Re: [PATCH v1] scsi: storvsc: Cap cmd_per_lun at can_queue
From: John Garry
Date: Tue Mar 09 2021 - 11:38:25 EST
On 09/03/2021 15:57, Michael Kelley wrote:
From: John Garry <john.garry@xxxxxxxxxx> Sent: Tuesday, March 9, 2021 2:10 AM
On 08/03/2021 17:56, Melanie Plageman wrote:
On Mon, Mar 08, 2021 at 02:37:40PM +0000, Michael Kelley wrote:
From: Melanie Plageman (Microsoft) <melanieplageman@xxxxxxxxx> Sent: Friday,
March 5, 2021 3:22 PM
The scsi_device->queue_depth is set to Scsi_Host->cmd_per_lun during
allocation.
Cap cmd_per_lun at can_queue to avoid dispatch errors.
Signed-off-by: Melanie Plageman (Microsoft) <melanieplageman@xxxxxxxxx>
---
drivers/scsi/storvsc_drv.c | 2 ++
1 file changed, 2 insertions(+)
diff --git a/drivers/scsi/storvsc_drv.c b/drivers/scsi/storvsc_drv.c
index 6bc5453cea8a..d7953a6e00e6 100644
--- a/drivers/scsi/storvsc_drv.c
+++ b/drivers/scsi/storvsc_drv.c
@@ -1946,6 +1946,8 @@ static int storvsc_probe(struct hv_device *device,
(max_sub_channels + 1) *
(100 - ring_avail_percent_lowater) / 100;
+ scsi_driver.cmd_per_lun = min_t(u32, scsi_driver.cmd_per_lun,
scsi_driver.can_queue);
+
I'm not sure what you mean by "avoid dispatch errors". Can you elaborate?
The scsi_driver.cmd_per_lun is set to 2048. Which is then used to set
Scsi_Host->cmd_per_lun in storvsc_probe().
In storvsc_probe(), when doing scsi_scan_host(), scsi_alloc_sdev() is
called and sets the scsi_device->queue_depth to the Scsi_Host's
cmd_per_lun with this code:
scsi_change_queue_depth(sdev, sdev->host->cmd_per_lun ?
sdev->host->cmd_per_lun : 1);
During dispatch, the scsi_device->queue_depth is used in
scsi_dev_queue_ready(), called by scsi_mq_get_budget() to determine
whether or not the device can queue another command.
On some machines, with the 2048 value of cmd_per_lun that was used to
set the initial scsi_device->queue_depth, commands can be queued that
are later not able to be dispatched after running out of space in the
ringbuffer.
On an 8 core Azure VM with 16GB of memory with a single 1 TiB SSD
(running an fio workload that I can provide if needed), storvsc_do_io()
ends up often returning SCSI_MLQUEUE_DEVICE_BUSY.
This is the call stack:
hv_get_bytes_to_write
hv_ringbuffer_write
vmbus_send_packet
storvsc_dio_io
storvsc_queuecommand
scsi_dispatch_cmd
scsi_queue_rq
dispatch_rq_list
Be aware that the calculation of "can_queue" in this driver is somewhat
flawed -- it should not be based on the size of the ring buffer, but instead on
the maximum number of requests Hyper-V will queue. And even then,
can_queue doesn't provide the cap you might expect because the blk-mq layer
allocates can_queue tags for each HW queue, not as a total.
The docs for scsi_mid_low_api document Scsi_Host can_queue this way:
can_queue
- must be greater than 0; do not send more than can_queue
commands to the adapter.
I did notice that in scsi_host.h, the comment for can_queue does say
can_queue is the "maximum number of simultaneous commands a single hw
queue in HBA will accept." However, I don't see it being used this way
in the code.
JFYI, the block layer ensures that no more than can_queue requests are
sent to the host. See scsi_mq_setup_tags(), and how the tagset queue
depth is set to shost->can_queue.
Thanks,
John
Agree on what's in scsi_mq_setup_tags(). But scsi_mq_setup_tags() calls
blk_mq_alloc_tag_set(), which in turn calls blk_mq_alloc_map_and_requests(),
which calls __blk_mq_alloc_rq_maps() repeatedly, reducing the tag
set queue_depth as needed until it succeeds.
The key thing is that __blk_mq_alloc_rq_maps() iterates over the
number of HW queues calling __blk_mq_alloc_map_and_request().
The latter function allocates the map and the requests with a count
of the tag set's queue_depth. There's no logic to apportion the
can_queue value across multiple HW queues. So each HW queue gets
can_queue tags allocated, and the SCSI host driver may see up to
(can_queue * # HW queues) simultaneous requests.
I'm certainly not an expert in this area, but that's what I see in the
code. We've run live experiments, and can see the number
simultaneous requests sent to the storvsc driver be greater than
can_queue when the # of HW queues is greater than 1, which seems
to be consistent with the code.
ah, ok. I assumed that # of HW queues = 1 here. So you're describing a
problem similar to
https://lore.kernel.org/linux-scsi/b3e4e597-779b-7c1e-0d3c-07bc3dab1bb5@xxxxxxxxxx/
So if you check nr_hw_queues comment in include/scsi/scsi_host.h, it reads:
the total queue depth per host is nr_hw_queues * can_queue. However, for
when host_tagset is set, the total queue depth is can_queue.
Setting .host_tagset will ensure at most can_queue requests will be sent
over all HW queues at any given time. A few SCSI MQ drivers set this now.
Thanks,
John
Michael
During dispatch, In scsi_target_queue_ready(), there is this code:
if (busy >= starget->can_queue)
goto starved;
And the scsi_target->can_queue value should be coming from Scsi_host as
mentioned in the scsi_target definition in scsi_device.h
/*
* LLDs should set this in the slave_alloc host template callout.
* If set to zero then there is not limit.
*/
unsigned int can_queue;
So, I don't really see how this would be per hardware queue.
I agree that the cmd_per_lun setting is also too big, but we should fix that in
the context of getting all of these different settings working together correctly,
and not piecemeal.
Capping Scsi_Host->cmd_per_lun to scsi_driver.can_queue during probe
will also prevent the LUN queue_depth from being set to a value that is
higher than it can ever be set to again by the user when
storvsc_change_queue_depth() is invoked.
Also in scsi_sysfs sdev_store_queue_depth() there is this check:
if (depth < 1 || depth > sdev->host->can_queue)
return -EINVAL;
I would also note that VirtIO SCSI in virtscsi_probe(), Scsi_Host->cmd_per_lun
is set to the min of the configured cmd_per_lun and
Scsi_Host->can_queue:
shost->cmd_per_lun = min_t(u32, cmd_per_lun, shost->can_queue);
Best,
Melanie
.