linux-kernel - Re: wake_wide mechanism clarification

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Message-ID: <CAJWu+ookYPvXrEF=bDW_aui30NV91hMBawy=+GZn_xEiriU0Ag@mail.gmail.com>
Date:   Sat, 29 Jul 2017 15:41:56 -0700
From:   Joel Fernandes <joelaf@...gle.com>
To:     Mike Galbraith <umgwanakikbuti@...il.com>
Cc:     Josef Bacik <josef@...icpanda.com>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Juri Lelli <Juri.Lelli@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Brendan Jackman <brendan.jackman@....com>,
        Chris Redpath <Chris.Redpath@....com>,
        Michael Wang <wangyun@...ux.vnet.ibm.com>,
        Matt Fleming <matt@...eblueprint.co.uk>
Subject: Re: wake_wide mechanism clarification

On Sat, Jul 29, 2017 at 3:28 PM, Joel Fernandes <joelaf@...gle.com> 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 :)

thanks,

-Joel