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 E8DC52D9EF0 for ; Wed, 11 Feb 2026 15:43:10 +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=1770824592; cv=none; b=IyBX5/kHmXIsMPWhLHdSLA69+AgxQhp4G1YfrAViSj5ScPzNc1K+uZeZA8E6lpqY0NfpZ+HNSNtG1jSViTknlORj5Y209/c5D6CVae8x7UxU3J0y67Do8QnY6vpG6xlOq/+27+AbWr6DCL8XgUVFvqZdul74hA+gtisG+mYlbJk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770824592; c=relaxed/simple; bh=f8P4Oik573PqSqcMl2nrdc4aRqLgKVEZJJEx7z4rc54=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=WJeL+BUMIrNaYsxK76G4lNai66BkH/A3D9NreVMKz7bL+uZBB9XE8iMdjHt5xF7BQT0PQBRkpABwvNEUb03P9NMbAuPN3NuGriNtPQRqYUqLGaGigAm68lACEtPhDysenNDfuLz6Nuqt9fcb4P8wTH0v9dnAqd/FGr5NgOYjFqs= 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=fVc7/mIe; 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="fVc7/mIe" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1770824590; 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=QAsCSJyvhO4BKRpaZq8qCLo9xLrC1Bu2FU5B/EJQXzA=; b=fVc7/mIejKqxg3q6sUgGOM1cUtTs8f7Kr4rU/MDJp0BH8eIjDdgx5nIf8wcyGyzGF8F2vU I2/i7SsoRT2+iRD3Ti2wcmrcyvPNaHapAzuycKNZ734wO0lH7+g7zS2nJpTzQfXYteLQAE 0DZBJo9/8/uQ3leV7VO6YKKBgO9C6pI= 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-608-VY_sHjiSNkmUh9yogSGtRA-1; Wed, 11 Feb 2026 10:43:06 -0500 X-MC-Unique: VY_sHjiSNkmUh9yogSGtRA-1 X-Mimecast-MFC-AGG-ID: VY_sHjiSNkmUh9yogSGtRA_1770824584 Received: from mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.12]) (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 DA8E7180057E; Wed, 11 Feb 2026 15:43:03 +0000 (UTC) Received: from tpad.localdomain (unknown [10.96.133.3]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 0104D19560B7; Wed, 11 Feb 2026 15:43:02 +0000 (UTC) Received: by tpad.localdomain (Postfix, from userid 1000) id 5CE154036F1D0; Wed, 11 Feb 2026 09:01:12 -0300 (-03) Date: Wed, 11 Feb 2026 09:01:12 -0300 From: Marcelo Tosatti To: Michal Hocko Cc: 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> Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 On Tue, Feb 10, 2026 at 03:01:10PM +0100, Michal Hocko wrote: > On Fri 06-02-26 11:34:30, Marcelo Tosatti wrote: > > 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 PREEMPT_RT=n this interfaces just wraps the current > > local_locks + WorkQueue behavior, so no expected change in runtime. > > > > If PREEMPT_RT=y, or CONFIG_QPW=y, 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. > > 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). 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 ? Thanks