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 59F0E3CFF4F for ; Tue, 10 Mar 2026 22:14:31 +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=1773180871; cv=none; b=kfWYsoTfRLaqY3UpWN6BD11b4tdiy4di298IgrAdBgrTzkKyEsU1Byx7jJ7qAG5f3/InFh4Wbp8PcTqGCbez+b472MIJ7uT4PInhSrN7O5RmFw6wCFxsKCzac7rabkF1Gey4xrWwl3D+5vJc/FlVhy4G6Wg28xkhJrZB1mDi22M= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773180871; c=relaxed/simple; bh=2LNNTUmBqtO5SmWcgIzMSfKem41SdrpfZxaik99VIGo=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=JTuLBD23fagTpmt5zQLqgR9ot+2h2wgFIC1EFzn7aJCaRCn/OL5qnM3TeSDIfvarbWtT9QIQREIFoZDN4CX+1VaB5xGNHS6UwwsRaeXjUWPaIGcXVhaFjTrAzU+RRtb6lPs2ATIikoyiJu9c3uuhpyIWdmMTO+/7IpzMtCNBqqg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=H3xEWPdp; 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="H3xEWPdp" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D6610C19423; Tue, 10 Mar 2026 22:14:30 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1773180871; bh=2LNNTUmBqtO5SmWcgIzMSfKem41SdrpfZxaik99VIGo=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=H3xEWPdp48GuEXQBuaHPzU5b12T4494AsA+9u3S8buE6SakGwCFJFLDQCPwxiwQTI 9HG5eLkD9b5DE5yrFkpAr/IOXDb5MMfXK9rffO4JV+vnpGkC+or4AASmZqK1+xCY5B hHZZvKcowjwvto1MpcywIrH6PUuE07Mq8fcgByTglOQRFcydCKsBNo2BIy8LSjmeL3 z9l6XX/Eu6yyiLAXXLnUe5g3TBfiPQX20CTao2vVR6xKKet5FwGbzANKYlxOlwRZrc oiiRBrokQPzAwHNVSPvjDGPkeHmgTudSorj54jgc3pclQkDSdM5ontmjyXmp9UWDjq WO/o4+coy4+3Q== Date: Tue, 10 Mar 2026 23:14:28 +0100 From: Frederic Weisbecker To: Marcelo Tosatti Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Johannes Weiner , Michal Hocko , 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 Feun Subject: Re: [PATCH v2 0/5] Introduce QPW for per-cpu operations (v2) Message-ID: References: <20260302154945.143996316@redhat.com> 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=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Le Tue, Mar 10, 2026 at 02:12:03PM -0300, Marcelo Tosatti a écrit : > Hi Frederic, > > On Thu, Mar 05, 2026 at 05:55:12PM +0100, Frederic Weisbecker wrote: > > Le Mon, Mar 02, 2026 at 12:49:45PM -0300, Marcelo Tosatti a écrit : > > > The problem: > > > Some places in the kernel implement a parallel programming strategy > > > consisting on local_locks() for most of the work, and some rare remote > > > operations are scheduled on target cpu. This keeps cache bouncing low since > > > cacheline tends to be mostly local, and avoids the cost of locks in non-RT > > > kernels, even though the very few remote operations will be expensive due > > > to scheduling overhead. > > > > > > On the other hand, for RT workloads this can represent a problem: getting > > > an important workload scheduled out to deal with remote requests is > > > sure to introduce unexpected deadline misses. > > > > > > The idea: > > > Currently with PREEMPT_RT=y, local_locks() become per-cpu spinlocks. > > > In this case, instead of scheduling work on a remote cpu, it should > > > be safe to grab that remote cpu's per-cpu spinlock and run the required > > > work locally. That major cost, which is un/locking in every local function, > > > already happens in PREEMPT_RT. > > > > > > Also, there is no need to worry about extra cache bouncing: > > > The cacheline invalidation already happens due to schedule_work_on(). > > > > > > This will avoid schedule_work_on(), and thus avoid scheduling-out an > > > RT workload. > > > > > > Proposed solution: > > > A new interface called Queue PerCPU Work (QPW), which should replace > > > Work Queue in the above mentioned use case. > > > > > > If CONFIG_QPW=n this interfaces just wraps the current > > > local_locks + WorkQueue behavior, so no expected change in runtime. > > > > > > If CONFIG_QPW=y, and qpw kernel boot option =1, > > > queue_percpu_work_on(cpu,...) will lock that cpu's per-cpu structure > > > and perform work on it locally. This is possible because on > > > functions that can be used for performing remote work on remote > > > per-cpu structures, the local_lock (which is already > > > a this_cpu spinlock()), will be replaced by a qpw_spinlock(), which > > > is able to get the per_cpu spinlock() for the cpu passed as parameter. > > > > So let me summarize what are the possible design solutions, on top of our discussions, > > so we can compare: > > > > 1) Never queue remotely but always queue locally and execute on userspace > > return via task work. > > How can you "queue locally" if the request is visible on a remote CPU? > > That is, the event which triggers the manipulation of data structures > which need to be performed by the owner CPU (owner of the data > structures) is triggered on a remote CPU. > > This is confusing. > > Can you also please give a practical example of such case ? Right so in the case of LRU batching, it consists in always queue locally as soon as there is something to do. Then no remote queueing is necessary. Like here: https://lwn.net/ml/all/20250703140717.25703-7-frederic@kernel.org/ > > > Pros: > > - Simple and easy to maintain. > > > > Cons: > > - Need a case by case handling. > > > > - Might be suitable for full userspace applications but not for > > some HPC usecases. In the best world MPI is fully implemented in > > userspace but that doesn't appear to be the case. > > > > 2) Queue locally the workqueue right away > > Again, the event which triggers the manipulation of data structures > by the owner CPU happens on a remote CPU. > So how can you queue it locally ? So that would be the same as above but instead of using task_work(), we would force queue a workqueue locally. It's more agressive. > > > or do it remotely (if it's > > really necessary) if the isolated CPU is in userspace, otherwise queue > > it for execution on return to kernel. The work will be handled by preemption > > to a worker or by a workqueue flush on return to userspace. > > > > Pros: > > - The local queue handling is simple. > > > > Cons: > > - The remote queue must synchronize with return to userspace and > > eventually postpone to return to kernel if the target is in userspace. > > Also it may need to differentiate IRQs and syscalls. > > > > - Therefore still involve some case by case handling eventually. > > > > - Flushing the global workqueues to avoid deadlocks is unadvised as shown > > in the comment above flush_scheduled_work(). It even triggers a > > warning. Significant efforts have been put to convert all the existing > > users. It's not impossible to sell in our case because we shouldn't > > hold a lock upon return to userspace. But that will restore a new > > dangerous API. > > > > - Queueing the workqueue / flushing involves a context switch which > > induce more noise (eg: tick restart) > > > > - As above, probably not suitable for HPC. > > > > 3) QPW: Handle the work remotely > > > > Pros: > > - Works on all cases, without any surprise. > > > > Cons: > > - Introduce new locking scheme to maintain and debug. > > > > - Needs case by case handling. > > > > Thoughts? > > Can you please be more verbose, mindful of lesser cognitive powers ? :-) Arguably verbosity is not my most developed skill :o) > > Note: i also dislike the added layers (and multiple cases) QPW adds. > > But there is precedence with local locks... > > Code would be less complex in case spinlocks were added: > > 01b44456a7aa7c3b24fa9db7d1714b208b8ef3d8 mm/page_alloc: replace local_lock with normal spinlock > 4b23a68f953628eb4e4b7fe1294ebf93d4b8ceee mm/page_alloc: protect PCP lists with a spinlock > > But people seem to reject that in the basis of performance > degradation. And that makes sense. Anyway, we have lockdep to help. Thanks. -- Frederic Weisbecker SUSE Labs