From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: From: Ming Lei To: Jens Axboe Cc: linux-block@vger.kernel.org, Ming Lei , Omar Sandoval , Andrew Jones , Bart Van Assche , linux-scsi@vger.kernel.org, "Martin K. Petersen" , Christoph Hellwig Subject: [PATCH 5/5] blk-mq: avoid to synchronize rcu inside blk_cleanup_queue() Date: Mon, 25 Jun 2018 19:31:49 +0800 Message-Id: <20180625113149.29449-6-ming.lei@redhat.com> In-Reply-To: <20180625113149.29449-1-ming.lei@redhat.com> References: <20180625113149.29449-1-ming.lei@redhat.com> List-ID: SCSI probing may synchronously create and destroy a lot of request_queues for non-existent devices. Any synchronize_rcu() in queue creation or destroy path may introduce long latency during booting, see detailed description in comment of blk_register_queue(). This patch removes one synchronize_rcu() inside blk_cleanup_queue() for this case, commit c2856ae2f315d75(blk-mq: quiesce queue before freeing queue) needs synchronize_rcu() for implementing blk_mq_quiesce_queue(), but when queue isn't initialized, it isn't necessary to do that since only pass-through requests are involved, no original issue in scsi_execute() at all. Without this patch and previous one, it may take more 20+ seconds for virtio-scsi to complete disk probe. With the two patches, the time becomes less than 100ms. Fixes: c2856ae2f315d75 ("blk-mq: quiesce queue before freeing queue") Reported-by: Andrew Jones Cc: Omar Sandoval Cc: Andrew Jones Cc: Bart Van Assche Cc: linux-scsi@vger.kernel.org Cc: "Martin K. Petersen" Cc: Christoph Hellwig Signed-off-by: Ming Lei --- block/blk-core.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/block/blk-core.c b/block/blk-core.c index afd2596ea3d3..222d4fc0e524 100644 --- a/block/blk-core.c +++ b/block/blk-core.c @@ -762,9 +762,13 @@ void blk_cleanup_queue(struct request_queue *q) * make sure all in-progress dispatch are completed because * blk_freeze_queue() can only complete all requests, and * dispatch may still be in-progress since we dispatch requests - * from more than one contexts + * from more than one contexts. + * + * No need to quiesce queue if it isn't initialized yet since + * blk_freeze_queue() should be enough for cases of passthrough + * request. */ - if (q->mq_ops) + if (q->mq_ops && blk_queue_init_done(q)) blk_mq_quiesce_queue(q); /* for synchronous bio-based driver finish in-flight integrity i/o */ -- 2.9.5