linux-kernel - Re: CFS scheduler unfairly prefers pinned tasks

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Message-Id: <201510061006.t96A69I8007672@como.maths.usyd.edu.au>
Date:	Tue, 6 Oct 2015 21:06:09 +1100
From:	paul.szabo@...ney.edu.au
To:	umgwanakikbuti@...il.com
Cc:	linux-kernel@...r.kernel.org
Subject: Re: CFS scheduler unfairly prefers pinned tasks

Dear Mike,

>> .. CFS ... unfairly gives more CPU time to [pinned] tasks ...
>
> If they can all migrate, load balancing can move any of them to try to
> fix the permanent imbalance, so they'll all bounce about sharing a CPU
> with some other hog, and it all kinda sorta works out.
>
> When most are pinned, to make it work out long term you'd have to be
> short term unfair, walking the unpinned minority around the box in a
> carefully orchestrated dance... and have omniscient powers that assure
> that none of the tasks you're trying to equalize is gonna do something
> rude like leave, sleep, fork or whatever, and muck up the grand plan.

Could not your argument be turned around: for a pinned task it is harder
to find an idle CPU, so they should get less time?

But really... those pinned tasks do not hog the CPU forever. Whatever
kicks them off: could not that be done just a little earlier?

And further... the CFS is meant to be fair, using things like vruntime
to preempt, and throttling. Why are those pinned tasks not preempted or
throttled?

Thanks, Paul

Paul Szabo   psz@...hs.usyd.edu.au   http://www.maths.usyd.edu.au/u/psz/
School of Mathematics and Statistics   University of Sydney    Australia
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