Re: [PATCH BUGFIX V3] block, bfq: add requeue-request hook

Paolo Valente
2018-02-23T15:41:43Z

> Il giorno 23 feb 2018, alle ore 16:07, Ming Lei <ming.lei@redhat.com> ha scritto:
>
> Hi Paolo,
>
> On Wed, Feb 07, 2018 at 10:19:20PM +0100, Paolo Valente wrote:
>> Commit 'a6a252e64914 ("blk-mq-sched: decide how to handle flush rq via
>> RQF_FLUSH_SEQ")' makes all non-flush re-prepared requests for a device
>> be re-inserted into the active I/O scheduler for that device. As a
>
> No, this behaviour isn't related with commit a6a252e64914, and
> it has been there since blk_mq_requeue_request() is introduced.
>

Hi Ming,
actually, we wrote the above statement after simply following the call
chain that led to the failure. In this respect, the change in commit
a6a252e64914:

static bool blk_mq_sched_bypass_insert(struct blk_mq_hw_ctx *hctx,
+ bool has_sched,
struct request *rq)
{
- if (rq->tag == -1) {
+ /* dispatch flush rq directly */
+ if (rq->rq_flags & RQF_FLUSH_SEQ) {
+ spin_lock(&hctx->lock);
+ list_add(&rq->queuelist, &hctx->dispatch);
+ spin_unlock(&hctx->lock);
+ return true;
+ }
+
+ if (has_sched) {
rq->rq_flags |= RQF_SORTED;
- return false;
+ WARN_ON(rq->tag != -1);
}

- /*
- * If we already have a real request tag, send directly to
- * the dispatch list.
- */
- spin_lock(&hctx->lock);
- list_add(&rq->queuelist, &hctx->dispatch);
- spin_unlock(&hctx->lock);
- return true;
+ return false;
}

makes blk_mq_sched_bypass_insert return false for all non-flush
requests. From that, the anomaly described in our commit follows, for
bfq any stateful scheduler that waits for the completion of requests
that passed through it. I'm elaborating again a little bit on this in
my replies to your next points below.

I don't mean that this change is an error, it simply sends a stateful
scheduler in an inconsistent state, unless the scheduler properly
handles the requeue that precedes the re-insertion into the
scheduler.

If this clarifies the situation, but there is still some misleading
statement in the commit, just let me better understand, and I'll be
glad to rectify it, if possible somehow.

> And you can see blk_mq_requeue_request() is called by lots of drivers,
> especially it is often used in error handler, see SCSI's example.
>
>> consequence, I/O schedulers may get the same request inserted again,
>> even several times, without a finish_request invoked on that request
>> before each re-insertion.
>>

...

>> @@ -5426,7 +5482,8 @@ static struct elevator_type iosched_bfq_mq = {
>> .ops.mq = {
>> .limit_depth = bfq_limit_depth,
>> .prepare_request = bfq_prepare_request,
>> - .finish_request = bfq_finish_request,
>> + .requeue_request = bfq_finish_requeue_request,
>> + .finish_request = bfq_finish_requeue_request,
>> .exit_icq = bfq_exit_icq,
>> .insert_requests = bfq_insert_requests,
>> .dispatch_request = bfq_dispatch_request,
>
> This way may not be correct since blk_mq_sched_requeue_request() can be
> called for one request which won't enter io scheduler.
>

Exactly, there are two cases: requeues that lead to subsequent
re-insertions, and requeues that don't. The function
bfq_finish_requeue_request handles both, and both need to be handled,
to inform bfq that it has not to wait for the completion of rq any
longer.

One special case is when bfq_finish_requeue_request gets invoked even
if rq has nothing to do with any scheduler. In that case,
bfq_finish_requeue_request exists immediately.

> __blk_mq_requeue_request() is called for two cases:
>
> - one is that the requeued request is added to hctx->dispatch, such
> as blk_mq_dispatch_rq_list()

yes

> - another case is that the request is requeued to io scheduler, such as
> blk_mq_requeue_request().
>

yes

> For the 1st case, blk_mq_sched_requeue_request() shouldn't be called
> since it is nothing to do with scheduler,

No, if that rq has been inserted and then extracted from the scheduler
through a dispatch_request, then it has. The scheduler is stateful,
and keeps state for rq, because it must do so, until a completion or a
requeue arrive. In particular, bfq may decide that no other
bfq_queues must be served before rq has been completed, because this
is necessary to preserve its target service guarantees. If bfq is not
informed, either through its completion or its requeue hook, then it
will wait forever.

> seems we only need to do that
> for 2nd case.
>

> So looks we need the following patch:
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 23de7fd8099a..a216f3c3c3ce 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -712,7 +714,6 @@ static void __blk_mq_requeue_request(struct request *rq)
>
> trace_block_rq_requeue(q, rq);
> wbt_requeue(q->rq_wb, &rq->issue_stat);
> - blk_mq_sched_requeue_request(rq);
>

By doing so, if rq has 'passed' through bfq, then you block again bfq
forever.

Of course, I may be wrong, as I don't have a complete mental model of
all what blk-mq does around bfq. But I thin that you can quickly
check whether a hang occurs again if you add this change. In
particular, it should happen in the USB former failure case.

Thanks,
Paolo

> if (blk_mq_rq_state(rq) != MQ_RQ_IDLE) {
> blk_mq_rq_update_state(rq, MQ_RQ_IDLE);
> @@ -725,6 +726,9 @@ void blk_mq_requeue_request(struct request *rq, bool kick_requeue_list)
> {
> __blk_mq_requeue_request(rq);
>
> + /* this request will be re-inserted to io scheduler queue */
> + blk_mq_sched_requeue_request(rq);
> +
> BUG_ON(blk_queued_rq(rq));
> blk_mq_add_to_requeue_list(rq, true, kick_requeue_list);
> }
>
>
> Thanks,
> Ming