Re: [PATCH] USB: uas: Fix slave queue_depth not being set

Tom Yan
2016-05-25T14:06:55Z

What if a UAS bridge requires specific SCSI command (e.g. UNMAP) to be
issued unqueued/untagged? Would track_queue_depth help?

On 25 May 2016 at 19:04, Hans de Goede <hdegoede@redhat.com> wrote:
> Hi,
>
>
> On 24-05-16 14:44, James Bottomley wrote:
>>
>> On Tue, 2016-05-24 at 08:53 +0200, Hans de Goede wrote:
>>>
>>> Hi,
>>>
>>> On 23-05-16 19:36, James Bottomley wrote:
>>>>
>>>> On Mon, 2016-05-23 at 13:49 +0200, Hans de Goede wrote:
>>>>>
>>>>> Commit 198de51dbc34 ("USB: uas: Limit qdepth at the scsi-host
>>>>> level")
>>>>> removed the scsi_change_queue_depth() call from
>>>>> uas_slave_configure() assuming that the slave would inherit the
>>>>> host's queue_depth, which that commit sets to the same value.
>>>>>
>>>>> This is incorrect, without the scsi_change_queue_depth() call the
>>>>> slave's queue_depth defaults to 1, introducing a performance
>>>>> regression.
>>>>>
>>>>> This commit restores the call, fixing the performance regression.
>>>>>
>>>>> Cc: stable@vger.kernel.org
>>>>> Fixes: 198de51dbc34 ("USB: uas: Limit qdepth at the scsi-host
>>>>> level")
>>>>> Reported-by: Tom Yan <tom.ty89@gmail.com>
>>>>> Signed-off-by: Hans de Goede <hdegoede@redhat.com>
>>>>> ---
>>>>> drivers/usb/storage/uas.c | 1 +
>>>>> 1 file changed, 1 insertion(+)
>>>>>
>>>>> diff --git a/drivers/usb/storage/uas.c
>>>>> b/drivers/usb/storage/uas.c
>>>>> index 16bc679..ecc7d4b 100644
>>>>> --- a/drivers/usb/storage/uas.c
>>>>> +++ b/drivers/usb/storage/uas.c
>>>>> @@ -835,6 +835,7 @@ static int uas_slave_configure(struct
>>>>> scsi_device
>>>>> *sdev)
>>>>> if (devinfo->flags & US_FL_BROKEN_FUA)
>>>>> sdev->broken_fua = 1;
>>>>>
>>>>> + scsi_change_queue_depth(sdev, devinfo->qdepth - 2);
>>>>
>>>>
>>>> Are you sure about this? For spinning rust, experiments imply that
>>>> the optimal queue depth per device is somewhere between 2 and 4.
>>>> Obviously that's not true for SSDs, so it depends on your use
>>>> case. Plus, for ATA NCQ devices (which I believe most UAS is
>>>> bridged to) you have a maximum NCQ depth of 31.
>>>
>>>
>>> So this value is the same as host.can_queue, and is what uas has
>>> always used, basically this says it is ok to queue as much as the
>>> bridge can handle. We've seen a few rare multi-lun devices, but
>>> typically almost all uas devices have one lun, what I really want to
>>> do here is give a maximum and let say the sd driver lower that if it
>>> is sub-optimal.
>>
>>
>> If that's what you actually want, you should be setting sdev
>> ->max_queue_depth and .track_queue_depth = 1 in the template.
>
>
> Hmm, I've been looking into this, but that does not seem right.
>
> max_queue_depth is never set by drivers, it is set to sdev->queue_depth
> in scsi_scan.c: scsi_add_lun() after calling the host drivers'
> slave_configure callback. So it seems that the right way to set
> max_queue_depth is to actually set queue_depth, or iow restore the
> call to scsi_change_queue_depth() as the patch we're discussing does.
>
> As for track_queue_depth = 1 that seems to be only set by some drivers
> under drivers/scsi and is never set by any drivers under drivers/ata,
> and we're almost exclusively dealing with sata disks in uas. It seems
> that track_queue_depth = 1 is mostly used for iscsi and fibre channel
> iow enterprise class storage stuff, so looking at existing drivers
> usage of this flag using it does not seem a good idea.
>
> Anyways this patch is a (partial) revert of a previous bug-fix patch
> (which has also gone to stable) so for now I would really like to
> move forward with this patch and get it upstream and in stable
> to fix the performance regressions people are seeing caused by
> me wrongly dropping the scsi_change_queue_depth() call.
>
> Then if we want to tweak the queuing further we can do that
> on top of this fix, and put that in next and let it get some testing
> first.
>
> So are you ok with moving this patch forward ?
>
> Regards,
>
> Hans
>
>
>
>
>> You might also need to add calls to scsi_track_queue_full() but only if
>> the devices aren't responding QUEUE_FULL correctly.
>>
>> James
>>
>>> Also notice that uas is used a lot with ssd-s, that is mostly what
>>> I want to optimize for, but it is definitely also used with spinning
>>> rust.
>>>
>>> And yes almost all uas devices are bridged sata devices (this may
>>> change in the near future though, with ssd-s specifically designed
>>> for usb-3 attachment, although sofar these all seem to use an
>>> embbeded sata bridge), so from this pov an upper limit of 31 makes
>>> sense, I guess, but I've not seen any bridges which actually do more
>>> then 32 streams anyways.
>>>
>>> Still this is a bug-fix patch, essentially a partial revert, to
>>> address performance regressions, so lets get this out as is and take
>>> our time to come up with some tweaks (if necessary) for the say 4.8.
>>>
>>>> There's a good reason why you don't want a queue deeper than you
>>>> can handle: it tends to interfere with writeback because you build
>>>> up a lot of pending I/O in the queue which can't be issued (it's
>>>> very similar to why bufferbloat is a problem in networks). In
>>>> theory, as long as your devices return the correct indicator
>>>> (QUEUE_FULL status), we'll handle most of this in the mid-layer by
>>>> plugging the block queue, but given what I've seen from UAS
>>>> devices, that's less than probable.
>>>
>>>
>>> So any smart ideas how to be nicer to spinning rust, without
>>> negatively impacting ssd-s? As said if I've to choice I think we
>>> should chose optimizing ssd-s, as that is where uas is used a lot
>>> (although usb attached harddisks are switching over to it too).
>>>
>>> Note I just checked the 1TB sata/ahci harddisk in my workstation and
>>> it is using a queue_depth of 31 too, so this really does seem like a
>>> mid-layer problem to me.
>>>
>>> Regards,
>>>
>>> Hans
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>>
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