[PATCH V3] blk-mq: don't schedule block kworker on isolated CPUs

[Ming Lei]

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@xxxxxxxxxx>
Cc: Andrew Theurer <atheurer@xxxxxxxxxx>
Cc: Joe Mario <jmario@xxxxxxxxxx>
Cc: Sebastian Jug <sejug@xxxxxxxxxx>
Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
Cc: Bart Van Assche <bvanassche@xxxxxxx>
Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
---

V3:
	- avoid to check invalid cpu as reported by Bart
	- take current cpu(to be offline, not done yet) into account
	- simplify blk_mq_hctx_has_online_cpu()

V2:
	- remove module parameter, meantime use queue map to check if
	all cpus in one hctx are offline

 block/blk-mq.c | 51 ++++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 41 insertions(+), 10 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index e2d11183f62e..4556978ce71b 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>
 
@@ -2158,7 +2159,11 @@ 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)
+	/*
+	 * In case of single queue or no allowed CPU for scheduling
+	 * worker, don't bound our worker with any CPU
+	 */
+	if (hctx->queue->nr_hw_queues == 1 || next_cpu >= nr_cpu_ids)
 		return WORK_CPU_UNBOUND;
 
 	if (--hctx->next_cpu_batch <= 0) {
@@ -3459,14 +3464,30 @@ 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,
+		unsigned int this_cpu)
 {
-	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;
+	enum hctx_type type = hctx->type;
+	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_online_cpu(cpu) {
+		struct blk_mq_hw_ctx *h = blk_mq_map_queue_type(hctx->queue,
+				type, cpu);
+
+		if (h != hctx)
+			continue;
+
+		/* this current CPU isn't put offline yet */
+		if (this_cpu != cpu)
+			return true;
+	}
+
+	return false;
 }
 
 static int blk_mq_hctx_notify_offline(unsigned int cpu, struct hlist_node *node)
@@ -3474,8 +3495,7 @@ 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, cpu))
 		return 0;
 
 	/*
@@ -3883,6 +3903,8 @@ 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.
@@ -3909,6 +3931,15 @@ 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
 		 */
-- 
2.41.0