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Date: Thu, 4 Sep 2014 07:04:24 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>, Kautuk Consul <consul.kautuk@gmail.com>,
        Ingo Molnar <mingo@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
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        Ionut Alexa <ionut.m.alexa@gmail.com>,
        Guillaume Morin <guillaume@morinfr.org>, linux-kernel@vger.kernel.org,
        Kirill Tkhai <tkhai@yandex.ru>
Subject: Re: [PATCH 1/1] do_exit(): Solve possibility of BUG() due to race
 with try_to_wake_up()
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* Peter Zijlstra <peterz@infradead.org> wrote:

> > Ah, I simply do not know what is cheaper, even on x86. Well, 
> > we need to enable/disable irqs, but again I do not really 
> > know how much does this cost.
> 
> Ah good point about that IRQ thing, yes that's horribly 
> expensive.

Enabling/disabling local IRQs is not really expensive (it's a 
flat cost essentially - below 10 cycles on modern x86 CPUs) - 
especially if we consider the 100x-1000x frequency difference 
between schedule() and exit(), on typical systems:

  $ grep -E 'ctxt|processes' /proc/stat 
  ctxt 47166536
  processes 91876

And that's from a system that emphatically does not schedule 
much. On others the difference is much larger.

So please don't push complexity into the scheduler from 
lower-freq areas of the kernel!

Thanks,

	Ingo