linux-kernel - Re: [PATCH] clock: Fix smp_processor

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Message-Id: <20170316210630.GB29977@linux.vnet.ibm.com>
Date:   Thu, 16 Mar 2017 14:06:30 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, mingo@...hat.com, fweisbec@...il.com
Subject: Re: [PATCH] clock: Fix smp_processor_id() in preemptible bug

On Thu, Mar 16, 2017 at 02:00:02PM -0700, Paul E. McKenney wrote:
> On Thu, Mar 16, 2017 at 09:57:48AM -0700, Paul E. McKenney wrote:
> > On Thu, Mar 16, 2017 at 04:53:10PM +0100, Peter Zijlstra wrote:
> > > On Wed, Mar 15, 2017 at 03:58:17PM -0700, Paul E. McKenney wrote:
> > > > On Mon, Mar 13, 2017 at 01:46:21PM +0100, Peter Zijlstra wrote:
> > > > > On Thu, Mar 09, 2017 at 10:37:32AM -0800, Paul E. McKenney wrote:
> > > > > > And it does pass light testing.  I will hammer it harder this evening.
> > > > > > 
> > > > > > So please send a formal patch!
> > > > > 
> > > > > Changed it a bit...
> > > > > 
> > > > > ---
> > > > > Subject: sched/clock: Some clear_sched_clock_stable() vs hotplug wobbles
> > > > > 
> > > > > Paul reported two independent problems with clear_sched_clock_stable().
> > > > > 
> > > > >  - if we tickle it during hotplug (even though the sched_clock was
> > > > >    already marked unstable) we'll attempt to schedule_work() and
> > > > >    this explodes because RCU isn't watching the new CPU quite yet.
> > > > > 
> > > > >  - since we run all of __clear_sched_clock_stable() from workqueue
> > > > >    context, there's a preempt problem.
> > > > > 
> > > > > Cure both by only doing the static_branch_disable() from a workqueue,
> > > > > and only when it's still stable.
> > > > > 
> > > > > This leaves the problem what to do about hotplug actually wrecking TSC
> > > > > though, because if it was stable and now isn't, then we will want to run
> > > > > that work, which then will prod RCU the wrong way.  Bloody hotplug.
> > > > 
> > > > Would it help to do the same trick tglx applied to the hot-unplug path,
> > > > that is IPIing some other CPU to schedule the workqueue?
> > > 
> > > So I've been looking again; and I don't think its a problem anymore.
> > > 
> > > The problem you reported here:
> > > 
> > >  https://lkml.kernel.org/r/20170308221656.GA11949@linux.vnet.ibm.com
> > > 
> > > Should not happen after commit:
> > > 
> > >   f94c8d116997 ("sched/clock, x86/tsc: Rework the x86 'unstable' sched_clock() interface")
> > > 
> > > which landed around 4.11-rc2; so _after_ your kernel (which reported
> > > itself as -rc1).
> > 
> > Very good!  I am re-runnning rcutorture on -rc2 (without your patch)
> > and will let you know how it goes.
> 
> And -rc2 works just fine on rcutorture, thank you again!
> 
> 							Thanx, Paul
> 
> > > Because since that commit we'll never call clear_sched_clock_stable() if
> > > tsc_unstable is set.
> > > 
> > > So I'll have to amend the Changelog somewhat.
> > 
> > Sounds good!

Ah, and I dropped this patch when rebasing to v4.11-rc2 in anticipation
of your Changelog amendments:

	sched/clock: Some clear_sched_clock_stable() vs hotplug wobbles

Please let me know if you would instead prefer me to keep carrying the
current version.

							Thanx, Paul