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

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Message-Id: <20120503220050.e91938418f882b4075526e08@gmail.com>
Date:	Thu, 3 May 2012 22:00:50 +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 14:29:10 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Thu, 2012-05-03 at 21:22 +0900, Takuya Yoshikawa wrote:
> > 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. 
> 
> Firstly, if you can hold a lock that long, it shouldn't be a spinlock,

I agree with you in principle, but isn't cond_resched_lock() there for that?

> secondly why isn't TIF_RESCHED being set if its running that long? That
> should still make cond_resched_lock() break.

I see.

I did some tests using spin_is_contended() and need_resched() and saw
that need_resched() was called as often as spin_is_contended(), so
experimentally I understand your point.

But as I could not see why spin_needbreak() was differently implemented
depending on CONFIG_PREEMPT, I wanted to understand the meaning.

Thanks,
	Takuya
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