Re: [bug report] block: Non-NCQ commands will never be executed while fio is continuously running

[Yu Kuai]

Hi,

在 2024/09/11 6:38, Damien Le Moal 写道:
> On 9/10/24 20:27, Niklas Cassel wrote:
> > On Tue, Sep 10, 2024 at 02:34:06PM +0800, yangxingui wrote:
> > > On 2024/9/10 12:45, Damien Le Moal wrote:
> > > > On 9/10/24 10:09 AM, yangxingui wrote:
> > > > > On 2024/9/9 21:21, Damien Le Moal wrote:
> > > > > > On 9/9/24 22:10, yangxingui wrote:
> > > > > > > Hello axboe & John,
> > > > > > >
> > > > > > > After the driver exposes all HW queues to the block layer, non-NCQ
> > > > > > > commands will never be executed while fio is continuously running, such
> > > > > > > as a smartctl command.
> > > > > > >
> > > > > > > The cause of the problem is that other hctx used by the NCQ command is
> > > > > > > still active and can continue to issue NCQ commands to the sata disk.
> > > > > > > And the pio command keeps retrying in its corresponding hctx because
> > > > > > > qc_defer() always returns true.
> > > > > > >
> > > > > > > hctx0: ncq, pio, ncq
> > > > > > > hctx1：ncq, ncq, ...
> > > > > > > ...
> > > > > > > hctxn: ncq, ncq, ...
> > > > > > >
> > > > > > > Is there any good solution for this?
> > > > > >
> > > > > > SATA devices are single queue so how can you have multiple queues ?
> > > > > > What adapter are you using ?
> > > > >
> > > > > In the following patch, we expose the host's 16 hardware queues to the block
> > > > > layer. And when connecting to a sata disk, 16 hctx are used.
> > > > >
> > > > > 8d98416a55eb ("scsi: hisi_sas: Switch v3 hw to MQ")
> > > >
> > > > OK, so the HBA is a hisi one, using libsas...
> > > > What is the device ? An SSD ? and HDD ?
> > > Both SATA SSD and SATA HDD have this problem.
> > >
> > > > Do you set a block I/O scheduler for the drive, e.g. mq-deadline. If not, does
> > > > setting a scheduler resolve the issue ?
> > > Currently, the default configuration mq-deadline is used, and the same
> > > phenomenon occurs when I try setting it to none. It seems to have nothing to
> > > do with the scheduling strategy.
> > >
> > > > I do not have any hisi HBA. I use a lot of mpt3sas and mpi3mr HBAs which also
> > > > have multiple queues with a shared tagset. Never seen the issue you are
> > > > reporting though using HDDs with mq-deadline or bfq as the scheduler.
> > > Unlike libsas, as these hosts don't use qc_defer()?
> >
> > mpt3sas and mpi3mr do not use any libata code at all, the SCSI to ATA
> > Translation (SAT) is done completely by the HBA, so from a Linux
> > perspective, we are issuing SCSI commands to the HBA.
>
> Yes, but we still can get requeue happening. Though for a SATA drive, that is
> unlikely since the max queue depth is clearly defined, unlike for SAS drives
>
> > We can see that libsas uses ata_std_qc_defer() as its .qc_defer callback:
> > https://github.com/torvalds/linux/blob/v6.11-rc7/drivers/scsi/libsas/sas_ata.c#L566
>
> And that may be the issue. More on this below.
>
> > Without considering if it is a good idea or not, it should be possible to
> > translate some commands to instead use the "NCQ encapsulated" variant of
> > the ATA command that was used in the "ATA-16 passthrough" SCSI command.
>
> That would be way too much work on the user side, and likely open up a can of
> device bugs unseen until now.
>
> > To be able to send a non-queued command, there has to be no NCQ commands queued
> > on the device. I guess you could implement a scheduler that would be quiescing
> > the queue, processes the non-queued command, and then thaw the queue, but that
> > would essentially make non-queued commands high priority commands, and could
> > thus be used to seriously limit throughput by just sending some non-queued
> > commands every now and then :)
>
> Passthrough commands do not go through the scheduler and are submitted directly
> to the dispatch queue, generally at the head of it (see blk_mq_insert_request()).
>
> So for a single queue device, even if ata_qc_defer causes a requeue, the
> passthrough command ends up back at the top of the dispatch queue. After
> repeating this a few times, all in-flight NCQ commands complete and the
> passthrough command goes through.
>
> But I feel this is very fragile given that the block layer requeue is done
> through a work item, so in parallel to an application submitting IOs. So in
> theory, I think that the requeue for the passthrough command could happen forever...
>
> And for a multi-queue setup like with the hisi adapter, that is what is happening.
>
> I do not have any good idea how to fix that yet. We need to find something.
> scsi_queue_rq() and the budget/host or device blocked state management may help
> with that, or we have a bug there... In any case, I do not think it is a block
> layer issue as the block layer knows nothing about NCQ vs non-NCQ.

Does libata return a specific value in this case? If so, maybe we can
stop other hctx untill this IO is handled.

For now, I think libata should use single hctx, it just doesn't support
multiple hctx yet.

Thanks,
Kuai

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