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 F00092747B for ; Fri, 27 Jun 2025 00:49:02 +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=1750985344; cv=none; b=UoQsiZNpTImG/VscIU0nUJN6DlDMDbPd5fcbe/ylxpQ79XdWS2GCRO/YXXkm8ETwc5u/uWCGXsQQZGpu9wK2qAV3cjYhuzgWOQK2cb45DAW4V6VuzO4VfAbXeW4TdixKDgfpxOdbUvYshTtMEjsS8Uj+DY9tD1aHX80y8RUZnLU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750985344; c=relaxed/simple; bh=y3AmZRizhCYu2Dn2RysfKaGz8kN/CPxFAiMyRDa+2Ds=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=VXL1dWWE4Mu1pqnfhq6MZgynTipHW45DhqzxUGPWRZVatTI9NXV6N1bCERxoJ+29NYWE2d8ZMu30to2kUEebvESHVE3OzvlifDdh8yKDKlqJMm9EE+a168T7fyWA53MU1fydrI7r5dJvSLvqoKa8SypceSfAN9Jwm/ktNGEiaBw= 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=FayOwSZp; 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="FayOwSZp" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1750985342; 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=CLA+OYU1ylbvB7vJ0ft6aI2rAS7z7d9CDwxTqg9DYJE=; b=FayOwSZpYGW/Vr0YI74vsPyuzFrzssmG7uVyd0EIBpmZHKcsqe7VBQPDRiwpGcg9JiUWSP qv5ey//dutrhpb68LnoN5AgYdlX4qepXf/d2oyalgN8RzwY3dhFyqrM1tItsh3YOtMNHfY lJNiLUFihRzCssmBXkPagjt60f/8PXQ= Received: from mx-prod-mc-04.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-398-oZyMqZ-7M8inndEbKaFkmQ-1; Thu, 26 Jun 2025 20:48:58 -0400 X-MC-Unique: oZyMqZ-7M8inndEbKaFkmQ-1 X-Mimecast-MFC-AGG-ID: oZyMqZ-7M8inndEbKaFkmQ_1750985337 Received: from mx-prod-int-02.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-02.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.15]) (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-04.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 843FB19560B9; Fri, 27 Jun 2025 00:48:56 +0000 (UTC) Received: from pauld.westford.csb (unknown [10.22.82.105]) by mx-prod-int-02.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 58736195609D; Fri, 27 Jun 2025 00:48:54 +0000 (UTC) Date: Thu, 26 Jun 2025 20:48:51 -0400 From: Phil Auld To: Waiman Long Cc: Frederic Weisbecker , LKML , Ingo Molnar , Marco Crivellari , Michal Hocko , Peter Zijlstra , Tejun Heo , Thomas Gleixner , Vlastimil Babka Subject: Re: [PATCH 02/27] sched/isolation: Introduce housekeeping per-cpu rwsem Message-ID: <20250627004851.GB222768@pauld.westford.csb> References: <20250620152308.27492-1-frederic@kernel.org> <20250620152308.27492-3-frederic@kernel.org> <3bf95ee2-1340-41b1-9f5c-1563f953c6eb@redhat.com> <20250625121850.GA57862@pauld.westford.csb> <20250625155017.GC57862@pauld.westford.csb> 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 3.0 on 10.30.177.15 On Thu, Jun 26, 2025 at 08:11:54PM -0400 Waiman Long wrote: > On 6/25/25 11:50 AM, Phil Auld wrote: > > On Wed, Jun 25, 2025 at 04:34:18PM +0200 Frederic Weisbecker wrote: > > > Le Wed, Jun 25, 2025 at 08:18:50AM -0400, Phil Auld a écrit : > > > > Hi Waiman, > > > > > > > > On Mon, Jun 23, 2025 at 01:34:58PM -0400 Waiman Long wrote: > > > > > On 6/20/25 11:22 AM, Frederic Weisbecker wrote: > > > > > > The HK_TYPE_DOMAIN isolation cpumask, and further the > > > > > > HK_TYPE_KERNEL_NOISE cpumask will be made modifiable at runtime in the > > > > > > future. > > > > > > > > > > > > The affected subsystems will need to synchronize against those cpumask > > > > > > changes so that: > > > > > > > > > > > > * The reader get a coherent snapshot > > > > > > * The housekeeping subsystem can safely propagate a cpumask update to > > > > > > the susbsytems after it has been published. > > > > > > > > > > > > Protect against readsides that can sleep with per-cpu rwsem. Updates are > > > > > > expected to be very rare given that CPU isolation is a niche usecase and > > > > > > related cpuset setup happen only in preparation work. On the other hand > > > > > > read sides can occur in more frequent paths. > > > > > > > > > > > > Signed-off-by: Frederic Weisbecker > > > > > Thanks for the patch series and it certainly has some good ideas. However I > > > > > am a bit concern about the overhead of using percpu-rwsem for > > > > > synchronization especially when the readers have to wait for the completion > > > > > on the writer side. From my point of view, during the transition period when > > > > > new isolated CPUs are being added or old ones being removed, the reader will > > > > > either get the old CPU data or the new one depending on the exact timing. > > > > > The effect the CPU selection may persist for a while after the end of the > > > > > critical section. > > > > > > > > > > Can we just rely on RCU to make sure that it either get the new one or the > > > > > old one but nothing in between without the additional overhead? > > > > > > > > > > My current thinking is to make use CPU hotplug to enable better CPU > > > > > isolation. IOW, I would shut down the affected CPUs, change the housekeeping > > > > > masks and then bring them back online again. That means the writer side will > > > > > take a while to complete. > > > > The problem with this approach is that offlining a cpu effects all the other > > > > cpus and causes latency spikes on other low latency tasks which may already be > > > > running on other parts of the system. > > > > > > > > I just don't want us to finally get to dynamic isolation and have it not > > > > usable for the usecases asking for it. > > > We'll have to discuss that eventually because that's the plan for nohz_full. > > > We can work around the stop machine rendez-vous on nohz_full if that's the > > > problem. If the issue is not to interrupt common RT-tasks, then that's a > > > different problem for which I don't have a solution. > > > > > My understanding is that it's the stop machine issue. If you have a way > > around that then great! > > My current thinking is to just run a selected set of CPUHP teardown and > startup methods relevant to housekeeping cpumasks usage without calling the > full set from CPUHP_ONLINE to CPUHP_OFFLINE. I don't know if it is possible > or not or how much additional changes will be needed to make that possible. > That will skip the CPUHP_TEARDOWN_CPU teardown method that is likely the > cause of most the latency spike experienced by other CPUs. > Yes, CPUHP_TEARDOWN_CPU is the source of the stop_machine I believe. It'll be interesting to see if you can safely use the cpuhp machinery selectively like that :) Cheers, Phil > Cheers, > Longman > --