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

[Yu Kuai]

Hi,

在 2025/08/11 8:44, Damien Le Moal 写道:
> On 8/6/25 17:57, Yu Kuai wrote:
> > From: Yu Kuai <yukuai3@xxxxxxxxxx>
> >
> > Currently, both mq-deadline and bfq have global spin lock that will be
> > grabbed inside elevator methods like dispatch_request, insert_requests,
> > and bio_merge. And the global lock is the main reason mq-deadline and
> > bfq can't scale very well.
> >
> > While dispatching request, blk_mq_get_disatpch_budget() and
> > blk_mq_get_driver_tag() must be called, and they are not ready to be called
> > inside elevator methods, hence introduce a new method like
> > dispatch_requests is not possible.
> >
> > Hence introduce a new high level elevator lock, currently it is protecting
> > dispatch_request only. Following patches will convert mq-deadline and bfq
> > to use this lock and finally support request batch dispatching by calling
> > the method multiple time while holding the lock.
> >
> > Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> > ---
> >   block/blk-mq-sched.c |  9 ++++++++-
> >   block/elevator.c     |  1 +
> >   block/elevator.h     | 14 ++++++++++++--
> >   3 files changed, 21 insertions(+), 3 deletions(-)
> >
> > 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.

Thanks,
Kuai

> > +		} else {
> > +			rq = e->type->ops.dispatch_request(hctx);
> > +		}
> > +
> >   		if (!rq) {
> >   			blk_mq_put_dispatch_budget(q, budget_token);
> >   			/*
> > diff --git a/block/elevator.c b/block/elevator.c
> > index 88f8f36bed98..45303af0ca73 100644
> > --- a/block/elevator.c
> > +++ b/block/elevator.c
> > @@ -144,6 +144,7 @@ struct elevator_queue *elevator_alloc(struct request_queue *q,
> >   	eq->type = e;
> >   	kobject_init(&eq->kobj, &elv_ktype);
> >   	mutex_init(&eq->sysfs_lock);
> > +	spin_lock_init(&eq->lock);
> >   	hash_init(eq->hash);
> >   
> >   	return eq;
> > diff --git a/block/elevator.h b/block/elevator.h
> > index a07ce773a38f..81f7700b0339 100644
> > --- a/block/elevator.h
> > +++ b/block/elevator.h
> > @@ -110,12 +110,12 @@ struct request *elv_rqhash_find(struct request_queue *q, sector_t offset);
> >   /*
> >    * each queue has an elevator_queue associated with it
> >    */
> > -struct elevator_queue
> > -{
> > +struct elevator_queue {
> >   	struct elevator_type *type;
> >   	void *elevator_data;
> >   	struct kobject kobj;
> >   	struct mutex sysfs_lock;
> > +	spinlock_t lock;
> >   	unsigned long flags;
> >   	DECLARE_HASHTABLE(hash, ELV_HASH_BITS);
> >   };
> > @@ -124,6 +124,16 @@ struct elevator_queue
> >   #define ELEVATOR_FLAG_DYING		1
> >   #define ELEVATOR_FLAG_ENABLE_WBT_ON_EXIT	2
> >   
> > +#define elevator_lock(e)		spin_lock(&(e)->lock)
> > +#define elevator_unlock(e)		spin_unlock(&(e)->lock)
> > +#define elevator_lock_irq(e)		spin_lock_irq(&(e)->lock)
> > +#define elevator_unlock_irq(e)		spin_unlock_irq(&(e)->lock)
> > +#define elevator_lock_irqsave(e, flags) \
> > +	spin_lock_irqsave(&(e)->lock, flags)
> > +#define elevator_unlock_irqrestore(e, flags) \
> > +	spin_unlock_irqrestore(&(e)->lock, flags)
> > +#define elevator_lock_assert_held(e)	lockdep_assert_held(&(e)->lock)
> > +
> >   /*
> >    * block elevator interface
> >    */