Re: wake_wide mechanism clarification

[Joel Fernandes]

Hi Josef,

On Mon, Jul 31, 2017 at 5:21 AM, Josef Bacik <josef@xxxxxxxxxxxxxx> wrote:
> On Sat, Jul 29, 2017 at 03:41:56PM -0700, Joel Fernandes wrote:
>> On Sat, Jul 29, 2017 at 3:28 PM, Joel Fernandes <joelaf@xxxxxxxxxx> wrote:
>> <snip>
>> >>>> Again I didn't follow why the second condition couldn't just be:
>> >>>> waker->nr_wakee_switch > factor, or, (waker->nr_wakee_switch +
>> >>>> wakee->nr_wakee_switch) > factor, based on the above explanation from
>> >>>> Micheal Wang that I quoted.
>> >>>> and why he's instead doing the whole multiplication thing there that I
>> >>>> was talking about earlier: "factor * wakee->nr_wakee_switch".
>> >>>>
>> >>>> Rephrasing my question in another way, why are we talking the ratio of
>> >>>> master/slave instead of the sum when comparing if its > factor? I am
>> >>>> surely missing something here.
>> >>>
>> >>> Because the heuristic tries to not demolish 1:1 buddies.  Big partner
>> >>> flip delta means the pair are unlikely to be a communicating pair,
>> >>> perhaps at high frequency where misses hurt like hell.
>> >>
>> >> But it does seem to me to demolish the N:N communicating pairs from a
>> >> latency/load balancing standpoint. For he case of N readers and N
>> >> writers, the ratio (master/slave) comes down to 1:1 and we wake
>> >> affine. Hopefully I didn't miss something too obvious about that.
>> >
>> > I think wake_affine() should correctly handle the case (of
>> > overloading) I bring up here where wake_wide() is too conservative and
>> > does affine a lot, (I don't have any data for this though, this just
>> > from code reading), so I take this comment back for this reason.
>>
>> aargh, nope :( it still runs select_idle_sibling although on the
>> previous CPU even if want_affine is 0 (and doesn't do the wider
>> wakeup..), so the comment still applies.. its easy to get lost into
>> the code with so many if statements :-\  sorry about the noise :)
>>
>
> I've been working in this area recently because of a cpu imbalance problem.
> Wake_wide() definitely makes it so we're waking affine way too often, but I
> think messing with wake_waide to solve that problem is the wrong solution.  This
> is just a heuristic to see if we should wake affine, the simpler the better.  I
> solved the problem of waking affine too often like this
>
> https://marc.info/?l=linux-kernel&m=150003849602535&w=2

Thanks! Cool!

>
> So why do you care about wake_wide() anyway?  Are you observing some problem
> that you suspect is affected by the affine wakeup stuff?  Or are you just trying

I am dealing with an affine wake up issue, yes.

> to understand what is going on for fun?  Cause if you are just doing this for
> fun you are a very strange person, thanks,

Its not just for fun :) Let me give you some background about me, I
work in the Android team and one of the things I want to do is to take
an out of tree patch that's been carried for some time and post a more
upstreamable solution - this is related to wake ups from the binder
driver which does sync wake ups (WF_SYNC). I can't find the exact out
of tree patch publicly since it wasn't posted to a list, but the code
is here [1]. What's worse is I have recently found really bad issues
with this patch itself where runnable times are increased. I should
have provided this background earlier (sorry that I didn't, my plan
was to trigger a separate discussion about the binder sync wake up
thing as a part of a patch/proposal I want to post - which I plan to
do so). Anyway, as a part of this effort, I want to understand
wake_wide() better and "respect" it since it sits in the wake up path
and I wanted to my proposal to work well with it, especially since I
want to solve this problem in an upstream-friendly way.

The other reason to trigger the discussion, is, I have seen
wake_wide() enough number of times and asked enough number of folks
how it works that it seems sensible to ask about it here (I was also
suggested to ask about wake_wide on LKML because since few people
seemingly understand how it works) and hopefully now its a bit better
understood.

I agree with you that instead of spending insane amounts of time on
wake_wide itself, its better to directly work on a problem and collect
some data - which is also what I'm doing, but I still thought its
worth doing some digging into wake_wide() during some free time I had,
thanks.

Cheers,

-Joel

[1] https://android.googlesource.com/kernel/msm.git/+/377e6e28b6097b3d6de7245d3d3def45fc8c9ffc/kernel/sched/fair.c#5492