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 454BD8F5B for ; Thu, 20 Feb 2025 15:52:41 +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=1740066764; cv=none; b=Rq3Doio6N6d+0TaMA0I+DcuRf4v1EoxuewG15zk8rRHo83YcyPO3MYPMAtyucVoTHSc7eZq/W0McIO928NagJQDplce/1m8KU7w0OExEb4N+2nTTYvWdMMxwWxIYM1TDZWJTMnxWxoo6+zIluwn4rdCd43BaP+HddxYtj6Qbc38= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1740066764; c=relaxed/simple; bh=nqMLoYyUSVteUWyDESp588+OG8stwgoB6g2nzJC/p08=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=pmP83BT44NWMjNF9JEDO28mhvzPDcMre3NomyQnr/EU/2/UHgfjGGoY8WZs5LBSINVcywWHWH6l3qE3FJY6KCAdGhx5mi3rMlvBZ2IC5RklATLV1Z2ajBgICEfFumG/rykNJbw+JQDMWeY4TMylNemPoz/T3WtCQGwHQ/93dDxU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none 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=cmEBz/mi; arc=none smtp.client-ip=170.10.133.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none 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="cmEBz/mi" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1740066760; 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=vXixf/+e+DbNzE964SK6i49ancXo4iPqgPiTeOrCAsc=; b=cmEBz/miRYA/FqknuaI6TtjNBlT6nhpITtQgqu2jOiSZ/0zmW1OftM28KK2diA/nJ49iHT 3hneWRCI3q+alTLBcMB8rf/6Eu3g5svG+ypDUIBz31IzoVMtqh4Qu2mawA2QKjv81XFPMx XVsvd38i1MzyzA62BW6GLgdnwuVh0j8= Received: from mx-prod-mc-08.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-84-bKOnWXlBNDqNLJT5yxcJGg-1; Thu, 20 Feb 2025 10:52:35 -0500 X-MC-Unique: bKOnWXlBNDqNLJT5yxcJGg-1 X-Mimecast-MFC-AGG-ID: bKOnWXlBNDqNLJT5yxcJGg_1740066754 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-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 5BF0A1801A10; Thu, 20 Feb 2025 15:52:34 +0000 (UTC) Received: from pauld.westford.csb (unknown [10.22.80.224]) by mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 823281800D96; Thu, 20 Feb 2025 15:52:32 +0000 (UTC) Date: Thu, 20 Feb 2025 10:52:29 -0500 From: Phil Auld To: Vishal Chourasia Cc: Madadi Vineeth Reddy , Peter Zijlstra , Juri Lelli , Frederic Weisbecker , Waiman Long , linux-kernel@vger.kernel.org Subject: Re: [CHANGE 1/2] sched/isolation: Make use of more than one housekeeping cpu Message-ID: <20250220155229.GB89684@pauld.westford.csb> References: <20250211140104.420739-1-pauld@redhat.com> <4a8d54e7-fa29-4ce4-9023-3cdffa0807e6@linux.ibm.com> <20250213142653.GA472203@pauld.westford.csb> <20250218150059.GC547103@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=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.93 Hi Vishal, On Thu, Feb 20, 2025 at 02:50:40PM +0530 Vishal Chourasia wrote: > On Tue, Feb 18, 2025 at 10:00:59AM -0500, Phil Auld wrote: > > Hi Vishal. > > > > On Fri, Feb 14, 2025 at 11:08:19AM +0530 Vishal Chourasia wrote: > > > Hi Phil, Vineeth > > > > > > On Thu, Feb 13, 2025 at 09:26:53AM -0500, Phil Auld wrote: > > > > On Thu, Feb 13, 2025 at 10:14:04AM +0530 Madadi Vineeth Reddy wrote: > > > > > Hi Phil Auld, > > > > > > > > > > On 11/02/25 19:31, Phil Auld wrote: > > > > > > The exising code uses housekeeping_any_cpu() to select a cpu for > > > > > > a given housekeeping task. However, this often ends up calling > > > > > > cpumask_any_and() which is defined as cpumask_first_and() which has > > > > > > the effect of alyways using the first cpu among those available. > > > > > > > > > > > > The same applies when multiple NUMA nodes are involved. In that > > > > > > case the first cpu in the local node is chosen which does provide > > > > > > a bit of spreading but with multiple HK cpus per node the same > > > > > > issues arise. > > > > > > > > > > > > Spread the HK work out by having housekeeping_any_cpu() and > > > > > > sched_numa_find_closest() use cpumask_any_and_distribute() > > > > > > instead of cpumask_any_and(). > > > > > > > > > > > > > > > > Got the overall intent of the patch for better load distribution on > > > > > housekeeping tasks. However, one potential drawback could be that by > > > > > spreading HK work across multiple CPUs might reduce the time that > > > > > some cores can spend in deeper idle states which can be beneficial for > > > > > power-sensitive systems. > > > > > > > > > > Thoughts? > > > > > > > > NOHZ_full setups are not generally used in power sensitive systems I think. > > > > They aren't in our use cases at least. > > > > > > > > In cases with many cpus a single housekeeping cpu can not keep up. Having > > > > other HK cpus in deep idle states while the one in use is overloaded is > > > > not a win. > > > > > > To me, an overloaded CPU sounds like where more than one tasks are ready > > > to run, and a HK CPU is one receiving periodic scheduling clock > > > ticks, so HP CPU is bound to comes out of any power-saving state it is in. > > > > If the overload is caused by HK and interrupts there is nothing in the > > system to help. Tasks, sure, can get load balanced. > > > > And as you say, the HK cpus will have generally ticks happening anyway. > > > > > > > > > > If your single HK cpu can keep up then only configure that one HK cpu. > > > > The others will go idle and stay there. And since they are nohz_full > > > > might get to stay idle even longer. > > > While it is good to distribute the load across each HK CPU in the HK > > > cpumask (queuing jobs on different CPUs each time), this can cause > > > jitter in virtualized environments. Unnecessaryily evicting other > > > tenants, when it's better to overload a VP than to wake up other VPs of a > > > tenant. > > > > > > > Sorry I'm not sure I understand your setup. Are your running virtual > > tenants on the HK cpus? nohz_full in the guests? Maybe you only need > > on HK then it won't matter. > > > Firstly, I am unaware if nohz_full is being used in virtualized environment. > Please correct me it is. I am not saying it can't or shouldn't be used, it's just > I don't know if anybody is using it. > I've seen some people trying it in various ways and to varying degrees of success if I recall correctly. > nohz_full in guests would mean that tick is disabled inside the guest but > the host might still be getting ticks. So, I am unsure, whether it is a good > idea to have nohz_full in virtualized environment. > > Nevertheless, the idea of nohz_full is to reduce to the kernel interference > for CPUs marked as nohz_full. And, it does help with guest interference. > > I would like to mention, In SPLPAR environment, scheduling work on > different HK CPU each time can caused VM preemption in a multi-tenant > setup in cases where CPUs in HK cpumask spans across VPs, its better to > consolidate them within few VPs. > > VP is virtual core/processor. I have a hard time reconciling you saying you are not using virtualization and then talking about VMs and VPs ;) Sometimes I forget how interesting PPC can be... > > > My concern is that currently there is no point in having more than > > one HK cpu (per node in a NUMA case). The code as currently implemented > > is just not doing what it needs to. > > > > We have numerous cases where a single HK cpu just cannot keep up and > > the remote_tick warning fires. It also can lead to the other things > > (orchastration sw, HA keepalives etc) on the HK cpus getting starved > > which leads to other issues. In these cases we recommend increasing > > the number of HK cpus. But... that only helps the userspace tasks > > somewhat. It does not help the actual housekeeping part. > > > > It seems clear to me that the intent of the cpumask_any_and() calls > > is to pick _any_ cpu in the hk mask. Not just the first, otherwise > > it would just use cpumask_first_and(). > > > > I'm open to alternate suggestions of how to fix this. > Your approach looks good to me. > > I wanted to mention the case of overcommitted multi-tenant setup > where this will cause noisy neighbour sort of situation, but this can be > avoided by carefully selecting HK CPUs. Yes, it can be configured away, hopefully. Still, this could easily be a sched feat if that make sense to people. Thanks for the review! Cheers, Phil > > Thanks, > vishalc > > > > > > Cheers, > > Phil > > > > > > > > > > I do have a patch that has this controlled by a sched feature if that > > > > is of interest. Then it could be disabled if you don't want it. > > > > > > Vishal > > > > > > > > Cheers, > > > > Phil > > > > > > > > > > > > > > Thanks, > > > > > Madadi Vineeth Reddy > > > > > > > > > > > Signed-off-by: Phil Auld > > > > > > Cc: Peter Zijlstra > > > > > > Cc: Juri Lelli > > > > > > Cc: Frederic Weisbecker > > > > > > Cc: Waiman Long > > > > > > Cc: linux-kernel@vger.kernel.org > > > > > > --- > > > > > > kernel/sched/isolation.c | 2 +- > > > > > > kernel/sched/topology.c | 2 +- > > > > > > 2 files changed, 2 insertions(+), 2 deletions(-) > > > > > > > > > > > > diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c > > > > > > index 81bc8b329ef1..93b038d48900 100644 > > > > > > --- a/kernel/sched/isolation.c > > > > > > +++ b/kernel/sched/isolation.c > > > > > > @@ -40,7 +40,7 @@ int housekeeping_any_cpu(enum hk_type type) > > > > > > if (cpu < nr_cpu_ids) > > > > > > return cpu; > > > > > > > > > > > > - cpu = cpumask_any_and(housekeeping.cpumasks[type], cpu_online_mask); > > > > > > + cpu = cpumask_any_and_distribute(housekeeping.cpumasks[type], cpu_online_mask); > > > > > > if (likely(cpu < nr_cpu_ids)) > > > > > > return cpu; > > > > > > /* > > > > > > diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c > > > > > > index c49aea8c1025..94133f843485 100644 > > > > > > --- a/kernel/sched/topology.c > > > > > > +++ b/kernel/sched/topology.c > > > > > > @@ -2101,7 +2101,7 @@ int sched_numa_find_closest(const struct cpumask *cpus, int cpu) > > > > > > for (i = 0; i < sched_domains_numa_levels; i++) { > > > > > > if (!masks[i][j]) > > > > > > break; > > > > > > - cpu = cpumask_any_and(cpus, masks[i][j]); > > > > > > + cpu = cpumask_any_and_distribute(cpus, masks[i][j]); > > > > > > if (cpu < nr_cpu_ids) { > > > > > > found = cpu; > > > > > > break; > > > > > > > > > > > > > -- > > > > > > > > > > > -- > > > --