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

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Date:	Thu, 03 May 2012 15:47:26 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Takuya Yoshikawa <takuya.yoshikawa@...il.com>, mingo@...e.hu,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	kvm@...r.kernel.org, mtosatti@...hat.com,
	yoshikawa.takuya@....ntt.co.jp
Subject: Re: [RFC] sched: make callers check lock contention for cond_resched_lock()

On 05/03/2012 03:29 PM, Peter Zijlstra 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,

In fact with your mm preemptibility work it can be made into a mutex, if
the entire mmu notifier path can be done in task context.  However it
ends up a strange mutex - you can sleep while holding it but you may not
allocate, because you might recurse into an mmu notifier again.

Most uses of the lock only involve tweaking some bits though.

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


-- 
error compiling committee.c: too many arguments to function

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