools don't need to flush on every read - only when making > critical decisions like OOM adjustments, container placement, or resource > limit enforcement. The opt-in nature of this mechanism allows us to pay the > flush cost only when precision is truly required. > >>> >>> 3. Integration flexibility: Monitoring scripts can decide when to pay >>> the flush cost based on their specific accuracy requirements >> >> [...] >>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c >>> index c34029e92bab..d6a5d872fbcb 100644 >>> --- a/mm/memcontrol.c >>> +++ b/mm/memcontrol.c >>> @@ -4531,6 +4531,17 @@ int memory_stat_show(struct seq_file *m, void *v) >>> return 0; >>> } >>> >>> +int memory_stat_write(struct cgroup_subsys_state *css, struct cftype *cft, u64 val) >>> +{ >>> + if (val != 1) >>> + return -EINVAL; >>> + >>> + if (css) >>> + css_rstat_flush(css); >> >> This is a kfunc. You can do this right now from a bpf program without >> any kernel changes. >> > > While css_rstat_flush() is indeed available as a BPF kfunc, the practical > challenge is determining when to call it. The natural hook point would be > memory_stat_show() using fentry, but this runs into a BPF verifier > limitation: the function's 'struct seq_file *' argument doesn't provide a > trusted path to obtain the 'struct cgroup_subsys_state *css' pointer > required by css_rstat_flush(). Ok, I see this would only work on the css for base stats. SEC("iter.s/cgroup") int cgroup_memcg_query(struct bpf_iter__cgroup *ctx) { struct cgroup *cgrp = ctx->cgroup; struct cgroup_subsys_state *css; if (!cgrp) return 1; /* example of flushing css for base cpu stats * css = container_of(cgrp, struct cgroup_subsys_state, cgroup); * if (!css) * return 1; * css_rstat_flush(css); */ /* get css for memcg stats */ css = cgrp->subsys[memory_cgrp_id]; if (!css) return 1; css_rstat_flush(css); <- confirm untrusted pointer arg error ... > > I attempted to implement this via BPF (code below), but it fails > verification because deriving the css pointer through > seq->private->kn->parent->priv results in an untrusted scalar that the > verifier rejects for the kfunc call: > > R1 invalid mem access 'scalar' > > The verifier error occurs because: > 1. seq->private is rdonly_untrusted_mem > 2. Dereferencing through kernfs_node internals produces untracked pointers > 3. css_rstat_flush() requires a trusted css pointer per its kfunc definition > > A direct userspace interface (memory.stat_refresh) avoids these verifier > limitations and provides a cleaner, more maintainable solution that doesn't > require BPF expertise or complex workarounds. This is subjective. After hearing more about your use case and how you mention making critical decisions, you should have a look at the work being done on BPF OOM [0][1]. I think you would benefit from this series. Specifically for your case it provides the ability to flush memcg on demand and also fetch stats. [0] https://lore.kernel.org/all/20251027231727.472628-1-roman.gushchin@linux.dev/ [1] https://lore.kernel.org/all/20251027232206.473085-2-roman.gushchin@linux.dev/