Re: [PATCH v2 2/5] mq-deadline: switch to use elevator lock

[Damien Le Moal]

On 7/31/25 3:32 PM, Yu Kuai wrote:
> Hi,
> 
> 在 2025/07/31 14:22, Damien Le Moal 写道:
>> On 7/31/25 3:20 PM, Hannes Reinecke wrote:
>>> On 7/30/25 10:22, Yu Kuai wrote:
>>>> From: Yu Kuai <yukuai3@xxxxxxxxxx>
>>>>
>>>> Replace the internal spinlock 'dd->lock' with the new spinlock in
>>>> elevator_queue, there are no functional changes.
>>>>
>>>> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
>>>> ---
>>>>    block/mq-deadline.c | 58 +++++++++++++++++++++------------------------
>>>>    1 file changed, 27 insertions(+), 31 deletions(-)
>>>>
>>>> diff --git a/block/mq-deadline.c b/block/mq-deadline.c
>>>> index 9ab6c6256695..2054c023e855 100644
>>>> --- a/block/mq-deadline.c
>>>> +++ b/block/mq-deadline.c
>>>> @@ -101,7 +101,7 @@ struct deadline_data {
>>>>        u32 async_depth;
>>>>        int prio_aging_expire;
>>>>    -    spinlock_t lock;
>>>> +    spinlock_t *lock;
>>>>    };
>>>>      /* Maps an I/O priority class to a deadline scheduler priority. */
>>>> @@ -213,7 +213,7 @@ static void dd_merged_requests(struct request_queue *q,
>>>> struct request *req,
>>>>        const u8 ioprio_class = dd_rq_ioclass(next);
>>>>        const enum dd_prio prio = ioprio_class_to_prio[ioprio_class];
>>>>    -    lockdep_assert_held(&dd->lock);
>>>> +    lockdep_assert_held(dd->lock);
>>>>          dd->per_prio[prio].stats.merged++;
>>>>    @@ -253,7 +253,7 @@ static u32 dd_queued(struct deadline_data *dd, enum
>>>> dd_prio prio)
>>>>    {
>>>>        const struct io_stats_per_prio *stats = &dd->per_prio[prio].stats;
>>>>    -    lockdep_assert_held(&dd->lock);
>>>> +    lockdep_assert_held(dd->lock);
>>>>          return stats->inserted - atomic_read(&stats->completed);
>>>>    }
>>>> @@ -323,7 +323,7 @@ static struct request *__dd_dispatch_request(struct
>>>> deadline_data *dd,
>>>>        enum dd_prio prio;
>>>>        u8 ioprio_class;
>>>>    -    lockdep_assert_held(&dd->lock);
>>>> +    lockdep_assert_held(dd->lock);
>>>>          if (!list_empty(&per_prio->dispatch)) {
>>>>            rq = list_first_entry(&per_prio->dispatch, struct request,
>>>> @@ -434,7 +434,7 @@ static struct request
>>>> *dd_dispatch_prio_aged_requests(struct deadline_data *dd,
>>>>        enum dd_prio prio;
>>>>        int prio_cnt;
>>>>    -    lockdep_assert_held(&dd->lock);
>>>> +    lockdep_assert_held(dd->lock);
>>>>          prio_cnt = !!dd_queued(dd, DD_RT_PRIO) + !!dd_queued(dd,
>>>> DD_BE_PRIO) +
>>>>               !!dd_queued(dd, DD_IDLE_PRIO);
>>>> @@ -466,10 +466,9 @@ static struct request *dd_dispatch_request(struct
>>>> blk_mq_hw_ctx *hctx)
>>>>        struct request *rq;
>>>>        enum dd_prio prio;
>>>>    -    spin_lock(&dd->lock);
>>>>        rq = dd_dispatch_prio_aged_requests(dd, now);
>>>>        if (rq)
>>>> -        goto unlock;
>>>> +        return rq;
>>>>          /*
>>>>         * Next, dispatch requests in priority order. Ignore lower priority
>>>> @@ -481,9 +480,6 @@ static struct request *dd_dispatch_request(struct
>>>> blk_mq_hw_ctx *hctx)
>>>>                break;
>>>>        }
>>>>    -unlock:
>>>> -    spin_unlock(&dd->lock);
>>>> -
>>>>        return rq;
>>>>    }
>>>>    @@ -538,9 +534,9 @@ static void dd_exit_sched(struct elevator_queue *e)
>>>>            WARN_ON_ONCE(!list_empty(&per_prio->fifo_list[DD_READ]));
>>>>            WARN_ON_ONCE(!list_empty(&per_prio->fifo_list[DD_WRITE]));
>>>>    -        spin_lock(&dd->lock);
>>>> +        spin_lock(dd->lock);
>>>>            queued = dd_queued(dd, prio);
>>>> -        spin_unlock(&dd->lock);
>>>> +        spin_unlock(dd->lock);
>>>>              WARN_ONCE(queued != 0,
>>>>                  "statistics for priority %d: i %u m %u d %u c %u\n",
>>>
>>> Do you still need 'dd->lock'? Can't you just refer to the lock from the
>>> elevator_queue structure directly?
>>
>> Indeed. Little inline helpers for locking/unlocking q->elevator->lock would be
>> nice.
> 
> How about the first patch to factor out inline helpers like dd_lock()
> and dd_unlock(), still use dd->lock without any functional changes, and
> then switch to use q->elevator->lock in the next patch? (same for bfq)

Patch one can introduce elv->lock and the helpers, then patch 2 use the helpers
to replace dd->lock. Just don't say "no functional change" in the commit
message and rather explain that things keep working the same way as before, but
using a different lock. That will address Bart's comment too.
And same for bfq in patch 3.


-- 
Damien Le Moal
Western Digital Research