Re: [PATCH V4] blk-mq: don't schedule block kworker on isolated CPUs

[Ming Lei <ming.lei@redhat.com>]

On Wed, Mar 20, 2024 at 10:34:46AM +0800, Ming Lei wrote:
> Kernel parameter of `isolcpus=` or 'nohz_full=' are used to isolate CPUs
> for specific task, and it isn't expected to let block IO disturb these CPUs.
> blk-mq kworker shouldn't be scheduled on isolated CPUs. Also if isolated
> CPUs is run for blk-mq kworker, long block IO latency can be caused.
> 
> Kernel workqueue only respects CPU isolation for WQ_UNBOUND, for bound
> WQ, the responsibility is on user because CPU is specified as WQ API
> parameter, such as mod_delayed_work_on(cpu), queue_delayed_work_on(cpu)
> and queue_work_on(cpu).
> 
> So not run blk-mq kworker on isolated CPUs by removing isolated CPUs
> from hctx->cpumask. Meantime use queue map to check if all CPUs in this
> hw queue are offline instead of hctx->cpumask, this way can avoid any
> cost in fast IO code path, and is safe since hctx->cpumask are only
> used in the two cases.
> 
> Cc: Tim Chen <tim.c.chen@linux.intel.com>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> Cc: Andrew Theurer <atheurer@redhat.com>
> Cc: Joe Mario <jmario@redhat.com>
> Cc: Sebastian Jug <sejug@redhat.com>
> Cc: Frederic Weisbecker <frederic@kernel.org>
> Cc: Bart Van Assche <bvanassche@acm.org>
> Cc: Tejun Heo <tj@kernel.org>
> Tested-by: Joe Mario <jmario@redhat.com>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
> V4:
> 	- improve comment & commit log as suggested by Tim

Hello Jens, Tejun and Guys,

This patch fixes one issue in OpenShift low latency environment, I appreciate
you may take a look at the patch and merge it if you are fine.


Thanks,
Ming