From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (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 447373A4F2A for ; Tue, 10 Mar 2026 17:13:51 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773162832; cv=none; b=LIpnmFfi9NGY5EdF4e9xKzTZCxvYaUkjtXrwRxxOsYYlPeMxlQ2ef8ry9lGrRlOfOqbFVHu/KuyLTk4UMRaRV7xfxQBOgxxZgukLoFYzHO5G2YY4nLanslJ6lAEEDCW2XUCbnQ7Bdqlj4Xpfzo1p2runzyvgHiZ0wgpb6qjjDZo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773162832; c=relaxed/simple; bh=KDR0B5TX7uvt90JLgBm+S/9qteOgd/8K23iL3P6TtrM=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=OlIu6FHfD0cwy7ZQ4dBaASkbeEMnDS6YMewUEu3fCEHhKbKx/AVolTIp/bV5aO40F1/xbmuLcXvUxWmTrQ07kypKNlFqnuSeVJuDa3gB/B5DVGGLYcfJ9HIi+BTDAmiXSHrzaj58sA3Tqsz6Rrgx5lXTDv4LS7YSnjqNZfxLx9w= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=Muqwupi7; arc=none smtp.client-ip=170.10.133.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="Muqwupi7" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1773162830; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=sXudXV+vIcCe/FaMWV94uAIHSh+hgQuxJ4MUz1MgDPw=; b=Muqwupi7d9Jo7oA/bo572s2O4QsqH4I9jZmZbuqqca7yXA549dXXP07PnPh+wfVrgfYgEk R+99pXKlJ3EJrciWKF8YF2FQo2db9cKK+2S8nT7g+DNj/HDI2B2yVQSbJRnaYRN/n+hRJg j+oFMxolOpcHxvDwdEnQzkhIuNoyMSU= 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-614-7P6qnvrTPXugEWadv_84rA-1; Tue, 10 Mar 2026 13:13:44 -0400 X-MC-Unique: 7P6qnvrTPXugEWadv_84rA-1 X-Mimecast-MFC-AGG-ID: 7P6qnvrTPXugEWadv_84rA_1773162822 Received: from mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.4]) (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 B4BAE18005B9; Tue, 10 Mar 2026 17:13:40 +0000 (UTC) Received: from tpad.localdomain (unknown [10.96.133.4]) by mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id E99B430002D2; Tue, 10 Mar 2026 17:13:38 +0000 (UTC) Received: by tpad.localdomain (Postfix, from userid 1000) id CBB5D40205ECE; Tue, 10 Mar 2026 14:12:03 -0300 (-03) Date: Tue, 10 Mar 2026 14:12:03 -0300 From: Marcelo Tosatti To: Frederic Weisbecker 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: X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 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 ? > 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 ? > 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 ? :-) 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.