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

[Frederic Weisbecker]

On Tue, Mar 31, 2015 at 09:07:11AM -0600, Jens Axboe wrote:
> On 03/31/2015 08:27 AM, 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;
> 
> Is this CPU ever going to queue IO? If yes, then it needs to be mapped. If
> userspace never runs on it and submits IO, then we'll never run completions
> on it nor schedule the associated workqueue. So I really don't see how it
> doesn't already work, as-is.

Well, it's fairly possible that full dynticks CPUs do IO of any sort. Is it possible
to affine these asynchronous works to specific CPU? The usual scheme of full dynticks
is to have CPU 0 handling any kind of housekeeping and other CPUs doing latency or performance
sensitive works that don't want to be disturbed.
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