Re: [PATCH RFC] nohz,blk-mq: do not create blk-mq workqueues on nohz dedicated CPUs

[Jens Axboe]

On 03/31/2015 05:17 PM, Marcelo Tosatti wrote:
> On Tue, Mar 31, 2015 at 10:27:26AM -0400, Rik van Riel wrote:
> > CPUs with nohz_full do not want disruption from timer interrupts,
> > or other random system things.  This includes block mq work.
> >
> > There is another issue with block mq vs. realtime tasks that run
> > 100% of the time, which is not uncommon on systems that have CPUs
> > dedicated to real time use with isolcpus= and nohz_full=
> >
> > Specifically, on systems like that, a block work item may never
> > get to run, which could lead to filesystems getting stuck forever.
> >
> > We can avoid both issues by not scheduling blk-mq workqueues on
> > cpus in nohz_full mode.
> >
> > Question for Jens: should we try to spread out the load for
> > currently offline and nohz CPUs across the remaining CPUs in
> > the system, to get the full benefit of blk-mq in these situations?
> >
> > If so, do you have any preference on how I should implement that?
> >
> > Cc: Frederic Weisbecker <fweisbec@xxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: Jens Axboe <axboe@xxxxxxxxxx>
> > Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
> > ---
> >   block/blk-mq.c | 5 +++++
> >   1 file changed, 5 insertions(+)
> >
> > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > index 4f4bea21052e..1004d6817fa4 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -21,6 +21,7 @@
> >   #include <linux/sched/sysctl.h>
> >   #include <linux/delay.h>
> >   #include <linux/crash_dump.h>
> > +#include <linux/tick.h>
> >
> >   #include <trace/events/block.h>
> >
> > @@ -1760,6 +1761,10 @@ static void blk_mq_init_cpu_queues(struct request_queue *q,
> >   		if (!cpu_online(i))
> >   			continue;
> >
> > +		/* Do not schedule work on nohz full dedicated CPUs. */
> > +		if (tick_nohz_full_cpu(i))
> > +			continue;
> > +
> >   		hctx = q->mq_ops->map_queue(q, i);
> >   		cpumask_set_cpu(i, hctx->cpumask);
> >   		hctx->nr_ctx++;
>
> Rik,
>
> I suppose any bound workqueue queued on isolated CPUs should be moved at
> queue time to other CPUs (sacrifficing performance).
>
> So that by doing "queue_work" on an isolated CPU would move that
> work somewhere else.

That wont work for blk-mq, we rely on the characteristics of bound 
workqueues. So it would have to be handled up front, like in the patch I 
sent out.

--
Jens Axboe

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