[PATCH] block: drop the cached plug time when blk_add_rq_to_plug() flushes

[PATCH] block: drop the cached plug time when blk_add_rq_to_plug() flushes

[Vasileios Almpanis]

blk_time_get_ns() caches the current time in the task's plug. The idea
is that the cache lives for one plug and is dropped when the plug is
flushed. __blk_flush_plug() and finish_task_switch() do that.

blk_add_rq_to_plug() also flushes, when the plug has too many requests
or the last request is at least BLK_PLUG_FLUSH_SIZE. But it calls
blk_mq_flush_plug_list() directly and never drops the cached time. So
a task that keeps submitting without sleeping stamps every request
after that with the time first cached in the plug. start_time_ns and
io_start_time_ns are stale and the disk stats charge each request for
the whole time since the plug began.

Drop the cached time after the flush, same as __blk_flush_plug() does.

Fixes: da4c8c3d0975 ("block: cache current nsec time in struct blk_plug")
Cc: stable@vger.kernel.org
Signed-off-by: Vasileios Almpanis <vasileios.almpanis@virtuozzo.com>
---
 block/blk-mq.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index a26a11c73ee3..7a37d9f1647d 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1384,6 +1384,7 @@ static void blk_add_rq_to_plug(struct blk_plug *plug, struct request *rq)
 		   (!blk_queue_nomerges(rq->q) &&
 		    blk_rq_bytes(last) >= BLK_PLUG_FLUSH_SIZE)) {
 		blk_mq_flush_plug_list(plug, false);
+		blk_plug_invalidate_ts();
 		last = NULL;
 		trace_block_plug(rq->q);
 	}

---
base-commit: 551c722f40809618230001baccf219193e22fc5a
change-id: 20260930-blk-plug-ts-flush-365cde9630d8

-- 
Best regards, Vasileios Almpanis
Software Developer, Virtuozzo.

Re: [PATCH] block: drop the cached plug time when blk_add_rq_to_plug() flushes

[Jens Axboe]

On Wed, 30 Sep 2026 14:32:26 +0000, Vasileios Almpanis wrote:
> blk_time_get_ns() caches the current time in the task's plug. The idea
> is that the cache lives for one plug and is dropped when the plug is
> flushed. __blk_flush_plug() and finish_task_switch() do that.
> 
> blk_add_rq_to_plug() also flushes, when the plug has too many requests
> or the last request is at least BLK_PLUG_FLUSH_SIZE. But it calls
> blk_mq_flush_plug_list() directly and never drops the cached time. So
> a task that keeps submitting without sleeping stamps every request
> after that with the time first cached in the plug. start_time_ns and
> io_start_time_ns are stale and the disk stats charge each request for
> the whole time since the plug began.
> 
> [...]

Applied, thanks!

[1/1] block: drop the cached plug time when blk_add_rq_to_plug() flushes
      commit: e0f2224433b15a57125d6ac5d46182f99c6c9d08

Best regards,
-- 
Jens Axboe