Re: [PATCH v3 1/5] blk-mq-sched: introduce high level elevator lock

[Yu Kuai]

Hi,

在 2025/08/11 12:34, Damien Le Moal 写道:
> On 8/11/25 13:25, Yu Kuai wrote:
> > Hi,
> >
> > 在 2025/08/11 11:53, Damien Le Moal 写道:
> > > On 8/11/25 10:01, Yu Kuai wrote:
> > > > > > diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> > > > > > index 55a0fd105147..1a2da5edbe13 100644
> > > > > > --- a/block/blk-mq-sched.c
> > > > > > +++ b/block/blk-mq-sched.c
> > > > > > @@ -113,7 +113,14 @@ static int __blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
> > > > > >     		if (budget_token < 0)
> > > > > >     			break;
> > > > > >     
> > > > > > -		rq = e->type->ops.dispatch_request(hctx);
> > > > > > +		if (blk_queue_sq_sched(q)) {
> > > > > > +			elevator_lock(e);
> > > > > > +			rq = e->type->ops.dispatch_request(hctx);
> > > > > > +			elevator_unlock(e);
> > > > >
> > > > > I do not think this is safe for bfq since bfq uses the irqsave/irqrestore spin
> > > > > lock variant. If it is safe, this needs a big comment block explaining why
> > > > > and/or the rules regarding the scheduler use of this lock.
> > > >
> > > > It's correct, however, this patch doesn't change bfq yet, and it's like:
> > > >
> > > > elevator_lock
> > > > spin_lock_irq(&bfqd->lock)
> > > > spin_unlock_irq(&bfqd->lock)
> > > > elevator_unlock
> > > >
> > > > Patch 3 remove bfqd->lock and convert this to:
> > > >
> > > > elevator_lock_irq
> > > > elevator_unlock_irq.
> > >
> > > I do not understand. Since q->elevator->lock is already taken here, without IRQ
> > > disabled, how can bfq_dispatch_request method again take this same lock with IRQ
> > > disabled ? That cannot possibly work.
> >
> > Looks like there is still misunderstanding somehow :( After patch 3,
> > bfq_dispatch_work doesn't grab any lock, elevator lock is held before
> > calling into dispatch method.
> >
> > Before:
> >
> > elevator_lock
> > bfq_dispatch_request
> >    spin_lock_irq(&bfqd->lock)
> >    spin_unlock_irq(&bfqd->lock)
> > elevator_unlock
> >
> > After:
> > elevator_lock_irq
> > bfq_dispatch_request
> > elevator_unlock_irq
>
> Ah, yes, I see it now.
>
> But that is a nasty change that affects *all* schedulers, even those that do not
> need to disable IRQs because they are not using the lock in their completion
> path, e.g. mq-deadline. So I do not think that is acceptable.

OK, I did some benchmark and didn't found any performance regression, so
I decided use the irq version for deadline. Perhaps I didn't found such
test.

I can add a flag in elevator_queue->flags in addition like following to
fix this:

diff --git a/block/elevator.h b/block/elevator.h
index 81f7700b0339..f04b9b2ae344 100644
--- a/block/elevator.h
+++ b/block/elevator.h
@@ -123,6 +123,7 @@ struct elevator_queue {
 #define ELEVATOR_FLAG_REGISTERED       0
 #define ELEVATOR_FLAG_DYING            1
 #define ELEVATOR_FLAG_ENABLE_WBT_ON_EXIT       2
+#define ELEVATOR_FLAG_LOCK_AT_COMP     3

 #define elevator_lock(e)               spin_lock(&(e)->lock)
 #define elevator_unlock(e)             spin_unlock(&(e)->lock)
@@ -134,6 +135,22 @@ struct elevator_queue {
        spin_unlock_irqrestore(&(e)->lock, flags)
 #define elevator_lock_assert_held(e)   lockdep_assert_held(&(e)->lock)

+static inline void elevator_dispatch_lock(struct elevator_queue *e)
+{
+       if (test_bit(ELEVATOR_FLAG_LOCK_AT_COMP, &e->flags))
+               elevator_lock_irq(e);
+       else
+               elevator_lock(e);
+}
+
+static inline void elevator_dispatch_unlock(struct elevator_queue *e)
+{
+       if (test_bit(ELEVATOR_FLAG_LOCK_AT_COMP, &e->flags))
+               elevator_unlock_irq(e);
+       else
+               elevator_unlock(e);
+}
+

Thanks,
Kuai