linux-kernel - Re: sched: fix/optimise some issues

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Date:	Thu, 21 Jul 2011 17:08:15 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	stephan.baerwolf@...ilmenau.de
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	ncrao@...gle.com
Subject: Re: sched: fix/optimise some issues

On Wed, 2011-07-20 at 15:42 +0200, Stephan Bärwolf wrote:
> 
> I also implemented an 128bit vruntime support:
> Majorly on systems with many tasks and (for example) deep cgroups
> (or increased NICE0_LOAD/ SCHED_LOAD_SCALE as in commit
> c8b281161dfa4bb5d5be63fb036ce19347b88c63), a weighted timeslice
> (unsigned long) can become very large (on x86_64) and consumes a
> large part of the u64 vruntimes (per tick) when added.
> This might lead to missscheduling because of overflows. 

Right, so I've often wanted a [us]128 type, and gcc has some (broken?)
support for that, but overhead has always kept me from it.

There's also the non-atomicy thing to consider, see min_vruntime_copy
etc.

How horrid is the current vruntime situation?

As to your true-idle, there's a very good reason the current SCHED_IDLE
isnt' a true idle scheduler; it would create horrid priority inversion
problems, imagine the true idle task holding a mutex or is required to
complete something.

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