Re: [PATCH] SCSI: run queue if SCSI device queue isn't ready and queue is idle
From: Ming Lei <hidden>
Date: 2017-12-05 16:28:51
Also in:
linux-scsi, lkml
On Tue, Dec 05, 2017 at 04:08:20PM +0000, Bart Van Assche wrote:
On Tue, 2017-12-05 at 15:52 +0800, Ming Lei wrote:quoted
diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c index db9556662e27..1816dd8259b3 100644 --- a/drivers/scsi/scsi_lib.c +++ b/drivers/scsi/scsi_lib.c@@ -1967,6 +1967,8 @@ static bool scsi_mq_get_budget(struct blk_mq_hw_ctx *hctx) out_put_device: put_device(&sdev->sdev_gendev); out: + if (atomic_read(&sdev->device_busy) == 0 && !scsi_device_blocked(sdev)) + blk_mq_delay_run_hw_queue(hctx, SCSI_QUEUE_DELAY); return false; }This cannot work since multiple threads can call scsi_mq_get_budget()
That is exactly the way we are handling these cases before 0df21c86bdbf(scsi: implement .get_budget and .put_budget for blk-mq), so if it can't work, that is not fault of commit 0df21c86bdbf.
concurrently and hence it can happen that none of them sees atomic_read(&sdev->device_busy) == 0. BTW, the above patch is something I
If there is concurrent .get_budget(), one of them will see the counter becoming zero finally because each sdev->device_busy is inc/dec atomically. Or scsi_dev_queue_ready() return true. Anyway, we need this patch to avoid possible regression. If you think there are bugs in blk-mq RESTART, just root cause and and fix it.
quoted hunk ↗ jump to hunk
consider as a workaround. Have you considered to fix the root cause and to add blk_mq_sched_mark_restart_hctx() where these are missing, e.g. in blk_mq_sched_dispatch_requests()? The patch below is not a full solution but resulted in a significant improvement in my tests:diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c index 69e3226e66ca..9d86876ec503 100644 --- a/block/blk-mq-sched.c +++ b/block/blk-mq-sched.c@@ -226,6 +226,7 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx) * TODO: get more budgets, and dequeue more requests in * one time. */ + blk_mq_sched_mark_restart_hctx(hctx); blk_mq_do_dispatch_ctx(hctx); } else { blk_mq_flush_busy_ctxs(hctx, &rq_list);
This is still a workaround for RESTART, see my comment before: https://marc.info/?l=linux-block&m=151217500929341&w=2 -- Ming