linux-kernel - Re: [tip:timers/nohz] nohz: Move full nohz kick to its own IPI

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Date:	Mon, 5 May 2014 16:52:59 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org, hpa@...or.com,
	paulmck@...ux.vnet.ibm.com, akpm@...ux-foundation.org,
	khilman@...aro.org, tglx@...utronix.de, axboe@...com,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:timers/nohz] nohz: Move full nohz kick to its own IPI

On Mon, May 05, 2014 at 02:37:06PM +0200, Peter Zijlstra wrote:
> On Wed, Apr 16, 2014 at 12:40:01AM -0700, tip-bot for Frederic Weisbecker wrote:
> > Commit-ID:  72aacf0259bb7d53b7a3b5b2f7bf982acaa52b61
> > Gitweb:     http://git.kernel.org/tip/72aacf0259bb7d53b7a3b5b2f7bf982acaa52b61
> > Author:     Frederic Weisbecker <fweisbec@...il.com>
> > AuthorDate: Tue, 18 Mar 2014 21:12:53 +0100
> > Committer:  Frederic Weisbecker <fweisbec@...il.com>
> > CommitDate: Thu, 3 Apr 2014 18:05:21 +0200
> > 
> > nohz: Move full nohz kick to its own IPI
> > 
> > Now that we have smp_queue_function_single() which can be used to
> > safely queue IPIs when interrupts are disabled and without worrying
> > about concurrent callers, lets use it for the full dynticks kick to
> > notify a CPU that it's exiting single task mode.
> > 
> > This unbloats a bit the scheduler IPI that the nohz code was abusing
> > for its cool "callable anywhere/anytime" properties.
> > 
> > Reviewed-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > Cc: Ingo Molnar <mingo@...nel.org>
> > Cc: Jens Axboe <axboe@...com>
> > Cc: Kevin Hilman <khilman@...aro.org>
> > Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> 
> So I suspect this is the patch that makes Ingo's machines unhappy, they
> appear to get stuck thusly:
> 
> [10513.382910] RIP: 0010:[<ffffffff8112b7da>]  [<ffffffff8112b7da>] generic_exec_single+0x9a/0x180
> 
> [10513.481704]  [<ffffffff8112c092>] smp_queue_function_single+0x42/0xa0
> [10513.488251]  [<ffffffff81126ce0>] tick_nohz_full_kick_cpu+0x50/0x80
> [10513.494661]  [<ffffffff810f4b0e>] enqueue_task_fair+0x59e/0x6c0
> [10513.506469]  [<ffffffff810e3d6a>] enqueue_task+0x3a/0x60
> [10513.511836]  [<ffffffff810e8ac3>] __migrate_task+0x123/0x150
> [10513.523535]  [<ffffffff810e8b0d>] migration_cpu_stop+0x1d/0x30
> [10513.529401]  [<ffffffff81143460>] cpu_stopper_thread+0x70/0x120
> 
> I'm not entirely sure how yet, but this is by far the most likely
> candidate. Ingo, if you still have the vmlinuz matching this trace (your
> hang2.txt) could you have a peek where that RIP lands?
> 
> If that is indeed the csd_lock() function, then this is it and
> something's buggered.

Aye!

> 
> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index c9007f2..4771063 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -1225,7 +1225,7 @@ static inline void inc_nr_running(struct rq *rq)
> >  		if (tick_nohz_full_cpu(rq->cpu)) {
> >  			/* Order rq->nr_running write against the IPI */
> >  			smp_wmb();
> 
> FWIW that barrier is complete crap ;-)

Yeah, I'm queing the removal of that now :)

> 
> > -			smp_send_reschedule(rq->cpu);
> > +			tick_nohz_full_kick_cpu(rq->cpu);
> >  		}
> >         }
> >  #endif
> > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> > index 9f8af69..582d3f6 100644
> > --- a/kernel/time/tick-sched.c
> > +++ b/kernel/time/tick-sched.c
> > @@ -230,6 +230,27 @@ void tick_nohz_full_kick(void)
> >  		irq_work_queue(&__get_cpu_var(nohz_full_kick_work));
> >  }
> >  
> > +static void nohz_full_kick_queue(struct queue_single_data *qsd)
> > +{
> > +	__tick_nohz_full_check();
> > +}
> > +
> > +static DEFINE_PER_CPU(struct queue_single_data, nohz_full_kick_qsd) = {
> > +	.func = nohz_full_kick_queue,
> > +};
> > +
> > +void tick_nohz_full_kick_cpu(int cpu)
> > +{
> > +	if (!tick_nohz_full_cpu(cpu))
> > +		return;
> > +
> > +	if (cpu == smp_processor_id()) {
> > +		irq_work_queue(&__get_cpu_var(nohz_full_kick_work));
> > +	} else {
> > +		smp_queue_function_single(cpu, &per_cpu(nohz_full_kick_qsd, cpu));
> > +	}
> > +}
> 
> Should we instead do irq_work_queue_on() ?

I would really much prefer that yeah. But if we do that, expect some added overhead on the local
irq_work_queue() path though. irq_work_raise can't use local cmpxchg ops anymore.

Or we can have a different pending raise system for remote irq work.

I can try something.
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