From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 3C40CEB3641 for ; Tue, 3 Mar 2026 00:20:23 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 757D26B00F0; Mon, 2 Mar 2026 19:20:22 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 6DB286B00F2; Mon, 2 Mar 2026 19:20:22 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 5BCF96B00F4; Mon, 2 Mar 2026 19:20:22 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0010.hostedemail.com [216.40.44.10]) by kanga.kvack.org (Postfix) with ESMTP id 499756B00F0 for ; Mon, 2 Mar 2026 19:20:22 -0500 (EST) Received: from smtpin29.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay09.hostedemail.com (Postfix) with ESMTP id DAEEE8B8A1 for ; Tue, 3 Mar 2026 00:20:21 +0000 (UTC) X-FDA: 84502845042.29.46FBFBE Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by imf03.hostedemail.com (Postfix) with ESMTP id C79D02000C for ; Tue, 3 Mar 2026 00:20:19 +0000 (UTC) Authentication-Results: imf03.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=Q020LqpM; spf=pass (imf03.hostedemail.com: domain of mtosatti@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=mtosatti@redhat.com; dmarc=pass (policy=quarantine) header.from=redhat.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1772497220; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=+PxAs2WvNVOxCftZ/jr64yaJOBUiDO5P1MZH42ytmr4=; b=Kp7td/LjCNXAA10D4/AqGeEQ9s5KdSCouNh/xD0QLPfrY8GE/8g+d4YS/kG7bBFhA1lfru F0V2UjZFCHajHrmAszP9yjAbA0Z7d0ePfXEqNakxcclRtfiMmUcpfjZSSiIbPvTxPliz+i h+TgH4ivuZSHCqyrke5NSNz2vj2Gr7c= ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1772497220; a=rsa-sha256; cv=none; b=t/lzAjkdA5t9CbL4AvRTigkdlFTezbvj5domu+0wbffK3fJPMYBWMSRYuB0M5+qm7mprzu m/RrWsC5kG5TpgtVExyWsrCI2lYHPEbT9pJpw+fLv78bZ9skdeUzdPXIRLVn22XCuYJ5XC pcZmMFzL3RYtiTYqpDP/V9QZ5KNNq44= ARC-Authentication-Results: i=1; imf03.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=Q020LqpM; spf=pass (imf03.hostedemail.com: domain of mtosatti@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=mtosatti@redhat.com; dmarc=pass (policy=quarantine) header.from=redhat.com DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1772497219; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=+PxAs2WvNVOxCftZ/jr64yaJOBUiDO5P1MZH42ytmr4=; b=Q020LqpM+CYjLP9qlMt+IWiBpBytDhOGDQH/ohlX/Ku9y6d3i+lu2hM/2Smo5JU0AmAHAr jb0sfNgSV9RoB32rbcDldWlURwwnEiiejnnN7D/pWrncFpm3qHVZFHL9P6CKMDJb1falN/ cD1P1y41NC7j/9idj2LMUiC/OBunNq4= Received: from mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-377-6OCu4L77MnWbFpTG33YaJw-1; Mon, 02 Mar 2026 19:20:15 -0500 X-MC-Unique: 6OCu4L77MnWbFpTG33YaJw-1 X-Mimecast-MFC-AGG-ID: 6OCu4L77MnWbFpTG33YaJw_1772497213 Received: from mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.93]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 1BBD21800366; Tue, 3 Mar 2026 00:20:12 +0000 (UTC) Received: from tpad.localdomain (unknown [10.96.133.6]) by mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id BC4521800348; Tue, 3 Mar 2026 00:20:09 +0000 (UTC) Received: by tpad.localdomain (Postfix, from userid 1000) id 17973401CFE28; Mon, 2 Mar 2026 21:19:44 -0300 (-03) Date: Mon, 2 Mar 2026 21:19:44 -0300 From: Marcelo Tosatti To: Leonardo Bras Cc: Michal Hocko , linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org, Johannes Weiner , Roman Gushchin , Shakeel Butt , Muchun Song , Andrew Morton , Christoph Lameter , Pekka Enberg , David Rientjes , Joonsoo Kim , Vlastimil Babka , Hyeonggon Yoo <42.hyeyoo@gmail.com>, Leonardo Bras , Thomas Gleixner , Waiman Long , Boqun Feng , Frederic Weisbecker Subject: Re: [PATCH 0/4] Introduce QPW for per-cpu operations Message-ID: References: <20260206143430.021026873@redhat.com> MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.93 X-Mimecast-MFC-PROC-ID: lBdhemGGYfXJcAolnA6vvSi1WddKqr_QR9y1QKGQAl0_1772497213 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: C79D02000C X-Stat-Signature: p95akfps53t96gz3nzpnxetmhck6mnud X-Rspam-User: X-HE-Tag: 1772497219-335899 X-HE-Meta: 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 6zhZjAsC tURATaTqRaY0zRbaUWldIS1BNxENGMLUNDh3MxWic7E9COGKurWLCa0moanzgdGbSx1s3uGveaDJZKO5ntgBIj1qb8q1Eh3cXDC99 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Feb 27, 2026 at 10:23:27PM -0300, Leonardo Bras wrote: > On Mon, Feb 23, 2026 at 10:06:32AM +0100, Michal Hocko wrote: > > On Fri 20-02-26 18:58:14, Leonardo Bras wrote: > > > On Mon, Feb 16, 2026 at 12:00:55PM +0100, Michal Hocko wrote: > > > > On Sat 14-02-26 19:02:19, Leonardo Bras wrote: > > > > > On Wed, Feb 11, 2026 at 05:38:47PM +0100, Michal Hocko wrote: > > > > > > On Wed 11-02-26 09:01:12, Marcelo Tosatti wrote: > > > > > > > On Tue, Feb 10, 2026 at 03:01:10PM +0100, Michal Hocko wrote: > > > > > > [...] > > > > > > > > What about !PREEMPT_RT? We have people running isolated workloads and > > > > > > > > these sorts of pcp disruptions are really unwelcome as well. They do not > > > > > > > > have requirements as strong as RT workloads but the underlying > > > > > > > > fundamental problem is the same. Frederic (now CCed) is working on > > > > > > > > moving those pcp book keeping activities to be executed to the return to > > > > > > > > the userspace which should be taking care of both RT and non-RT > > > > > > > > configurations AFAICS. > > > > > > > > > > > > > > Michal, > > > > > > > > > > > > > > For !PREEMPT_RT, _if_ you select CONFIG_QPW=y, then there is a kernel > > > > > > > boot option qpw=y/n, which controls whether the behaviour will be > > > > > > > similar (the spinlock is taken on local_lock, similar to PREEMPT_RT). > > > > > > > > > > > > My bad. I've misread the config space of this. > > > > > > > > > > > > > If CONFIG_QPW=n, or kernel boot option qpw=n, then only local_lock > > > > > > > (and remote work via work_queue) is used. > > > > > > > > > > > > > > What "pcp book keeping activities" you refer to ? I don't see how > > > > > > > moving certain activities that happen under SLUB or LRU spinlocks > > > > > > > to happen before return to userspace changes things related > > > > > > > to avoidance of CPU interruption ? > > > > > > > > > > > > Essentially delayed operations like pcp state flushing happens on return > > > > > > to the userspace on isolated CPUs. No locking changes are required as > > > > > > the work is still per-cpu. > > > > > > > > > > > > In other words the approach Frederic is working on is to not change the > > > > > > locking of pcp delayed work but instead move that work into well defined > > > > > > place - i.e. return to the userspace. > > > > > > > > > > > > Btw. have you measure the impact of preempt_disbale -> spinlock on hot > > > > > > paths like SLUB sheeves? > > > > > > > > > > Hi Michal, > > > > > > > > > > I have done some study on this (which I presented on Plumbers 2023): > > > > > https://lpc.events/event/17/contributions/1484/ > > > > > > > > > > Since they are per-cpu spinlocks, and the remote operations are not that > > > > > frequent, as per design of the current approach, we are not supposed to see > > > > > contention (I was not able to detect contention even after stress testing > > > > > for weeks), nor relevant cacheline bouncing. > > > > > > > > > > That being said, for RT local_locks already get per-cpu spinlocks, so there > > > > > is only difference for !RT, which as you mention, does preemtp_disable(): > > > > > > > > > > The performance impact noticed was mostly about jumping around in > > > > > executable code, as inlining spinlocks (test #2 on presentation) took care > > > > > of most of the added extra cycles, adding about 4-14 extra cycles per > > > > > lock/unlock cycle. (tested on memcg with kmalloc test) > > > > > > > > > > Yeah, as expected there is some extra cycles, as we are doing extra atomic > > > > > operations (even if in a local cacheline) in !RT case, but this could be > > > > > enabled only if the user thinks this is an ok cost for reducing > > > > > interruptions. > > > > > > > > > > What do you think? > > > > > > > > The fact that the behavior is opt-in for !RT is certainly a plus. I also > > > > do not expect the overhead to be really be really big. > > > > > > Awesome! Thanks for reviewing! > > > > > > > To me, a much > > > > more important question is which of the two approaches is easier to > > > > maintain long term. The pcp work needs to be done one way or the other. > > > > Whether we want to tweak locking or do it at a very well defined time is > > > > the bigger question. > > > > > > That crossed my mind as well, and I went with the idea of changing locking > > > because I was working on workloads in which deferring work to a kernel > > > re-entry would cause deadline misses as well. Or more critically, the > > > drains could take forever, as some of those tasks would avoid returning to > > > kernel as much as possible. > > > > Could you be more specific please? > > Hi Michal, > Sorry for the delay > > I think Marcelo covered some of the main topics earlier in this > thread: > > https://lore.kernel.org/all/aZ3ejedS7nE5mnva@tpad/ > > But in syntax: > - There are workloads that are projected not avoid as much as possible > return to kernelspace, as they are either cpu intensive, or latency > sensitive (RT workloads) such as low-latency automation. > > There are scenarios such as industrial automation in which > the applications are supposed to reply a request in less than 50us since it > was generated (IIRC), so sched-out, dealing with interruptions, or syscalls > are a no-go. In those cases, using cpu isolation is a must, and since it > can stay really long running in userspace, it may take a very long time to > do any syscall to actually perform the scheduled flush. > > - Other workloads may need to use syscalls, or rely in interrupts, such as > HPC, but it's also not interesting to take long on them, as the time spent > there is time not used for processing the required data. > > Let's say that for the sake of cpu isolation, a lot of different > requests made to given isolated cpu are batched to be run on syscall > entry/exit. It means the next syscall may take much longer than > usual. > - This may break other RT workloads such as sensor/sound/image sampling, > which could be generally ok with some of the faster syscalls for their > application, and now may perceive an error because one of those syscalls > took too long. > > While the qpw approach may cost a few extra cycles, it operates remotelly > and makes the system a bit more predictable. > > Also, when I was planning the mechanism, I remember it was meant to add > zero overhead in case of CONFIG_QPW=n, very little overhead in case of > CONFIG_QPW=y + qpw=0 (a couple of static branches, possibly with the > cost removed by the cpu branch predictor), and only add a few cycles in > case of qpw=1 + !RT. Which means we may be missing just a few adjustments > to get there. Leo, v2 of the patchset adds only 2 cycles to CONFIG_QPW=y + qpw=0. The larger overhead was due to migrate_disable, which is now (on v2) hidden inside the static branch. My bad. > BTW, if the numbers are not that great for your workloads, we could take a > look at adding an extra QPW mode in which local_locks are taken in > the fastpath and it allows the flush wq to be posponed to that point in > syscall return that you mentioned. What I mean is that we don't need to be > limitted to choosing between solutions, but instead allow the user (or > distro) to choose the desired behavior. > > Thanks! > Leo I think 2 cycles is acceptable.