linux-kernel - Re: [PATCH v2 1/2] sched/wait: Add wait

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Message-ID: <3e359937-5b19-8a4c-4243-ba2edff68504@gmail.com>
Date:   Mon, 23 Sep 2019 19:37:46 +0300
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Jens Axboe <axboe@...nel.dk>, Ingo Molnar <mingo@...hat.com>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] sched/wait: Add wait_threshold

Just in case duplicating a mail from the cover-letter thread:

It could be done with @cond indeed, that's how it works for now.
However, this addresses performance issues only.

The problem with wait_event_*() is that, if we have a counter and are
trying to wake up tasks after each increment, it would schedule each
waiting task O(threshold) times just for it to spuriously check @cond
and go back to sleep. All that overhead (memory barriers, registers
save/load, accounting, etc) turned out to be enough for some workloads
to slow down the system.

With this specialisation it still traverses a wait list and makes
indirect calls to the checker callback, but the list supposedly is
fairly  small, so performance there shouldn't be a problem, at least for
now.

Regarding semantics; It should wake a task when a value passed to
wake_up_threshold() is greater or equal then a task's threshold, that is
specified individually for each task in wait_threshold_*().

In pseudo code:
```
def wake_up_threshold(n, wait_queue):
	for waiter in wait_queue:
		waiter.wake_up_if(n >= waiter.threshold);
```

Any thoughts how to do it better? Ideas are very welcome.

BTW, this monster is mostly a copy-paste from wait_event_*(),
wait_bit_*(). We could try to extract some common parts from these
three, but that's another topic.

On 23/09/2019 10:19, Peter Zijlstra wrote:
> On Sun, Sep 22, 2019 at 11:08:50AM +0300, Pavel Begunkov (Silence) wrote:
>> From: Pavel Begunkov <asml.silence@...il.com>
>>
>> Add wait_threshold -- a custom wait_event derivative, that waits until
>> a value is equal to or greater than the specified threshold.
> 
> This is quite insufficient justification for this monster... what exact
> semantics do you want?
> 
> Why can't you do this exact same with a slightly more complicated @cond
> ?
> 

-- 
Yours sincerely,
Pavel Begunkov

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