Re: [PATCH v2 1/2] sched/wait: Add wait_threshold

[Peter Zijlstra <peterz@infradead.org>]

A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?
A: Top-posting.
Q: What is the most annoying thing in e-mail?

On Mon, Sep 23, 2019 at 07:37:46PM +0300, Pavel Begunkov wrote:
> Just in case duplicating a mail from the cover-letter thread:

Just because every patch should have a self contained and coherent
Changelog.

> 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.

I don't think that is another topic at all. It is a quality of
implementation issue. We already have too many copies of all that (3).

So basically you want to fudge the wake function to do the/a @cond test,
not unlike what wait_bit already does, but differenly.

I'm really rather annoyed with C for not having proper lambda functions;
that would make all this so much easier. Anyway, let me have a poke at
this in the morning, it's late already.

Also, is anything actually using wait_queue_entry::private ? I'm
not finding any in a hurry.