linux-kernel - Re: [PATCH 1/1] do_exit(): Solve possibility of BUG() due to race with try_to_wake

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Date:	Thu, 4 Sep 2014 07:04:24 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Kautuk Consul <consul.kautuk@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>,
	David Rientjes <rientjes@...gle.com>,
	Ionut Alexa <ionut.m.alexa@...il.com>,
	Guillaume Morin <guillaume@...infr.org>,
	linux-kernel@...r.kernel.org, Kirill Tkhai <tkhai@...dex.ru>
Subject: Re: [PATCH 1/1] do_exit(): Solve possibility of BUG() due to race
 with try_to_wake_up()


* Peter Zijlstra <peterz@...radead.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
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