From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (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 ED385310636 for ; Wed, 11 Feb 2026 15:43:11 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770824593; cv=none; b=F6ZjO3lWmOFDQECqZoMK0aH4qqqKJNxrt+an2zhkIqwpa6Jws+N/qUHCm0awW7HmtfPPQA27dRukHBf5nnc6k8dYTLd07N8Qt++hl5FYw4R7lkM4znCWRTMVYjtDyV2MQbfirvJh8hPcV8NKvhAYBLbtKeyNhXQXhqosvOcSSco= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770824593; c=relaxed/simple; bh=KASUW0pyryAgersxWpn6QLBWZqNmrD3crrlaDFyAuR0=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=C4Ll/ge2wzMtcQW/oqZ9nKo2LYtR2s3pofhhs4x9SqRkuWBoYy2//9u/Z1HPu8Ide9e54afbHybpJMCrI9hubZRELvQb9/4FAUWPhY40wjNUW7NZPbgiSTd7JX/Xps7tlQuexWXGbSTQNUmeyZRRkSWYWGYUeVmD5LKV8lD5Ees= 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=UDb9hAux; arc=none smtp.client-ip=170.10.129.124 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="UDb9hAux" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1770824591; 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=wE7la7K/IKWK0kSdZLLaAcmHE3mv3hvFTPaHbiBOxk8=; b=UDb9hAux+qJH2e75ZO/z/a2h9EOA84fAhccaZ1tiGzSSYLt9La2GZ/mI2gdyrN7Msr/2We KBM1XxMNVsylYQyFxMik0dIV3N2FFWfl4xem+EgbeSd4Om+e1PDfPMNP1r+imRVTYht2CM AojUOG4YviyG8UJA6zPGy62DQLl+ZpE= Received: from mx-prod-mc-03.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-61-CvMfWgf9MqKHQQmxAVGhcg-1; Wed, 11 Feb 2026 10:43:06 -0500 X-MC-Unique: CvMfWgf9MqKHQQmxAVGhcg-1 X-Mimecast-MFC-AGG-ID: CvMfWgf9MqKHQQmxAVGhcg_1770824584 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-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 0E0861955F28; Wed, 11 Feb 2026 15:43:04 +0000 (UTC) Received: from tpad.localdomain (unknown [10.96.133.3]) by mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 0F7911800668; Wed, 11 Feb 2026 15:43:03 +0000 (UTC) Received: by tpad.localdomain (Postfix, from userid 1000) id 7DAC1400DF589; Wed, 11 Feb 2026 09:11:21 -0300 (-03) Date: Wed, 11 Feb 2026 09:11:21 -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 3.4.1 on 10.30.177.93 On Wed, Feb 11, 2026 at 09:01:12AM -0300, Marcelo Tosatti wrote: > 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. OK, this is not true. There is only CONFIG_QPW and the qpw=yes/no kernel boot option for control. CONFIG_PREEMPT_RT should probably select CONFIG_QPW=y and CONFIG_QPW_DEFAULT=y. > 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 >