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 E230B22ACEF for ; Wed, 25 Jun 2025 12:19:01 +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=1750853943; cv=none; b=uJXbteU8PDaRzTsPjVJtdCvhXy/OKhc4L2V9dGKs2v5GVvFVbwpWkCFlcOHdKaoio4rnNzWYa2T9/JJlDXdSddxKLEj8ou/38u7FVTCZyVkQuIf7+fqYhwVJxOJfUfBh2gWYSiUghuytHUWSc/nke570Livs1xVxPLNRQu0VJHA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750853943; c=relaxed/simple; bh=THpLSUo1zbryMD/lIPlz85eBltoAEkKOC0tgM66ddBc=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=Hoej8zyunzpMkyyeelLXP52HEPIFq40d7WZj6C82lZ7qIgLsppKW/uqdVFxpD0TbR1hI9FJ+VnktChuaMo3yjYqWdbM+LpPp4VchubbZwoqs53Z2eBjXoTypRmUm6PTtAkTfCd7yzokonzE20X1QIoa3hXjRS/lqRfq6pT/p8XA= 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=COhCUnlu; 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="COhCUnlu" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1750853940; 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=dQj2nZA5hc1BOiPdMuCgn0JCpLJnPCr5nB5ytIjhmS4=; b=COhCUnluQR/GlhfPGWkbmXZSe7lyFaH/rJHg9Eysg8W55IfDoKBJwFjZHwL14F0GosD4j5 ZO6tA+BW92hJb1YBEBqGJHWs5szYJ8E+p+Vp/6/GtAB2keGa58Q2/+5H6+M85WoZpHyNF8 73bw+wzZhg7XyMEywxfPwK9DKEKtKX0= 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-621-sN56K696PjybWWhJmgARIA-1; Wed, 25 Jun 2025 08:18:57 -0400 X-MC-Unique: sN56K696PjybWWhJmgARIA-1 X-Mimecast-MFC-AGG-ID: sN56K696PjybWWhJmgARIA_1750853936 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-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 15C0819560B5; Wed, 25 Jun 2025 12:18:56 +0000 (UTC) Received: from pauld.westford.csb (unknown [10.22.88.237]) by mx-prod-int-02.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 84C9E195E340; Wed, 25 Jun 2025 12:18:53 +0000 (UTC) Date: Wed, 25 Jun 2025 08:18:50 -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: <20250625121850.GA57862@pauld.westford.csb> References: <20250620152308.27492-1-frederic@kernel.org> <20250620152308.27492-3-frederic@kernel.org> <3bf95ee2-1340-41b1-9f5c-1563f953c6eb@redhat.com> 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: <3bf95ee2-1340-41b1-9f5c-1563f953c6eb@redhat.com> X-Scanned-By: MIMEDefang 3.0 on 10.30.177.15 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. 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 > > --