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Subject: Re: [PATCH v2 0/2] Optimise io_uring completion waiting
To:     Peter Zijlstra <peterz@infradead.org>,
        Pavel Begunkov <asml.silence@gmail.com>
Cc:     Ingo Molnar <mingo@kernel.org>, Ingo Molnar <mingo@redhat.com>,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
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From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <b66cc383-ceca-b7dd-b3a3-eb998e865cea@kernel.dk>
Date:   Tue, 24 Sep 2019 14:57:01 +0200
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On 9/24/19 5:43 AM, Peter Zijlstra wrote:
> On Tue, Sep 24, 2019 at 02:11:29PM +0300, Pavel Begunkov wrote:
> 
>> @@ -2717,15 +2757,18 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
>>   			return ret;
>>   	}
>>   
>> +	iowq.nr_timeouts = atomic_read(&ctx->cq_timeouts);
>> +	prepare_to_wait_exclusive(&ctx->wait, &iowq.wq, TASK_INTERRUPTIBLE);
>> +	do {
>> +		if (io_should_wake(&iowq))
>> +			break;
>> +		schedule();
>> +		if (signal_pending(current))
>> +			break;
>> +		set_current_state(TASK_INTERRUPTIBLE);
>> +	} while (1);
>> +	finish_wait(&ctx->wait, &iowq.wq);
> 
> It it likely OK, but for paranoia, I'd prefer this form:
> 
> 	for (;;) {
> 		prepare_to_wait_exclusive(&ctx->wait, &iowq.wq, TASK_INTERRUPTIBLE);
> 		if (io_should_wake(&iowq))
> 			break;
> 		schedule();
> 		if (signal_pending(current))
> 			break;
> 	}
> 	finish_wait(&ctx->wait, &iowq.wq);
> 
> The thing is, if we ever succeed with io_wake_function() (that CPU
> observes io_should_wake()), but when waking here, we do not observe
> is_wake_function() and go sleep again, we might never wake up if we
> don't put ourselves back on the wait-list again.

Might be paranoia indeed, but especially after this change, we don't
expect to make frequent roundtrips there. So better safe than sorry,
I'll make the change.

-- 
Jens Axboe