Date: Thu, 26 May 2011 13:31:37 +0100
From: Russell King - ARM Linux <linux@arm.linux.org.uk>
To: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <peterz@infradead.org>, Marc Zyngier <Marc.Zyngier@arm.com>,
        Frank Rowand <frank.rowand@am.sony.com>,
        Oleg Nesterov <oleg@redhat.com>, linux-kernel@vger.kernel.org,
        Yong Zhang <yong.zhang0@gmail.com>,
        linux-arm-kernel@lists.infradead.org
Subject: Re: [BUG] "sched: Remove rq->lock from the first half of ttwu()"
	locks up on ARM
Message-ID: <20110526123137.GG24876@n2100.arm.linux.org.uk>
References: <1306260792.27474.133.camel@e102391-lin.cambridge.arm.com> <1306272750.2497.79.camel@laptop> <1306343335.21578.65.camel@twins> <1306358128.21578.107.camel@twins> <BANLkTi=XbTXQsu3jUEvQyCfBy6-aRnqSpw@mail.gmail.com> <1306405979.1200.63.camel@twins> <1306407759.27474.207.camel@e102391-lin.cambridge.arm.com> <1306409575.1200.71.camel@twins> <1306412511.1200.90.camel@twins> <20110526122623.GA11875@elte.hu>
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On Thu, May 26, 2011 at 02:26:23PM +0200, Ingo Molnar wrote:
> 
> * Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > Sort this by reverting to the old behaviour for this situation and 
> > perform a full remote wake-up.
> 
> Btw., ARM should consider switching most of its subarchitectures to 
> !__ARCH_WANT_INTERRUPTS_ON_CTXSW - enabling irqs during context 
> switches is silly and now expensive as well.

Not going to happen.  The reason we do it is because most of the CPUs
have to (slowly) flush their caches during switch_mm(), and to have
IRQs off over the cache flush means that we lose IRQs.

So it's not silly at all, bit a technical requirement imposed by the
cache architecture.

If it's become expensive through development, it suggests that the
development did not take account of the issues we have on ARM.
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