RE: [bug report] shared tags causes IO hang and performance drop

From: Kashyap Desai <kashyap.desai@broadcom.com>
To: Ming Lei <ming.lei@redhat.com>
Cc: John Garry <john.garry@huawei.com>,
	linux-block@vger.kernel.org, linux-scsi@vger.kernel.org,
	"Martin K. Petersen" <martin.petersen@oracle.com>,
	Jens Axboe <axboe@kernel.dk>,
	Douglas Gilbert <dgilbert@interlog.com>
Subject: RE: [bug report] shared tags causes IO hang and performance drop
Date: Wed, 14 Apr 2021 19:29:07 +0530	[thread overview]
Message-ID: <b41586781cffea03c5fd6b0849e2b9e4@mail.gmail.com> (raw)
In-Reply-To: <YHbOOfGNHwO4SMS7@T590>

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> > I tried both - 5.12.0-rc1 and 5.11.0-rc2+ and there is a same
behavior.
> > Let me also check  megaraid_sas and see if anything generic or this is
> > a special case of scsi_debug.
>
> As I mentioned, it could be one generic issue wrt. SCHED_RESTART.
> shared tags might have to restart all hctx since all share same tags.

Ming - I tried many combination on MR shared host tag driver but there is
no single instance of IO hang.
I will keep trying, but when I look at scsi_debug driver code I found
below odd settings in scsi_debug driver.
can_queue of adapter is set to 128 but queue_depth of sdev is set to 255.

If I apply below patch, scsi_debug driver's hang is also resolved. Ideally
sdev->queue depth cannot exceed shost->can_queue.
Not sure why cmd_per_lun is 255 in scsi_debug driver which can easily
exceed can_queue.  I will simulate something similar in MR driver and see
how it behaves w.r.t IO hang issue.

diff --git a/drivers/scsi/scsi_debug.c b/drivers/scsi/scsi_debug.c
index 70165be10f00..dded762540ee 100644
--- a/drivers/scsi/scsi_debug.c
+++ b/drivers/scsi/scsi_debug.c
@@ -218,7 +218,7 @@ static const char *sdebug_version_date = "20200710";
  */
 #define SDEBUG_CANQUEUE_WORDS  3       /* a WORD is bits in a long */
 #define SDEBUG_CANQUEUE  (SDEBUG_CANQUEUE_WORDS * BITS_PER_LONG)
-#define DEF_CMD_PER_LUN  255
+#define DEF_CMD_PER_LUN  SDEBUG_CANQUEUE

 /* UA - Unit Attention; SA - Service Action; SSU - Start Stop Unit */
 #define F_D_IN                 1       /* Data-in command (e.g. READ) */
@@ -7558,6 +7558,7 @@ static int sdebug_driver_probe(struct device *dev)
        sdbg_host = to_sdebug_host(dev);

        sdebug_driver_template.can_queue = sdebug_max_queue;
+       sdebug_driver_template.cmd_per_lun = sdebug_max_queue;
        if (!sdebug_clustering)
                sdebug_driver_template.dma_boundary = PAGE_SIZE - 1;
>
>
> Thanks,
> Ming

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