Re: [PATCH v3 3/4] blk-mq: Rerun dispatching in the case of budget contention

[Ming Lei]

On Tue, Apr 07, 2020 at 03:00:04PM -0700, Douglas Anderson wrote:
> If ever a thread running blk-mq code tries to get budget and fails it
> immediately stops doing work and assumes that whenever budget is freed
> up that queues will be kicked and whatever work the thread was trying
> to do will be tried again.
> 
> One path where budget is freed and queues are kicked in the normal
> case can be seen in scsi_finish_command().  Specifically:
> - scsi_finish_command()
>   - scsi_device_unbusy()
>     - # Decrement "device_busy", AKA release budget
>   - scsi_io_completion()
>     - scsi_end_request()
>       - blk_mq_run_hw_queues()
> 
> The above is all well and good.  The problem comes up when a thread
> claims the budget but then releases it without actually dispatching
> any work.  Since we didn't schedule any work we'll never run the path
> of finishing work / kicking the queues.
> 
> This isn't often actually a problem which is why this issue has
> existed for a while and nobody noticed.  Specifically we only get into
> this situation when we unexpectedly found that we weren't going to do
> any work.  Code that later receives new work kicks the queues.  All
> good, right?
> 
> The problem shows up, however, if timing is just wrong and we hit a
> race.  To see this race let's think about the case where we only have
> a budget of 1 (only one thread can hold budget).  Now imagine that a
> thread got budget and then decided not to dispatch work.  It's about
> to call put_budget() but then the thread gets context switched out for
> a long, long time.  While in this state, any and all kicks of the
> queue (like the when we received new work) will be no-ops because
> nobody can get budget.  Finally the thread holding budget gets to run
> again and returns.  All the normal kicks will have been no-ops and we
> have an I/O stall.
> 
> As you can see from the above, you need just the right timing to see
> the race.  To start with, the only case it happens if we thought we
> had work, actually managed to get the budget, but then actually didn't
> have work.  That's pretty rare to start with.  Even then, there's
> usually a very small amount of time between realizing that there's no
> work and putting the budget.  During this small amount of time new
> work has to come in and the queue kick has to make it all the way to
> trying to get the budget and fail.  It's pretty unlikely.
> 
> One case where this could have failed is illustrated by an example of
> threads running blk_mq_do_dispatch_sched():
> 
> * Threads A and B both run has_work() at the same time with the same
>   "hctx".  Imagine has_work() is exact.  There's no lock, so it's OK
>   if Thread A and B both get back true.
> * Thread B gets interrupted for a long time right after it decides
>   that there is work.  Maybe its CPU gets an interrupt and the
>   interrupt handler is slow.
> * Thread A runs, get budget, dispatches work.
> * Thread A's work finishes and budget is released.
> * Thread B finally runs again and gets budget.
> * Since Thread A already took care of the work and no new work has
>   come in, Thread B will get NULL from dispatch_request().  I believe
>   this is specifically why dispatch_request() is allowed to return
>   NULL in the first place if has_work() must be exact.
> * Thread B will now be holding the budget and is about to call
>   put_budget(), but hasn't called it yet.
> * Thread B gets interrupted for a long time (again).  Dang interrupts.
> * Now Thread C (maybe with a different "hctx" but the same queue)
>   comes along and runs blk_mq_do_dispatch_sched().
> * Thread C won't do anything because it can't get budget.

Thread C will re-run queue in this case:

Just thought scsi_mq_get_budget() does handle the case via re-run queue:

        if (atomic_read(&sdev->device_busy) == 0 && !scsi_device_blocked(sdev))
                blk_mq_delay_run_hw_queue(hctx, SCSI_QUEUE_DELAY);

So looks no such race.

> * Finally Thread B will run again and put the budget without kicking
>   any queues.
> 
> Even though the example above is with blk_mq_do_dispatch_sched() I
> believe the race is possible any time someone is holding budget but
> doesn't do work.
> 
> Unfortunately, the unlikely has become more likely if you happen to be
> using the BFQ I/O scheduler.  BFQ, by design, sometimes returns "true"
> for has_work() but then NULL for dispatch_request() and stays in this
> state for a while (currently up to 9 ms).  Suddenly you only need one
> race to hit, not two races in a row.  With my current setup this is
> easy to reproduce in reboot tests and traces have actually shown that
> we hit a race similar to the one describe above.
> 
> In theory we could choose to just fix blk_mq_do_dispatch_sched() to
> kick the queues when it puts budget.  That would fix the BFQ case and
> one could argue that all the other cases are just theoretical.  While
> that is true, for all the other cases it should be very uncommon to
> run into the case where we need put_budget().  Having an extra queue
> kick for safety there shouldn't affect much and keeps the race at bay.
> 
> One last note is that (at least in the SCSI case) budget is shared by
> all "hctx"s that have the same queue.  Thus we need to make sure to
> kick the whole queue, not just re-run dispatching on a single "hctx".
> 
> Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> ---
> 
> Changes in v3:
> - Always kick when putting the budget.
> - Delay blk_mq_do_dispatch_sched() kick by 3 ms for inexact has_work().
> - Totally rewrote commit message.
> 
> Changes in v2:
> - Replace ("scsi: core: Fix stall...") w/ ("blk-mq: Rerun dispatch...")
> 
>  block/blk-mq.h | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/block/blk-mq.h b/block/blk-mq.h
> index 10bfdfb494fa..1270505367ab 100644
> --- a/block/blk-mq.h
> +++ b/block/blk-mq.h
> @@ -180,12 +180,24 @@ unsigned int blk_mq_in_flight(struct request_queue *q, struct hd_struct *part);
>  void blk_mq_in_flight_rw(struct request_queue *q, struct hd_struct *part,
>  			 unsigned int inflight[2]);
>  
> +#define BLK_MQ_BUDGET_DELAY	3		/* ms units */
> +
>  static inline void blk_mq_put_dispatch_budget(struct blk_mq_hw_ctx *hctx)
>  {
>  	struct request_queue *q = hctx->queue;
>  
> -	if (q->mq_ops->put_budget)
> +	if (q->mq_ops->put_budget) {
>  		q->mq_ops->put_budget(hctx);
> +
> +		/*
> +		 * The only time we call blk_mq_put_dispatch_budget() is if
> +		 * we released the budget without dispatching.  Holding the
> +		 * budget could have blocked any "hctx"s with the same queue
> +		 * and if we didn't dispatch then there's no guarantee anyone
> +		 * will kick the queue.  Kick it ourselves.
> +		 */
> +		blk_mq_delay_run_hw_queues(q, BLK_MQ_BUDGET_DELAY);

No, please don't do that un-conditionally we just need to re-run queue
when there has work to do.

Thanks,
Ming