Re: [RFC] io-wq: decouple work_list protection from the big wqe->lock

[Jens Axboe <axboe@kernel.dk>]

On 11/4/21 5:35 AM, Hao Xu wrote:
> 在 2021/11/4 上午3:10, Jens Axboe 写道:
>> On 10/31/21 4:49 AM, Hao Xu wrote:
>>> @@ -380,10 +382,14 @@ static void io_wqe_dec_running(struct io_worker *worker)
>>>   	if (!(worker->flags & IO_WORKER_F_UP))
>>>   		return;
>>>   
>>> +	raw_spin_lock(&acct->lock);
>>>   	if (atomic_dec_and_test(&acct->nr_running) && io_acct_run_queue(acct)) {
>>> +		raw_spin_unlock(&acct->lock);
>>>   		atomic_inc(&acct->nr_running);
>>>   		atomic_inc(&wqe->wq->worker_refs);
>>>   		io_queue_worker_create(worker, acct, create_worker_cb);
>>> +	} else {
>>> +		raw_spin_unlock(&acct->lock);
>>>   	}
>>>   }
>>
>> I think this may be more readable as:
>>
>> static void io_wqe_dec_running(struct io_worker *worker)
>> 	__must_hold(wqe->lock)
>> {
>> 	struct io_wqe_acct *acct = io_wqe_get_acct(worker);
>> 	struct io_wqe *wqe = worker->wqe;
>>
>> 	if (!(worker->flags & IO_WORKER_F_UP))
>> 		return;
>> 	if (!atomic_dec_and_test(&acct->nr_running))
>> 		return;
>>
>> 	raw_spin_lock(&acct->lock);
>> 	if (!io_acct_run_queue(acct)) {
>> 		raw_spin_unlock(&acct->lock);
>> 		return;
>> 	}
>>
>> 	raw_spin_unlock(&acct->lock);
>> 	atomic_inc(&acct->nr_running);
>> 	atomic_inc(&wqe->wq->worker_refs);
>> 	io_queue_worker_create(worker, acct, create_worker_cb);
>> }
>>
>> ?
>>
>> Patch looks pretty sane to me, but there's a lot of lock shuffling going
>> on for it. Like in io_worker_handle_work(), and particularly in
>> io_worker_handle_work(). I think it'd be worthwhile to spend some time
>> to see if that could be improved. These days, lock contention is more
>> about frequency of lock grabbing rather than hold time. Maybe clean
>> nesting of wqe->lock -> acct->lock (which would be natural) can help
>> that?
> Sure, I'm working on reduce the lock contension further, will
> update it and send the whole patchset later.

Sounds good, thanks!

-- 
Jens Axboe