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 049D21A4E6E for ; Fri, 6 Sep 2024 12:36:59 +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=1725626220; cv=none; b=Z+YCWZuBfXtwKqnVWa9mgmmqkh04xHEWXFIAw/dBPXUZzrF3KTnTBA0YDhq9CmL0Uyqm7aMymvKvSN8MQKkOsRhzoT3hzUigPKVsRSohBwsn0zv+gMmr5TE3h4nY/KULi2YAUDOUXSzqpkF/6HuZb3cep/Vm5iyzz6tLsMn0zpY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1725626220; c=relaxed/simple; bh=tJ9daY7WGj0ycxrojbLCpIa6HPeBgYgJObTvkAZ1yDU=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=nkdKJJ4Y5Mw40G1IghAWCsBfAD1F66lKgaFSaj9WkgNkMqDlFp6WYta5RZuOAhNnck6Tb+z5Dl6+0lewwWfHAAe867RM6QvR5KD8jtVSbcwgJsdLtOBz7oAFs9d7w1YY/MU7h3gXD4jnczf6oylU+b24kjCxaxGkjLa4Gcw15eU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=G2QLrCDm; 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="G2QLrCDm" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 25C7EC4CEC7; Fri, 6 Sep 2024 12:36:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1725626219; bh=tJ9daY7WGj0ycxrojbLCpIa6HPeBgYgJObTvkAZ1yDU=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=G2QLrCDmMzLSNe5ilBgAQtx2I77NBNaxen3D1zOOhkbp+RXNVlMZfbw5+rzDZ5iEz m3dTp7FCjuEJBSI9vHZGApiGRMfKOWL79xEjxVzREO7/VXqYCSz6t+OsN6Juyh702s soHEfg8Ed6T0PmlxUV5AQyr5ZpdRq+YmjnpPZcJimbAlvIvPKUItZwhyIgriC6NUYn y6Vq6ky0o3zYNbzhiAYZqJoWi0/D1uBnUhfXXEdiS5LDzhq7P4YrbBkm7kQCtQ3puz tbNRKX8Fd5ZmixojkamuL5h4hAwx8W2/8WxyUUtYSmm4JK9/BRJSpDsC9s7yz/hhQb H2N0BzA4nkysw== Date: Fri, 6 Sep 2024 14:36:56 +0200 From: Frederic Weisbecker To: Peter Zijlstra Cc: Waiman Long , Ingo Molnar , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/3] sched/isolation: Add HK_FLAG_SCHED to nohz_full Message-ID: References: <20240818234520.90186-1-longman@redhat.com> <20240818234520.90186-2-longman@redhat.com> <7fa3dbd5-7c2e-4614-a5f4-258546cb090b@redhat.com> <4822d111-b02d-469a-a457-46392c35021f@redhat.com> <20240904130445.GI4723@noisy.programming.kicks-ass.net> 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: <20240904130445.GI4723@noisy.programming.kicks-ass.net> Le Wed, Sep 04, 2024 at 03:04:45PM +0200, Peter Zijlstra a écrit : > On Wed, Sep 04, 2024 at 02:44:26PM +0200, Frederic Weisbecker wrote: > > Le Tue, Sep 03, 2024 at 09:23:53PM -0400, Waiman Long a écrit : > > > > After discussing with Peter lately, the rules should be: > > > > > > > > 1) If a nohz_full CPU is part of a multi-CPU domain, then it should > > > > be part of load balancing. Peter even says that nohz_full should be > > > > forbidden in this case, because the tick plays a role in the > > > > load balancing. > > > > > > My understand is that most users will use nohz_full together with isolcpus. > > > So nohz_full CPUs are also isolated and not in a sched domain. There may > > > still be user setting nohz_full without isolcpus though, but that should be > > > relatively rare. > > > > Apparently there are users wanting to use isolation along with automatic > > containers deployments such as kubernetes, which doesn't seem to work > > well with isolcpus... > > I've been proposing to get rid of isolcpus for at least the last 15 > years or so. There just isn't a good reason to ever use it. We were > close and then the whole NOHZ_FULL thing came along. > > You can create single CPU partitions using cpusets dynamically. I'm not sure we could have removed isolcpus= even back then. It has always been widely used and we would have broke someone's box. > > > > Anyway, all these nohz_full/kernel_nose setting will only apply to CPUs in > > > isolated cpuset partitions which will not be in a sched domain. > > > > > > > > > > > 2) Otherwise, if CPU is not part of a domain or it is the only CPU of all its > > > > domains, then it can be out of the load balancing machinery. > > > I am aware that a single-cpu domain is the same as being isolated with no > > > load balancing. > > > > By the way is it possible to have a single-cpu domain (sorry I'm a noob here) > > or do such CPU always end up on a null domain? > > IIRC they always end up with the null domain; but its been a while. It > simply doesn't make much sense to have a 1 cpu domain. The way the > topology code works is by always building the full domain tree, and then > throwing away all levels that do not contribute, and in the 1 cpu case, > that would be all of them. > > Look for 'degenerate' in kernel/sched/topology.c. Ok. > > > > > > > > > I'm a bit scared about rule 1) because I know there are existing users of > > > > nohz_full on multi-CPU domains... So I feel a bit trapped. > > > > > > As stated before, this is not a common use case. > > > > Not sure and anyway it's not a forbidden usecase. But this is anyway outside > > the scope of this patchset. > > Most crucially, it is a completely broken setup. It doesn't actually > work well. > > Taking it away will force people to fix their broken. That's a good > thing, no? I'm all for it but isn't the rule not to break userspace? > > > > The isolcpus boot option is deprecated, as stated in kernel-parameters.txt. > > > > We should undeprecate it, apparently it's still widely used. Perhaps by people > > who can't afford to use cpusets/cgroups. > > What is the actual problem with using cpusets? At the very least the > whole nohz_full thing needs to be moved into cpusets so it isn't a fixed > boot time thing anymore. Sure that's the plan. > > > > My plan is to deprecate nohz_full as well once we are able to make dynamic > > > CPU isolation via cpuset works almost as good as isolcpus + nohz_full. > > > > You can't really deprecate such a kernel boot option unfortunately. Believe me > > I wish we could. > > Why not? As I said, the only thing that's kept it around, and worse, > made it more popular again, is this nohz_full nonsense. That never > should've used isolcpus, but that's not something we can do anything > about now. > > Rigid, boot time only things are teh suck. I know... Thanks.