Re: [PATCH 4/6] io_uring: switch normal task_work to a mpscq

[Jens Axboe <axboe@kernel.dk>]

On 6/15/26 2:40 PM, Caleb Sander Mateos wrote:
>> @@ -34,10 +34,6 @@ void io_tctx_fallback_work(struct work_struct *work)
>>                                                   fallback_work);
>>         unsigned int count = 0;
>>
>> -       /* see tctx_task_work() - a set bit must always have a run coming */
>> -       clear_bit(0, &tctx->tw_pending);
>> -       smp_mb__after_atomic();
>> -
>>         /*
>>          * Run the entries directly. We're in PF_KTHRED context, hence
>>          * io_should_terminate_tw() is true and they will be marked as
>> @@ -101,6 +97,13 @@ void tctx_task_work_run(struct io_uring_task *tctx, unsigned int max_entries,
>>                                 io_poll_task_func, io_req_rw_complete,
>>                                 (struct io_tw_req){req}, ts);
>>                 (*count)++;
>> +               /*
>> +                * Break if most recent pop emptied the queue. This helps
>> +                * bound task_work run, and also protects the regular
>> +                * task_work addition.
>> +                */
>> +               if (mpscq_pop_emptied(&tctx->task_list, tctx->task_head))
>> +                       break;
> 
> I think we can now remove the "if (mpscq_empty(&tctx->task_list))
> break;" above? The queue must be nonempty initially, otherwise the
> task work wouldn't have been scheduled. And if the queue is empty
> after an attempted pop, the previous iteration of this loop must have
> successfully marked the queue as empty.

We could, but then we'd need to special case the SQPOLL side. I think
it's better if we just leave it somewhat defensive as-is, it's just a
single compare anyway, non-atomic.

-- 
Jens Axboe