Re: [PATCH 3/3] io_uring: tweak io_req_task_work_add

[Pavel Begunkov <asml.silence@gmail.com>]

On 6/30/21 11:11 PM, Jens Axboe wrote:
> On 6/30/21 4:10 PM, Pavel Begunkov wrote:
>> On 6/30/21 10:56 PM, Jens Axboe wrote:
>>> On 6/30/21 3:45 PM, Jens Axboe wrote:
>>>> On 6/30/21 3:38 PM, Pavel Begunkov wrote:
>>>>> On 6/30/21 10:22 PM, Jens Axboe wrote:
>>>>>> On 6/30/21 3:19 PM, Pavel Begunkov wrote:
>>>>>>> On 6/30/21 10:17 PM, Jens Axboe wrote:
>>>>>>>> On 6/30/21 2:54 PM, Pavel Begunkov wrote:
>>>>>>>>> Whenever possible we don't want to fallback a request. task_work_add()
>>>>>>>>> will be fine if the task is exiting, so don't check for PF_EXITING,
>>>>>>>>> there is anyway only a relatively small gap between setting the flag
>>>>>>>>> and doing the final task_work_run().
>>>>>>>>>
>>>>>>>>> Also add likely for the hot path.
>>>>>>>>
>>>>>>>> I'm not a huge fan of likely/unlikely, and in particular constructs like:
>>>>>>>>
>>>>>>>>> -	if (test_bit(0, &tctx->task_state) ||
>>>>>>>>> +	if (likely(test_bit(0, &tctx->task_state)) ||
>>>>>>>>>  	    test_and_set_bit(0, &tctx->task_state))
>>>>>>>>>  		return 0;
>>>>>>>>
>>>>>>>> where the state is combined. In any case, it should be a separate
>>>>>>>> change. If there's an "Also" paragraph in a patch, then that's also
>>>>>>>> usually a good clue that that particular change should've been
>>>>>>>> separate :-)
>>>>>>>
>>>>>>> Not sure what's wrong with likely above, but how about drop
>>>>>>> this one then?
>>>>>>
>>>>>> Yep I did - we can do the exiting change separately, the commit message
>>>>>
>>>>> I think 1-2 is good enough for 5.14, I'll just send it for-next
>>>>>
>>>>>> just needs to be clarified a bit on why it's ok to do now. And that
>>>>>
>>>>> It should have been ok to do before those 2 patches, but
>>>>> haven't tracked where it lost actuality.
>>>>
>>>> Right, I was thinking it was related to the swapping of the signal
>>>> exit and task work run ordering. But didn't look that far yet...
>>>
>>> BTW, in usual testing, even just the one hunk removing the exit check
>>> seems to result in quite a lot of memory leaks running
>>> test/poll-mshot-update. So something is funky with the patch.
>>
>> I guess you're positive that patches 1-2 have nothing to do
>> with that. Right?
> 
> I double checked, and seems fine with those two alone. Ran the test
> twice, saw massive amounts of leaks with patches 1-3, and none with
> patches 1-2 only.

I think there is a problem with the failing path of
io_req_task_work_add(), the removing back part. Will send a patch
tomorrow, but not able to test.

-- 
Pavel Begunkov