[PATCH V5] blk-mq: don't schedule block kworker on isolated CPUs

[Ming Lei]

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@xxxxxxxxxxxxxxx>
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>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Tesed-by: Joe Mario <jmario@xxxxxxxxxx>
Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
---
V5:
	- fix comment
	- add one helper for document benefit
	- all are suggested by Jens
V4:
	- improve comment & commit log as suggested by Tim
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 | 57 +++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 47 insertions(+), 10 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 555ada922cf0..06cd9c8068fc 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -28,6 +28,7 @@
 #include <linux/prefetch.h>
 #include <linux/blk-crypto.h>
 #include <linux/part_stat.h>
+#include <linux/sched/isolation.h>
 
 #include <trace/events/block.h>
 
@@ -2168,6 +2169,15 @@ static inline int blk_mq_first_mapped_cpu(struct blk_mq_hw_ctx *hctx)
 	return cpu;
 }
 
+/*
+ * ->next_cpu is always calculated from hctx->cpumask, so simply use
+ * it for speeding up the check
+ */
+static bool blk_mq_hctx_empty_cpumask(struct blk_mq_hw_ctx *hctx)
+{
+        return hctx->next_cpu >= nr_cpu_ids;
+}
+
 /*
  * It'd be great if the workqueue API had a way to pass
  * in a mask and had some smarts for more clever placement.
@@ -2179,7 +2189,8 @@ 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)
+	/* Switch to unbound if no allowable CPUs in this hctx */
+	if (hctx->queue->nr_hw_queues == 1 || blk_mq_hctx_empty_cpumask(hctx))
 		return WORK_CPU_UNBOUND;
 
 	if (--hctx->next_cpu_batch <= 0) {
@@ -3488,14 +3499,30 @@ 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 to rule out isolated CPUs, but userspace still
+	 * might submit IOs on these isolated CPUs, so use the 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 hctx has at least one online CPU */
+		if (this_cpu != cpu)
+			return true;
+	}
+
+	return false;
 }
 
 static int blk_mq_hctx_notify_offline(unsigned int cpu, struct hlist_node *node)
@@ -3503,8 +3530,7 @@ 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;
 
 	/*
@@ -3912,6 +3938,8 @@ 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.
@@ -3938,6 +3966,15 @@ 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 to avoid
+		 * running block kworker on isolated CPUs
+		 */
+		for_each_cpu(cpu, hctx->cpumask) {
+			if (cpu_is_isolated(cpu))
+				cpumask_clear_cpu(cpu, hctx->cpumask);
+		}
+
 		/*
 		 * Initialize batch roundrobin counts
 		 */
-- 
2.41.0