linux-kernel - Re: [PATCH 4.4 173/268] sched/rt: Fix rq->clock_update_flags < RQCF_ACT

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Date:   Thu, 14 Jun 2018 18:32:11 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Ben Hutchings <ben.hutchings@...ethink.co.uk>
Cc:     Davidlohr Bueso <dbueso@...e.de>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        stable@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mike Galbraith <efault@....de>,
        Thomas Gleixner <tglx@...utronix.de>, dave@...olabs.net,
        Ingo Molnar <mingo@...nel.org>,
        Sasha Levin <alexander.levin@...rosoft.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4.4 173/268] sched/rt: Fix rq->clock_update_flags <
 RQCF_ACT_SKIP warning

On Thu, 14 Jun 2018 22:55:56 +0100
Ben Hutchings <ben.hutchings@...ethink.co.uk> wrote:

> On Mon, 2018-05-28 at 12:02 +0200, Greg Kroah-Hartman wrote:
> > 4.4-stable review patch.  If anyone has any objections, please let me know.
> > 
> > ------------------
> > 
> > From: Davidlohr Bueso <dave@...olabs.net>
> > 
> > [ Upstream commit d29a20645d5e929aa7e8616f28e5d8e1c49263ec ]
> > 
> > While running rt-tests' pi_stress program I got the following splat:
> > 
> >   rq->clock_update_flags < RQCF_ACT_SKIP
> >   WARNING: CPU: 27 PID: 0 at kernel/sched/sched.h:960 assert_clock_updated.isra.38.part.39+0x13/0x20
> > 
> >   [...]
> > 
> >   <IRQ>
> >   enqueue_top_rt_rq+0xf4/0x150
> >   ? cpufreq_dbs_governor_start+0x170/0x170
> >   sched_rt_rq_enqueue+0x65/0x80
> >   sched_rt_period_timer+0x156/0x360
> >   ? sched_rt_rq_enqueue+0x80/0x80
> >   __hrtimer_run_queues+0xfa/0x260
> >   hrtimer_interrupt+0xcb/0x220
> >   smp_apic_timer_interrupt+0x62/0x120
> >   apic_timer_interrupt+0xf/0x20
> >   </IRQ>
> > 
> >   [...]
> > 
> >   do_idle+0x183/0x1e0
> >   cpu_startup_entry+0x5f/0x70
> >   start_secondary+0x192/0x1d0
> >   secondary_startup_64+0xa5/0xb0
> > 
> > We can get rid of it be the "traditional" means of adding an
> > update_rq_clock() call after acquiring the rq->lock in
> > do_sched_rt_period_timer().
> > 
> > The case for the RT task throttling (which this workload also hits)
> > can be ignored in that the skip_update call is actually bogus and
> > quite the contrary (the request bits are removed/reverted).
> > 
> > By setting RQCF_UPDATED we really don't care if the skip is happening
> > or not and will therefore make the assert_clock_updated() check happy.  
> 
> There is no such flag or assertion in 4.4 or 4.9, so does this change
> still make sense there?

I believe the assert was added to catch bugs like this.

Although the change log is a bit ambiguous in if it is fixing an actual
miss update, or if it is just quieting a false positive.

Davidlohr?

-- Steve


> 
> Ben.
> 
> > Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
> > Reviewed-by: Matt Fleming <matt@...eblueprint.co.uk>
> > Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> > Cc: Mike Galbraith <efault@....de>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: dave@...olabs.net
> > Cc: linux-kernel@...r.kernel.org
> > Cc: rostedt@...dmis.org
> > Link: http://lkml.kernel.org/r/20180402164954.16255-1-dave@stgolabs.net
> > Signed-off-by: Ingo Molnar <mingo@...nel.org>
> > Signed-off-by: Sasha Levin <alexander.levin@...rosoft.com>
> > Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > ---
> >  kernel/sched/rt.c |    2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > --- a/kernel/sched/rt.c
> > +++ b/kernel/sched/rt.c
> > @@ -822,6 +822,8 @@ static int do_sched_rt_period_timer(stru
> >  		struct rq *rq = rq_of_rt_rq(rt_rq);
> >  
> >  		raw_spin_lock(&rq->lock);
> > +		update_rq_clock(rq);
> > +
> >  		if (rt_rq->rt_time) {
> >  			u64 runtime;
> >  
> > 
> > 
> >