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

From: Jens Axboe
Date: Tue Mar 31 2015 - 11:43:24 EST

On 03/31/2015 09:33 AM, Frederic Weisbecker wrote:
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.

That'd be easy enough to do, that's how blk-mq handles offline CPUs as well. The attached patch is completely untested, but will handle offline or nohz CPUs in the same fashion - they will punt to hardware queue 0, which is mapped to CPU0 (and others, depending on the queue vs CPU ratio).

--
Jens Axboe

diff --git a/block/blk-mq-cpumap.c b/block/blk-mq-cpumap.c
index 5f13f4d0bcce..9cb20d14c6b9 100644
--- a/block/blk-mq-cpumap.c
+++ b/block/blk-mq-cpumap.c
@@ -51,7 +51,10 @@ int blk_mq_update_queue_map(unsigned int *map, unsigned int nr_queues)

queue = 0;
for_each_possible_cpu(i) {
- if (!cpu_online(i)) {
+ /*
+ * Offline or full nohz CPUs get mapped to CPU0
+ */
+ if (blk_mq_cpu_offline(i)) {
map[i] = 0;
continue;
}
diff --git a/block/blk-mq.c b/block/blk-mq.c
index b7b8933ec241..ec0de2871950 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -366,7 +366,7 @@ static void blk_mq_ipi_complete_request(struct request *rq)
if (!test_bit(QUEUE_FLAG_SAME_FORCE, &rq->q->queue_flags))
shared = cpus_share_cache(cpu, ctx->cpu);

- if (cpu != ctx->cpu && !shared && cpu_online(ctx->cpu)) {
+ if (cpu != ctx->cpu && !shared && !blk_mq_cpu_offline(ctx->cpu)) {
rq->csd.func = __blk_mq_complete_request_remote;
rq->csd.info = rq;
rq->csd.flags = 0;
@@ -1022,7 +1022,7 @@ void blk_mq_insert_request(struct request *rq, bool at_head, bool run_queue,
struct blk_mq_ctx *ctx = rq->mq_ctx, *current_ctx;

current_ctx = blk_mq_get_ctx(q);
- if (!cpu_online(ctx->cpu))
+ if (blk_mq_cpu_offline(ctx->cpu))
rq->mq_ctx = ctx = current_ctx;

hctx = q->mq_ops->map_queue(q, ctx->cpu);
@@ -1051,7 +1051,7 @@ static void blk_mq_insert_requests(struct request_queue *q,

current_ctx = blk_mq_get_ctx(q);

- if (!cpu_online(ctx->cpu))
+ if (blk_mq_cpu_offline(ctx->cpu))
ctx = current_ctx;
hctx = q->mq_ops->map_queue(q, ctx->cpu);

@@ -1757,7 +1757,7 @@ static void blk_mq_init_cpu_queues(struct request_queue *q,
__ctx->queue = q;

/* If the cpu isn't online, the cpu is mapped to first hctx */
- if (!cpu_online(i))
+ if (blk_mq_cpu_offline(i))
continue;

hctx = q->mq_ops->map_queue(q, i);
@@ -1789,7 +1789,7 @@ static void blk_mq_map_swqueue(struct request_queue *q)
*/
queue_for_each_ctx(q, ctx, i) {
/* If the cpu isn't online, the cpu is mapped to first hctx */
- if (!cpu_online(i))
+ if (blk_mq_cpu_offline(i))
continue;

hctx = q->mq_ops->map_queue(q, i);
diff --git a/block/blk-mq.h b/block/blk-mq.h
index 6a48c4c0d8a2..443dc8e0ea24 100644
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -1,6 +1,8 @@
#ifndef INT_BLK_MQ_H
#define INT_BLK_MQ_H

+#include <linux/tick.h>
+
struct blk_mq_tag_set;

struct blk_mq_ctx {
@@ -123,4 +125,13 @@ static inline bool blk_mq_hw_queue_mapped(struct blk_mq_hw_ctx *hctx)
return hctx->nr_ctx && hctx->tags;
}

+/*
+ * If the CPU is offline or is a nohz CPU, we will remap any IO processing
+ * to the first hardware queue.
+ */
+static inline bool blk_mq_cpu_offline(const unsigned int cpu)
+{
+ return !cpu_online(cpu) || tick_nohz_full_cpu(cpu);
+}
+
#endif