From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id CCAF939E167 for ; Thu, 9 Apr 2026 09:36:33 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775727393; cv=none; b=ezvi/o3xEODFNwnrW/IqzjhERh5yY9DkMfUqssAEFt1gVNb3x4Dk1TGfWW4uWvyDXNnwrChtly47UwD48pvTxYjYdLbim71R3dbc3ZtdytOawMAHxrPq9jWhhmflp92qJ5oAEiCDk0vVofT4wGhT1+45N+G843kzdkjQuFgB5tk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775727393; c=relaxed/simple; bh=U0RZzJ1RvS+lKgIqqq906rosP31H3ovBpVd8yaLJNuw=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=J4KaYGkR3UtgihY+QfcGvIOA1fKD+djJQCG8nrpOzh4enLpTm2dzp3Y8ATRKlpp8euNtNNrdIyR//w8yDj+WJxzc4ExnM6F4a094kre9kdk43dIRvaegVM9q198IlS6589G/dlB2SWu+CfASsiUkMZhKt5mXzteoDg3vwMaLX4w= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=gvazIDQ9; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="gvazIDQ9" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C5666C4CEF7; Thu, 9 Apr 2026 09:36:30 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775727393; bh=U0RZzJ1RvS+lKgIqqq906rosP31H3ovBpVd8yaLJNuw=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=gvazIDQ9nnc67MwDQU1usgufWROXpImnvTQ+UepFJ/vDiSOIPN5lTnz00rClKK1e6 mwodZY1o8sEOL3X0uq2QiRpUQ5pEJdTJ5XYjTVFIJpnT6AwrltVn5KwYtEGpisqAZo AyHOna1A8ZBDkWCGQeMZU1IFuNeylv6/3M++pT7RKiMVEkDAmSfSVV4u1YdZEY2nqh w6+LeyGglPXv40/969QoRowJ08PwIBnWJ7kqcazUVLFQ57szrXDse2LcQqtX3hWli2 M2XQfY5xLltUZvaMQIpigdRgZ6zx2wEqs80ir9VQIeP7MxFAISv/k2TTWITcSfvftG 0+3T397xYx4Zg== Message-ID: <78a063c5-286d-4eea-8aae-2c0f1d0847e9@kernel.org> Date: Thu, 9 Apr 2026 11:36:29 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/vmstat: spread vmstat_update requeue across the stat interval Content-Language: en-US To: Breno Leitao Cc: Andrew Morton , David Hildenbrand , Lorenzo Stoakes , "Liam R. Howlett" , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kas@kernel.org, shakeel.butt@linux.dev, usama.arif@linux.dev, kernel-team@meta.com References: <20260401-vmstat-v1-1-b68ce4a35055@debian.org> From: "Vlastimil Babka (SUSE)" In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 4/8/26 19:00, Breno Leitao wrote: > On Wed, Apr 08, 2026 at 08:13:43AM -0700, Breno Leitao wrote: >> On Wed, Apr 08, 2026 at 12:13:04PM +0200, Vlastimil Babka (SUSE) wrote: >> >> diff --git a/mm/vmstat.c b/mm/vmstat.c >> index 2370c6fb1fcd6..8d53242e7aa66 100644 >> --- a/mm/vmstat.c >> +++ b/mm/vmstat.c >> @@ -2139,8 +2139,12 @@ static void vmstat_shepherd(struct work_struct *w) >> if (cpu_is_isolated(cpu)) >> continue; >> >> - if (!delayed_work_pending(dw) && need_update(cpu)) >> + if (!delayed_work_pending(dw) && need_update(cpu)) { >> + WARN_ONCE(work_busy(&dw->work) & WORK_BUSY_RUNNING, >> + "cpu%d: vmstat_update already running, scheduling again\n", >> + cpu); >> queue_delayed_work_on(cpu, mm_percpu_wq, dw, 0); >> + } >> } >> >> cond_resched(); >> >> The fix is a one-line change: !delayed_work_pending(dw) → !work_busy(&dw->work) > > In my testing, this race condition occurs more frequently than expected, > likely due to the timer configurations we've been discussing throughout > this thread. > > I developed a diagnostic patch to monitor vmstat_update worker scheduling > frequency, and the results show consistently low values. Avoiding > rescheduling a worker that is already also reduces the contention on > stress-ng test case. > > commit d725f0664b70aa5c677215b0fc1abc0117aaf114 > Author: Breno Leitao > Date: Wed Apr 8 09:01:02 2026 -0700 > > mm/vmstat: fix vmstat_shepherd double-scheduling vmstat_update > > vmstat_shepherd uses delayed_work_pending() to check whether > vmstat_update is already scheduled for a given CPU before queuing it. > However, delayed_work_pending() only tests WORK_STRUCT_PENDING_BIT, > which is cleared the moment a worker thread picks up the work to > execute it. > > This means that while vmstat_update is actively running on a CPU, > delayed_work_pending() returns false. If need_update() also returns > true at that point (per-cpu counters not yet zeroed mid-flush), the > shepherd queues a second invocation with delay=0, causing vmstat_update > to run again immediately after finishing. > > On a 72-CPU system this race is readily observable: before the fix, > many CPUs show invocation gaps well below 500 jiffies (the minimum > round_jiffies_relative() can produce), with the most extreme cases > reaching 0 jiffies—vmstat_update called twice within the same jiffy. > > Fix this by replacing delayed_work_pending() with work_busy(), which > returns non-zero for both WORK_BUSY_PENDING (timer armed or work > queued) and WORK_BUSY_RUNNING (work currently executing). The shepherd > now correctly skips a CPU in all busy states. > > After the fix, all sub-jiffy and most sub-100-jiffie gaps disappear. > The remaining early invocations have gaps in the 700–999 jiffie range, > attributable to round_jiffies_relative() aligning to a nearer > jiffie-second boundary rather than to this race. > > Each spurious vmstat_update invocation has a measurable side effect: > refresh_cpu_vm_stats() calls decay_pcp_high() for every zone, which > drains idle per-CPU pages back to the buddy allocator via > free_pcppages_bulk(), taking the zone spinlock each time. Eliminating > the double-scheduling therefore reduces zone lock contention directly. > On a 72-CPU stress-ng workload measured with perf lock contention: > > free_pcppages_bulk contention count: ~55% reduction > free_pcppages_bulk total wait time: ~57% reduction > free_pcppages_bulk max wait time: ~47% reduction Great! > Note: work_busy() is inherently racy—between the check and the > subsequent queue_delayed_work_on() call, vmstat_update can finish > execution, leaving the work neither pending nor running. In that > narrow window the shepherd can still queue a second invocation. > After the fix, this residual race is rare and produces only occasional > small gaps, a significant improvement over the systematic > double-scheduling seen with delayed_work_pending(). > > Signed-off-by: Breno Leitao Reviewed-by: Vlastimil Babka (SUSE) > diff --git a/mm/vmstat.c b/mm/vmstat.c > index d59eff1582547..5489549241b51 100644 > --- a/mm/vmstat.c > +++ b/mm/vmstat.c > @@ -2156,7 +2156,7 @@ static void vmstat_shepherd(struct work_struct *w) > if (cpu_is_isolated(cpu)) > continue; > > - if (!delayed_work_pending(dw) && need_update(cpu)) > + if (!work_busy(&dw->work) && need_update(cpu)) > queue_delayed_work_on(cpu, mm_percpu_wq, dw, 0); > } >