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 E9E802BD01A for ; Wed, 25 Jun 2025 15:50:32 +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=1750866634; cv=none; b=FcvbwZssZds5jLoKdzToDAZ2yLcBBQsjlM+aHwi/JmsKy17G1IsUmsoRsJZS0wqBCZWnrb4uAAN4uzRlM8gp+ZzDGxligtgjnYdU7fKACg4tNwDiSSlgdmeVbH1JUpPOUfcVokKKjLJCkfM2XSxNj+pQQAm3FXDmYMyiK3GNEkw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750866634; c=relaxed/simple; bh=PQlCGUu9ew2BV9fRAhuf4/5kOypGE+Rg3sdeFr1eoAg=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=ZWVNxpaTEZOq28K1xVIpU32rwAMwnIAAgpMt/16pIXvZ1v/rnM6jQtj/DwhhW3YYkKCx4zxbjrGU1uEO7duIs1/UPGvCPjtAZm6k9d0LMXXckTwmtVbKq6Ac5lLhYlXJDImTCepmIns3TBySG9p4bZlPKw4ptCnE17Xr5KPX19Y= 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=XQ73U4XN; 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="XQ73U4XN" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1750866629; 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=uXch+EGZA2Bjd2coHpx+46DNNAsKlNH/dcmJmL/3v/g=; b=XQ73U4XNSTIKl0Vwj5QIsYcF25OBVmVymNsnjnfegUbWXHNcPdFpd1pfAxLgm6UjsegH0E UGtCIT6kTqZ2w8y4e0fqQSCQHCfT4lCY5cKIIwhQ7phQP8r8aOdIQmndq5mlx5uEwO7Afo q8N7phuH0BAFVRBabUQcmpjHXATP/Ts= Received: from mx-prod-mc-05.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-241-ufTUDgmFOs2ezFP1WpO2CA-1; Wed, 25 Jun 2025 11:50:24 -0400 X-MC-Unique: ufTUDgmFOs2ezFP1WpO2CA-1 X-Mimecast-MFC-AGG-ID: ufTUDgmFOs2ezFP1WpO2CA_1750866623 Received: from mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.40]) (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-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id D11291956095; Wed, 25 Jun 2025 15:50:22 +0000 (UTC) Received: from pauld.westford.csb (unknown [10.22.88.237]) by mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 4AC4D19560A3; Wed, 25 Jun 2025 15:50:20 +0000 (UTC) Date: Wed, 25 Jun 2025 11:50:17 -0400 From: Phil Auld To: Frederic Weisbecker Cc: Waiman Long , 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: <20250625155017.GC57862@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> 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 10.30.177.40 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! Cheers, Phil > Thanks. > > > > > Cheers, > > Phil > > > > > > > > Cheers, > > > Longman > > > > > > > --- > > > > include/linux/sched/isolation.h | 7 +++++++ > > > > kernel/sched/isolation.c | 12 ++++++++++++ > > > > kernel/sched/sched.h | 1 + > > > > 3 files changed, 20 insertions(+) > > > > > > > > diff --git a/include/linux/sched/isolation.h b/include/linux/sched/isolation.h > > > > index f98ba0d71c52..8de4f625a5c1 100644 > > > > --- a/include/linux/sched/isolation.h > > > > +++ b/include/linux/sched/isolation.h > > > > @@ -41,6 +41,9 @@ static inline bool housekeeping_cpu(int cpu, enum hk_type type) > > > > return true; > > > > } > > > > +extern void housekeeping_lock(void); > > > > +extern void housekeeping_unlock(void); > > > > + > > > > extern void __init housekeeping_init(void); > > > > #else > > > > @@ -73,6 +76,8 @@ static inline bool housekeeping_cpu(int cpu, enum hk_type type) > > > > return true; > > > > } > > > > +static inline void housekeeping_lock(void) { } > > > > +static inline void housekeeping_unlock(void) { } > > > > static inline void housekeeping_init(void) { } > > > > #endif /* CONFIG_CPU_ISOLATION */ > > > > @@ -84,4 +89,6 @@ static inline bool cpu_is_isolated(int cpu) > > > > cpuset_cpu_is_isolated(cpu); > > > > } > > > > +DEFINE_LOCK_GUARD_0(housekeeping, housekeeping_lock(), housekeeping_unlock()) > > > > + > > > > #endif /* _LINUX_SCHED_ISOLATION_H */ > > > > diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c > > > > index 83cec3853864..8c02eeccea3b 100644 > > > > --- a/kernel/sched/isolation.c > > > > +++ b/kernel/sched/isolation.c > > > > @@ -18,12 +18,24 @@ static cpumask_var_t housekeeping_cpumasks[HK_TYPE_MAX]; > > > > unsigned long housekeeping_flags; > > > > EXPORT_SYMBOL_GPL(housekeeping_flags); > > > > +DEFINE_STATIC_PERCPU_RWSEM(housekeeping_pcpu_lock); > > > > + > > > > bool housekeeping_enabled(enum hk_type type) > > > > { > > > > return !!(housekeeping_flags & BIT(type)); > > > > } > > > > EXPORT_SYMBOL_GPL(housekeeping_enabled); > > > > +void housekeeping_lock(void) > > > > +{ > > > > + percpu_down_read(&housekeeping_pcpu_lock); > > > > +} > > > > + > > > > +void housekeeping_unlock(void) > > > > +{ > > > > + percpu_up_read(&housekeeping_pcpu_lock); > > > > +} > > > > + > > > > int housekeeping_any_cpu(enum hk_type type) > > > > { > > > > int cpu; > > > > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h > > > > index 475bb5998295..0cdb560ef2f3 100644 > > > > --- a/kernel/sched/sched.h > > > > +++ b/kernel/sched/sched.h > > > > @@ -46,6 +46,7 @@ > > > > #include > > > > #include > > > > #include > > > > +#include > > > > #include > > > > #include > > > > #include > > > > > > > > > > -- > > > > -- > Frederic Weisbecker > SUSE Labs > --