On 05/05/16 20:47, Douglas Gilbert wrote:
On 2016-05-04 06:32 PM, Bart Van Assche wrote:
On 04/30/2016 07:44 PM, Douglas Gilbert wrote:
+static struct sdebug_queue *get_queue(void)
+{
+ struct sdebug_queue *sqp = sdebug_q_arr;
+
+ return sqp + (raw_smp_processor_id() % submit_queues);
+}
Does this function have the same purpose as blk_mq_map_queue()? If so,
why has
this function been introduced instead of using blk_mq_map_queue()?
No, it is copied from drivers/block/null_blk.c, the nullb_to_queue()
function. scsi_lib.c seems to be the only user of blk_mq_map_queue().
@@ -5001,6 +5158,10 @@ static int scsi_debug_queuecommand(struct
Scsi_Host
*shost,
bool has_wlun_rl;
scsi_set_resid(scp, 0);
+ if (sdebug_statistics) {
+ sqp = get_queue();
+ atomic_inc(&sqp->cmnd_count);
+ }
Why does scsi_debug_queuecommand() call get_queue() instead of
blk_mq_unique_tag() and blk_mq_unique_tag_to_hwq() which is what other
scsi-mq
drivers do?
Okay I have switched to using:
u32 tag = blk_mq_unique_tag(cmnd->request);
u16 hwq = blk_mq_unique_tag_to_hwq(tag);
return sqp + hwq;
and as far as I can tell it works just as well as:
return sqp + (raw_smp_processor_id() % submit_queues);
Thanks for the feedback. I was not yet aware that null_blk also follows
this approach. Apparently null_blk has its own prep function and that
prep function needs access to the submit queue. Maybe that is why
null_blk has its own submit queue selection function. However, I'd like
to see that kind of code being moved from the null_blk driver into the
blk-mq core.
Is the role of the sqp->cmnd_count counter identical to that of
blk_mq_hw_ctx.queued? If so, can sqp->cmnd_count be left out and can
blk_mq_hw_ctx.queued be used instead?
Not quite, it counts commands in, some of which may get
responded to in the same thread (e.g. when a SCSI illegal request
type error is being reported). But it is very close to my
"completions" count.
So I'll move those counters back into file scope which simplifies
scsi_debug's logic. The cmnd_counter is used in the injection of
pseudo errors "every_nth" command. Can blk_mq_hw_ctx.queued be
viewed from the user space?
As far as I can see, yes. From block/blk-mq-sysfs.c:
static ssize_t blk_mq_hw_sysfs_queued_show(struct blk_mq_hw_ctx *hctx,
char *page)
{
return sprintf(page, "%lu\n", hctx->queued);
}
@@ -5168,6 +5328,16 @@ static int sdebug_driver_probe(struct device *
dev)
+ if (sdebug_mq_available && (submit_queues > 1))
+ hpnt->nr_hw_queues = submit_queues;
There is already a submit_queues < 1 check in scsi_debug_init(). Is the
submit_queues > 1 check in sdebug_driver_probe() needed?
I noticed that the other (2) scsi mq LLDs take care only to set
hpnt->nr_hw_queues when the submit_queues > 1 .
You might have overlooked the ib_srp driver. That driver is a SCSI LLD
to which support for multiple hardware queues was added before it was
added to the lpfc and virtio_scsi drivers.
I haven't found any code in any of these three drivers that only
modifies nr_hw_queues if submit_queues > 1 ? Anyway, this is a detail
and something I do not consider important.
Bart.
--
To unsubscribe from this list: send the line "unsubscribe linux-scsi" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
