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

[Frederic Weisbecker <fweisbec@gmail.com>]

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;

I guess in this case, the queue for this CPU will be handled by another CPU?
Is this an unbound workqueue? I guess it's not but if it is, we should wait for
the workqueue affinity patchset.

Also, since we are doing a lot of kernel pre-setting behind nohz full, it would
be nice to warn the user about each of them in dmesg.

> +
>  		hctx = q->mq_ops->map_queue(q, i);
>  		cpumask_set_cpu(i, hctx->cpumask);
>  		hctx->nr_ctx++;
> --
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