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

[Peter Zijlstra <peterz@infradead.org>]

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@gmail.com>
> AuthorDate: Tue, 18 Mar 2014 21:12:53 +0100
> Committer:  Frederic Weisbecker <fweisbec@gmail.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@linux.vnet.ibm.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Jens Axboe <axboe@fb.com>
> Cc: Kevin Hilman <khilman@linaro.org>
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.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.

> 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 ;-)

> -			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() ?