Re: [RFC PATCH] sched: Invoke io_wq_worker_sleeping() with enabled preemption

[Jens Axboe <axboe@kernel.dk>]

On 8/19/20 6:15 AM, peterz@infradead.org wrote:
> On Wed, Aug 19, 2020 at 02:37:58PM +0200, Sebastian Andrzej Siewior wrote:
> 
>> I don't see a significant reason why this lock should become a
>> raw_spinlock_t therefore I suggest to move it after the
>> tsk_is_pi_blocked() check.
> 
>> Any feedback on this vs raw_spinlock_t?
>>
>> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
>> ---
>>  fs/io-wq.c          |  8 ++++----
>>  kernel/sched/core.c | 10 +++++-----
>>  2 files changed, 9 insertions(+), 9 deletions(-)
>>
> 
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 3bbb60b97c73c..b76c0f27bd95e 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -4694,18 +4694,18 @@ static inline void sched_submit_work(struct task_struct *tsk)
>>  	 * in the possible wakeup of a kworker and because wq_worker_sleeping()
>>  	 * requires it.
>>  	 */
>> -	if (tsk->flags & (PF_WQ_WORKER | PF_IO_WORKER)) {
>> +	if (tsk->flags & PF_WQ_WORKER) {
>>  		preempt_disable();
>> -		if (tsk->flags & PF_WQ_WORKER)
>> -			wq_worker_sleeping(tsk);
>> -		else
>> -			io_wq_worker_sleeping(tsk);
>> +		wq_worker_sleeping(tsk);
>>  		preempt_enable_no_resched();
>>  	}
>>  
>>  	if (tsk_is_pi_blocked(tsk))
>>  		return;
>>  
>> +	if (tsk->flags & PF_IO_WORKER)
>> +		io_wq_worker_sleeping(tsk);
>> +
> 
> Urgh, so this adds a branch in what is normally considered a fairly hot
> path.
>
> 
> I'm thinking that the raw_spinlock_t option would permit leaving that
> single:
> 
> 	if (tsk->flags & (PF_WQ_WORKER | PF_IO_WORKER))
> 
> branch intact?

Yes, the raw spinlock would do it, and leave the single branch intact
in the hot path. I'd be fine with going that route for io-wq.


-- 
Jens Axboe