4.14-stable review patch. If anyone has any objections, please let me know.
------------------ From: Ming Lei <[email protected]> [ Upstream commit b04f50ab8a74129b3041a2836c33c916be3c6667 ] Only attempt to merge bio iff the ctx->rq_list isn't empty, because: 1) for high-performance SSD, most of times dispatch may succeed, then there may be nothing left in ctx->rq_list, so don't try to merge over sw queue if it is empty, then we can save one acquiring of ctx->lock 2) we can't expect good merge performance on per-cpu sw queue, and missing one merge on sw queue won't be a big deal since tasks can be scheduled from one CPU to another. Cc: Laurence Oberman <[email protected]> Cc: Omar Sandoval <[email protected]> Cc: Bart Van Assche <[email protected]> Tested-by: Kashyap Desai <[email protected]> Reported-by: Kashyap Desai <[email protected]> Reviewed-by: Christoph Hellwig <[email protected]> Signed-off-by: Ming Lei <[email protected]> Signed-off-by: Jens Axboe <[email protected]> Signed-off-by: Sasha Levin <[email protected]> Signed-off-by: Greg Kroah-Hartman <[email protected]> --- block/blk-mq-sched.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) --- a/block/blk-mq-sched.c +++ b/block/blk-mq-sched.c @@ -236,7 +236,8 @@ bool __blk_mq_sched_bio_merge(struct req return e->type->ops.mq.bio_merge(hctx, bio); } - if (hctx->flags & BLK_MQ_F_SHOULD_MERGE) { + if ((hctx->flags & BLK_MQ_F_SHOULD_MERGE) && + !list_empty_careful(&ctx->rq_list)) { /* default per sw-queue merge */ spin_lock(&ctx->lock); ret = blk_mq_attempt_merge(q, ctx, bio);
