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Date: Tue, 13 Jan 2026 11:36:26 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Wangyang Guo <wangyang.guo@intel.com>,
	K Prateek Nayak <kprateek.nayak@amd.com>,
	Ingo Molnar <mingo@redhat.com>, Juri Lelli <juri.lelli@redhat.com>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	Valentin Schneider <vschneid@redhat.com>,
	linux-kernel@vger.kernel.org,
	Shrikanth Hegde <sshegde@linux.ibm.com>,
	Benjamin Lei <benjamin.lei@intel.com>,
	Tim Chen <tim.c.chen@linux.intel.com>,
	Tianyou Li <tianyou.li@intel.com>
Subject: Re: [PATCH v2] sched/clock: Avoid false sharing for
 sched_clock_irqtime
Message-ID: <20260113103626.GB830229@noisy.programming.kicks-ass.net>
References: <20260113074807.3404180-1-wangyang.guo@intel.com>
 <20260113100755.GV830755@noisy.programming.kicks-ass.net>
 <CAKfTPtDpdFuVo83ZbXFhTAbydq-5TxTAcf+JWeN8mK3nosJCQA@mail.gmail.com>
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In-Reply-To: <CAKfTPtDpdFuVo83ZbXFhTAbydq-5TxTAcf+JWeN8mK3nosJCQA@mail.gmail.com>

On Tue, Jan 13, 2026 at 11:14:54AM +0100, Vincent Guittot wrote:
> On Tue, 13 Jan 2026 at 11:08, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Tue, Jan 13, 2026 at 03:48:07PM +0800, Wangyang Guo wrote:
> > > Read-mostly sched_clock_irqtime may share the same cacheline with
> > > frequently updated nohz struct. Mark it as __read_mostly to avoid
> > > false sharing issue.
> > >
> >
> > Is there a reason that thing can't be a static_key or so?
> 
> We tried that but disable_sched_clock_irqtime can be called in atomic
> context with mark_tsc_unstable()
> https://lore.kernel.org/all/174161357383.14745.8770394914047302959.tip-bot2@tip-bot2/

Right, but if you note, that very same path has
clear_sched_clock_stable(), and that is a static_key too.

It's just that that punts by pushing it into a workqueue. So why not
lift that workqueue and do both keys or something?