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

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Message-ID: <f4429faf-65b7-244f-7cf5-18c08ce4964c@huaweicloud.com>
Date: Mon, 11 Aug 2025 12:25:19 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Damien Le Moal <dlemoal@...nel.org>, Yu Kuai <yukuai1@...weicloud.com>,
 hare@...e.de, jack@...e.cz, bvanassche@....org, tj@...nel.org,
 josef@...icpanda.com, axboe@...nel.dk
Cc: cgroups@...r.kernel.org, linux-block@...r.kernel.org,
 linux-kernel@...r.kernel.org, yi.zhang@...wei.com, yangerkun@...wei.com,
 johnny.chenyi@...wei.com, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH v3 1/5] blk-mq-sched: introduce high level elevator lock

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

> 
> The other side of this is that if you use elevator_lock(e) to call
> e->type->ops.dispatch_request(hctx), it means that the scheduler *can NOT* use
> that same lock in its completion path, since that can be called from IRQ
> context. This may not be needed/a problem right now, but I think this needs a
> comment in this patch to mention this.
> 

So, the first patch just grab elevator lock for dispatch method, bfq
dispatch still using spin_lock_irq(&bfqd->lock), and complete is still
using bfqd->lock, this is safe.

Later patch 3 remove bfqd->lock and use elevator lock instead, and
because elevator lock will be used in complete path now, elevator_lock
from dispatch path is converted to elevator_lock_irq.

Thanks,
Kuai