Re: [PATCH v6 2/2] sched/uclamp: Protect uclamp fast path code with static key

From: Qais Yousef <qais.yousef@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>,
	Valentin Schneider <valentin.schneider@arm.com>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	Patrick Bellasi <patrick.bellasi@matbug.net>,
	Chris Redpath <chris.redpath@arm.com>,
	Lukasz Luba <lukasz.luba@arm.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v6 2/2] sched/uclamp: Protect uclamp fast path code with static key
Date: Tue, 30 Jun 2020 20:28:22 +0100	[thread overview]
Message-ID: <20200630192821.xzg53b3mx7hvjmr4@e107158-lin.cambridge.arm.com> (raw)
In-Reply-To: <20200630190643.GC4817@hirez.programming.kicks-ass.net>

On 06/30/20 21:06, Peter Zijlstra wrote:
> On Tue, Jun 30, 2020 at 06:55:02PM +0100, Qais Yousef wrote:
> > On 06/30/20 19:07, Peter Zijlstra wrote:
> 
> > > There's a fun race described in 9107c89e269d ("perf: Fix race between
> > > event install and jump_labels"), are we sure this isn't also susceptible
> > > to something similar?
> > > 
> > > I suspect not, but I just wanted to make sure.
> > 
> > IIUC, the worry is that not all CPUs might have observed the change in the
> > static key state; hence could not be running the patched
> > enqueue/dequeue_task(), so we could end up with some CPUs accounting for
> > uclamp in the enqueue/dequeue path but not others?
> > 
> > I was hoping this synchronization is guaranteed by the static_branch_*() call.
> 
> It is, that isn't quite the the problem. Looking at it more I think
> commit 1dbb6704de91 ("jump_label: Fix concurrent
> static_key_enable/disable()") fixed some of it.
> 
> From what I can remember there were two parts to this problem, one being
> fixed by the above commit, the other being that if we enable while a
> task is running we miss the switch-in event (exactly how in this patch
> we miss the enqueue).
> 
> Due to the missing switch-in, the state is 'weird' and the subsequent
> IPI to install a remote event didn't quite work.
> 
> So I put that sync_sched() call in to guarantee all CPUs have done a
> schedule() cycle after having the key switched. This makes sure that
> every running task has seen the switch-in and thus the state is as
> expected.
> 
> But like I said, I think we're good, that one extra branch deals with
> the half-state.

Got it, thanks.

Yes, we should be good for currently running tasks.

Thanks

--
Qais Yousef