linux-kernel - Re: runqueue locks in schedule()

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Message-Id: <200801181928.43249.nickpiggin@yahoo.com.au>
Date:	Fri, 18 Jan 2008 19:28:42 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	"stephane eranian" <eranian@...glemail.com>
Cc:	"Peter Zijlstra" <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, ia64 <linux-ia64@...r.kernel.org>,
	"Stephane Eranian" <eranian@...il.com>,
	"Corey J Ashford" <cjashfor@...ibm.com>,
	"Ingo Molnar" <mingo@...e.hu>
Subject: Re: runqueue locks in schedule()

On Friday 18 January 2008 17:33, stephane eranian wrote:
> Nick,

> > It is arch specific. If an architecture wants interrupts on during
> > context switch, or runqueue unlocked, then they set it (btw
> > INTERRUPTS_ON_CTXSW also implies UNLOCKED_CTXSW).
>
> Yes , I noticed that. I am only interested in UNLOCKED_CTXSW.
> But it appears that the approach suggested my Peter does work. We are
> running some tests.

OK, that might be OK.


> > Although, eg on x86, you would hold off interrupts and runqueue lock for
> > slightly less time if you defined those, it results in _slightly_ more
> > complicated context switching... although I did once find a workload
> > where the reduced runqueue contention improved throughput a bit, it is
> > not much problem in general to hold the lock.
>
> By complicated you mean that now you'd have to make sure you don't
> need to access runqueue data?

Well, not speaking about the arch-specific code (which may involve
more complexities), but the core scheduler needs the
task_struct->oncpu variable wheras that isn't required if the
runqueue is locked while switching tasks.
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