[V2] blk-mq: don't schedule block kworker on isolated CPUs

  Kernel parameter of `isolcpus=` or 'nohz_full=' are used for isolating CPUs
for specific task, and user often won't want block IO to disturb these CPUs,
also long IO latency may be caused if blk-mq kworker is scheduled on these
isolated CPUs.

Kernel workqueue only respects this limit for WQ_UNBOUND, for bound wq,
the responsibility should be on wq user.

So don't not run block kworker on isolated CPUs by ruling out isolated CPUs
from hctx->cpumask. Meantime in cpuhp handler, use queue map to check if
all CPUs in this hw queue are offline, this way can avoid any cost in fast
IO code path.

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>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
V2:
	- remove module parameter, meantime use queue map to check if
	all cpus in one hctx are offline

 block/blk-mq.c | 42 +++++++++++++++++++++++++++++++++---------
 1 file changed, 33 insertions(+), 9 deletions(-)
  

Tested-by: Joe Mario <jmario@redhat.com>

Verified that block kworker threads no longer ran on isolated cpus.
Joe

On 10/13/23 8:47 AM, Ming Lei wrote:
> Kernel parameter of `isolcpus=` or 'nohz_full=' are used for isolating CPUs
> for specific task, and user often won't want block IO to disturb these CPUs,
> also long IO latency may be caused if blk-mq kworker is scheduled on these
> isolated CPUs.
> 
> Kernel workqueue only respects this limit for WQ_UNBOUND, for bound wq,
> the responsibility should be on wq user.
> 
> So don't not run block kworker on isolated CPUs by ruling out isolated CPUs
> from hctx->cpumask. Meantime in cpuhp handler, use queue map to check if
> all CPUs in this hw queue are offline, this way can avoid any cost in fast
> IO code path.
> 
> 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>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
> V2:
> 	- remove module parameter, meantime use queue map to check if
> 	all cpus in one hctx are offline
> 
>  block/blk-mq.c | 42 +++++++++++++++++++++++++++++++++---------
>  1 file changed, 33 insertions(+), 9 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index ec922c6bccbe..91055bdc4426 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -29,6 +29,7 @@
>  #include <linux/prefetch.h>
>  #include <linux/blk-crypto.h>
>  #include <linux/part_stat.h>
> +#include <linux/sched/isolation.h>
>  
>  #include <trace/events/block.h>
>  
> @@ -3476,14 +3477,27 @@ static bool blk_mq_hctx_has_requests(struct blk_mq_hw_ctx *hctx)
>  	return data.has_rq;
>  }
>  
> -static inline bool blk_mq_last_cpu_in_hctx(unsigned int cpu,
> -		struct blk_mq_hw_ctx *hctx)
> +static bool blk_mq_hctx_has_online_cpu(struct blk_mq_hw_ctx *hctx)
>  {
> -	if (cpumask_first_and(hctx->cpumask, cpu_online_mask) != cpu)
> -		return false;
> -	if (cpumask_next_and(cpu, hctx->cpumask, cpu_online_mask) < nr_cpu_ids)
> -		return false;
> -	return true;
> +	struct blk_mq_tag_set *tag_set = hctx->queue->tag_set;
> +	int cpu;
> +
> +	/*
> +	 * hctx->cpumask has rule out isolated CPUs, but userspace still
> +	 * might submit IOs on these isolated CPUs, so use queue map to
> +	 * check if all CPUs mapped to this hctx are offline
> +	 */
> +	for_each_possible_cpu(cpu) {
> +		unsigned idx = tag_set->map[hctx->type].mq_map[cpu];
> +
> +		if (idx != hctx->queue_num)
> +			continue;
> +
> +		if (cpu_online(cpu))
> +			return true;
> +	}
> +
> +	return false;
>  }
>  
>  static int blk_mq_hctx_notify_offline(unsigned int cpu, struct hlist_node *node)
> @@ -3491,8 +3505,7 @@ static int blk_mq_hctx_notify_offline(unsigned int cpu, struct hlist_node *node)
>  	struct blk_mq_hw_ctx *hctx = hlist_entry_safe(node,
>  			struct blk_mq_hw_ctx, cpuhp_online);
>  
> -	if (!cpumask_test_cpu(cpu, hctx->cpumask) ||
> -	    !blk_mq_last_cpu_in_hctx(cpu, hctx))
> +	if (blk_mq_hctx_has_online_cpu(hctx))
>  		return 0;
>  
>  	/*
> @@ -3900,6 +3913,8 @@ static void blk_mq_map_swqueue(struct request_queue *q)
>  	}
>  
>  	queue_for_each_hw_ctx(q, hctx, i) {
> +		int cpu;
> +
>  		/*
>  		 * If no software queues are mapped to this hardware queue,
>  		 * disable it and free the request entries.
> @@ -3926,6 +3941,15 @@ static void blk_mq_map_swqueue(struct request_queue *q)
>  		 */
>  		sbitmap_resize(&hctx->ctx_map, hctx->nr_ctx);
>  
> +		/*
> +		 * rule out isolated CPUs from hctx->cpumask for avoiding to
> +		 * run wq worker on isolated CPU
> +		 */
> +		for_each_cpu(cpu, hctx->cpumask) {
> +			if (cpu_is_isolated(cpu))
> +				cpumask_clear_cpu(cpu, hctx->cpumask);
> +		}
> +
>  		/*
>  		 * Initialize batch roundrobin counts
>  		 */
  

On Fri, Oct 13, 2023 at 08:47:58PM +0800, Ming Lei wrote:
> Kernel parameter of `isolcpus=` or 'nohz_full=' are used for isolating CPUs
> for specific task, and user often won't want block IO to disturb these CPUs,
> also long IO latency may be caused if blk-mq kworker is scheduled on these
> isolated CPUs.
> 
> Kernel workqueue only respects this limit for WQ_UNBOUND, for bound wq,
> the responsibility should be on wq user.
> 
> So don't not run block kworker on isolated CPUs by ruling out isolated CPUs
> from hctx->cpumask. Meantime in cpuhp handler, use queue map to check if
> all CPUs in this hw queue are offline, this way can avoid any cost in fast
> IO code path.
> 
> 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>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
> V2:
> 	- remove module parameter, meantime use queue map to check if
> 	all cpus in one hctx are offline

Hello Guys,

Ping...



Thanks,
Ming
  

On 10/13/23 05:47, Ming Lei wrote:
> @@ -3926,6 +3941,15 @@ static void blk_mq_map_swqueue(struct request_queue *q)
>   		 */
>   		sbitmap_resize(&hctx->ctx_map, hctx->nr_ctx);
>   
> +		/*
> +		 * rule out isolated CPUs from hctx->cpumask for avoiding to
> +		 * run wq worker on isolated CPU
> +		 */
> +		for_each_cpu(cpu, hctx->cpumask) {
> +			if (cpu_is_isolated(cpu))
> +				cpumask_clear_cpu(cpu, hctx->cpumask);
> +		}

What will happen if this code makes hctx->cpumask empty? Code like
blk_mq_first_mapped_cpu() and blk_mq_hctx_next_cpu() assumes that
hctx->cpumask is not empty. There may be other code that assumes that
hctx->cpumask is not empty.

Thanks,

Bart.
  

On Tue, Oct 24, 2023 at 09:53:49AM -0700, Bart Van Assche wrote:
> 
> On 10/13/23 05:47, Ming Lei wrote:
> > @@ -3926,6 +3941,15 @@ static void blk_mq_map_swqueue(struct request_queue *q)
> >   		 */
> >   		sbitmap_resize(&hctx->ctx_map, hctx->nr_ctx);
> > +		/*
> > +		 * rule out isolated CPUs from hctx->cpumask for avoiding to
> > +		 * run wq worker on isolated CPU
> > +		 */
> > +		for_each_cpu(cpu, hctx->cpumask) {
> > +			if (cpu_is_isolated(cpu))
> > +				cpumask_clear_cpu(cpu, hctx->cpumask);
> > +		}
> 
> What will happen if this code makes hctx->cpumask empty? Code like
> blk_mq_first_mapped_cpu() and blk_mq_hctx_next_cpu() assumes that
> hctx->cpumask is not empty. There may be other code that assumes that
> hctx->cpumask is not empty.

hctx->cpumask is only used for selecting next cpu to schedule run
queue now, so it is fine for hctx->cpumask to be empty.

But the patch has one hole, following the delta fix:


diff --git a/block/blk-mq.c b/block/blk-mq.c
index 0917f8eabab9..1d9a7ded27af 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2159,7 +2159,7 @@ static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx *hctx)
 	bool tried = false;
 	int next_cpu = hctx->next_cpu;
 
-	if (hctx->queue->nr_hw_queues == 1)
+	if (hctx->queue->nr_hw_queues == 1 || next_cpu >= nr_cpu_ids)
 		return WORK_CPU_UNBOUND;
 
 	if (--hctx->next_cpu_batch <= 0) {



Thanks,
Ming