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

Marcelo Tosatti
2015-04-01T02:13:40Z

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@redhat.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Jens Axboe <axboe@kernel.org>
> Signed-off-by: Rik van Riel <riel@redhat.com>
> ---
> 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.