linux-kernel - [PATCH RFT] blk-mq: optimize queue tag busy iter for shared

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Message-Id: <20211221123157.14052-1-kashyap.desai@broadcom.com>
Date:   Tue, 21 Dec 2021 18:01:57 +0530
From:   Kashyap Desai <kashyap.desai@...adcom.com>
To:     axboe@...nel.dk
Cc:     Kashyap Desai <kashyap.desai@...adcom.com>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        john.garry@...wei.com, ming.lei@...hat.com,
        sathya.prakash@...adcom.com
Subject: [PATCH RFT] blk-mq: optimize queue tag busy iter for shared_tags

In [0], CPU usage for blk_mq_queue_tag_busy_iter() was optimized, but
there are still periodic call of blk_mq_queue_tag_busy_iter() from
below context. Below context is used for block layer timer to find out
potential expired command (per request queue) which requires tag iteration
almost every 5 seconds(defined BLK_MAX_TIMEOUT) for each request queue.

kthread
        worker_thread
        process_one_work
        blk_mq_timeout_work
        blk_mq_queue_tag_busy_iter
        bt_iter
        blk_mq_find_and_get_req
        _raw_spin_lock_irqsave
        native_queued_spin_lock_slowpath

Changes in this patch optimize extra iterations of tags in case of
shared_tags. One iteration of shared_tags can give expected results for
iterate function.

Setup -  AMD64 Gen-4.0 Server.
64 Virtual Drive created using 16 Nvme drives + mpi3mr driver (in
shared_tags mode)

Test command -
fio 64.fio --rw=randread --bs=4K --iodepth=32 --numjobs=2 --ioscheduler=mq-deadline --disk_util=0

Without this patch on 5.16.0-rc5, mpi3mr driver in shared_tags mode can
give 4.0M IOPs vs expected to get ~6.0M.
Snippet of perf top

  25.42%  [kernel]                               [k] native_queued_spin_lock_slowpath
   3.95%  [kernel]                               [k] cpupri_set
   2.05%  [kernel]                               [k] __blk_mq_get_driver_tag
   1.67%  [kernel]                               [k] __rcu_read_unlock
   1.63%  [kernel]                               [k] check_preemption_disabled

After applying this patch on 5.16.0-rc5, mpi3mr driver in shared_tags
mode reach up to 5.8M IOPs.

Snippet of perf top

   7.95%  [kernel]                               [k] native_queued_spin_lock_slowpath
   5.61%  [kernel]                               [k] cpupri_set
   2.98%  [kernel]                               [k] acpi_processor_ffh_cstate_enter
   2.49%  [kernel]                               [k] read_tsc
   2.15%  [kernel]                               [k] check_preemption_disabled


[0] https://lore.kernel.org/all/9b092ca49e9b5415772cd950a3c12584@mail.gmail.com/ 


Cc: linux-block@...r.kernel.org
Cc: linux-kernel@...r.kernel.org
Cc: john.garry@...wei.com
Cc: ming.lei@...hat.com
Cc: sathya.prakash@...adcom.com
Signed-off-by: Kashyap Desai <kashyap.desai@...adcom.com>
---
 block/blk-mq-tag.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 995336abee33..3e0a8e79f966 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -253,7 +253,8 @@ static bool bt_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
 	if (!rq)
 		return true;
 
-	if (rq->q == hctx->queue && rq->mq_hctx == hctx)
+	if (rq->q == hctx->queue && (rq->mq_hctx == hctx ||
+				blk_mq_is_shared_tags(hctx->flags)))
 		ret = iter_data->fn(hctx, rq, iter_data->data, reserved);
 	blk_mq_put_rq_ref(rq);
 	return ret;
@@ -484,6 +485,14 @@ void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn,
 		if (tags->nr_reserved_tags)
 			bt_for_each(hctx, &tags->breserved_tags, fn, priv, true);
 		bt_for_each(hctx, &tags->bitmap_tags, fn, priv, false);
+		
+		/* In case of shared bitmap if shared_tags is allocated, it is not required
+		 * to iterate all the hctx. Looping one hctx is good enough.
+		 */
+		if (blk_mq_is_shared_tags(hctx->flags)) {
+			blk_queue_exit(q);
+			return;
+		}
 	}
 	blk_queue_exit(q);
 }
-- 
2.18.1


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