Re: [PATCH] io-wq: check max_worker limits if a worker transitions bound state

[Jens Axboe <axboe@kernel.dk>]

On 8/29/21 9:06 PM, Hao Xu wrote:
> 在 2021/8/30 上午6:19, Jens Axboe 写道:
>> For the two places where new workers are created, we diligently check if
>> we are allowed to create a new worker. If we're currently at the limit
>> of how many workers of a given type we can have, then we don't create
>> any new ones.
>>
>> If you have a mixed workload with various types of bound and unbounded
>> work, then it can happen that a worker finishes one type of work and
>> is then transitioned to the other type. For this case, we don't check
>> if we are actually allowed to do so. This can cause io-wq to temporarily
>> exceed the allowed number of workers for a given type.
>>
>> When retrieving work, check that the types match. If they don't, check
>> if we are allowed to transition to the other type. If not, then don't
>> handle the new work.
>>
>> Cc: stable@vger.kernel.org
>> Reported-by: Johannes Lundberg <johalun0@gmail.com>
>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>
>> ---
>>
>> diff --git a/fs/io-wq.c b/fs/io-wq.c
>> index 4b5fc621ab39..dced22288983 100644
>> --- a/fs/io-wq.c
>> +++ b/fs/io-wq.c
>> @@ -424,7 +424,31 @@ static void io_wait_on_hash(struct io_wqe *wqe, unsigned int hash)
>>   	spin_unlock(&wq->hash->wait.lock);
>>   }
>>   
>> -static struct io_wq_work *io_get_next_work(struct io_wqe *wqe)
>> +/*
>> + * We can always run the work if the worker is currently the same type as
>> + * the work (eg both are bound, or both are unbound). If they are not the
>> + * same, only allow it if incrementing the worker count would be allowed.
>> + */
>> +static bool io_worker_can_run_work(struct io_worker *worker,
>> +				   struct io_wq_work *work)
>> +{
>> +	struct io_wqe_acct *acct;
>> +
>> +	if ((worker->flags & IO_WORKER_F_BOUND) &&
>> +	    !(work->flags & IO_WQ_WORK_UNBOUND))
>> +		return true;
>> +	else if (!(worker->flags & IO_WORKER_F_BOUND) &&
>> +		 (work->flags & IO_WQ_WORK_UNBOUND))
>> +		return true;
> 
> How about:
> bool a = !(worker->flags & IO_WORKER_F_BOUND);
> bool b = !(work->flags & IO_WQ_WORK_UNBOUND);
> 
> if (a != b)
>      return true;

Yeah good point, I'll change it to be:

if (!(worker->flags & IO_WORKER_F_BOUND) !=                             
    !(work->flags & IO_WQ_WORK_UNBOUND))                                
         return 1;                                    

return acct->nr_workers < acct->max_workers;

-- 
Jens Axboe