linux-kernel - Re: [RFC] sched: make callers check lock contention for cond_resched

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Message-Id: <20120503212244.6abbfa8bc3f46a7f7a932bb7@gmail.com>
Date:	Thu, 3 May 2012 21:22:44 +0900
From:	Takuya Yoshikawa <takuya.yoshikawa@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...e.hu, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, kvm@...r.kernel.org, avi@...hat.com,
	mtosatti@...hat.com, yoshikawa.takuya@....ntt.co.jp
Subject: Re: [RFC] sched: make callers check lock contention for
 cond_resched_lock()

On Thu, 03 May 2012 10:35:27 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Thu, 2012-05-03 at 17:12 +0900, Takuya Yoshikawa wrote:
> > 
> > Although we can do that using spin_is_contended() and cond_resched(),
> > changing cond_resched_lock() to satisfy such a need is another option.
> > 
> Yeah, not a pretty patch. Changing all cond_resched_lock() sites just to
> change one and in such an ugly way too.
> 
> So what's the impact of making spin_needbreak() work for !PREEMPT?

Although the real use case is out of this RFC patch, we are now discussing
a case in which we may hold a spin_lock for long time, ms order, depending
on workload;  and in that case, other threads -- VCPU threads -- should be
given higher priority for that problematic lock.

I wanted to hear whether other users also have similar needs.  If so, it
may be worth making the API a bit more generic.

But I could not find a clean solution for that.  Do you think that using
spin_is_contended() directly is the way to go?

Thanks,
	Takuya
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