Re: [PATCH] scsi: core: move scsi_host_busy() out of host lock for waking up EH handler

[Ming Lei <ming.lei@redhat.com>]

On Fri, Jan 12, 2024 at 02:34:52PM -0500, Ewan Milne wrote:
> On Fri, Jan 12, 2024 at 7:43 AM Ming Lei <ming.lei@redhat.com> wrote:
> >
> > On Fri, Jan 12, 2024 at 12:12:57PM +0100, Hannes Reinecke wrote:
> > > On 1/12/24 08:00, Ming Lei wrote:
> > > > Inside scsi_eh_wakeup(), scsi_host_busy() is called & checked with host lock
> > > > every time for deciding if error handler kthread needs to be waken up.
> > > >
> > > > This way can be too heavy in case of recovery, such as:
> > > >
> > > > - N hardware queues
> > > > - queue depth is M for each hardware queue
> > > > - each scsi_host_busy() iterates over (N * M) tag/requests
> > > >
> > > > If recovery is triggered in case that all requests are in-flight, each
> > > > scsi_eh_wakeup() is strictly serialized, when scsi_eh_wakeup() is called
> > > > for the last in-flight request, scsi_host_busy() has been run for (N * M - 1)
> > > > times, and request has been iterated for (N*M - 1) * (N * M) times.
> > > >
> > > > If both N and M are big enough, hard lockup can be triggered on acquiring
> > > > host lock, and it is observed on mpi3mr(128 hw queues, queue depth 8169).
> > > >
> > > > Fix the issue by calling scsi_host_busy() outside host lock, and we
> > > > don't need host lock for getting busy count because host lock never
> > > > covers that.
> > > >
> > > Can you share details for the hard lockup?
> > > I do agree that scsi_host_busy() is an expensive operation, so it
> > > might not be ideal to call it under a spin lock.
> > > But I wonder where the lockup comes in here.
> > > Care to explain?
> >
> > Recovery happens when there is N * M inflight requests, then scsi_dec_host_busy()
> > can be called for each inflight request/scmnd from irq context.
> >
> > host lock serializes every scsi_eh_wakeup().
> >
> > Given each hardware queue has its own irq handler, so there could be one
> > request, scsi_dec_host_busy() is called and the host lock is spinned until
> > it is released from scsi_dec_host_busy() for all requests from all other
> > hardware queues.
> >
> > The spin time can be long enough to trigger the hard lockup if N and M
> > is big enough, and the total wait time can be:
> >
> >         (N - 1) * M * time_taken_in_scsi_host_busy().
> >
> > Meantime the same story happens on scsi_eh_inc_host_failed() which is
> > called from softirq context, so host lock spin can be much more worse.
> >
> > It is observed on mpi3mr with 128(N) hw queues and 8169(M) queue depth.
> >
> > >
> > > And if it leads to a lockup, aren't other instances calling scsi_host_busy()
> > > under a spinlock affected, as well?
> >
> > It is only possible when it is called in per-command situation.
> >
> >
> > Thanks,
> > Ming
> >
> 
> I can't see why this wouldn't work, or cause a problem with a lost wakeup,
> but the cost of iterating to obtain the host_busy value is still being paid,
> just outside the host_lock.  If this has triggered a hard lockup, should
> we revisit the algorithm, e.g. are we still delaying EH wakeup for a noticeable
> amount of time?

SCSI EH is designed to start handling until all in-flight commands are
failed, so it waits until all requests are failed first.

> O(n^2) algorithms in the kernel don't seem like the best idea.

It is actually O(n) because each hardware queue handles request
in parallel.

It is degraded to O(n^2) or O(n * m) just because of shared host lock.

Single or N scsi_host_busy() won't take too long without host lock, what
matters is actually the per-host lock spin time which can be accumulated
as too big.

> 
> In any case...
> Reviewed-by: Ewan D. Milne <emilne@redhat.com>

Thanks for the review!


-- 
Ming