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Date: Mon, 19 Oct 2015 11:04:05 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
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        Oleg Nesterov <oleg@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: Q: schedule() and implied barriers on arm64
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On Mon, Oct 19, 2015 at 09:06:05AM +0200, Ingo Molnar wrote:
> 
> * Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > In any case, its all moot now, since Paul no longer requires schedule() to imply 
> > a full barrier.
> >
> > [...]
> 
> Nevertheless from a least-surprise POV it might be worth guaranteeing it, because 
> I bet there's tons of code that assumes that schedule() is a heavy operation and 
> it's such an easy mistake to make. Since we are so close to having that guarantee, 
> we might as well codify it?
> 
> Just like system calls are assumed to be barriers in general - 

Are they? I know they are on some platforms, but I'm not sure we've
audited them all and established this.

> and system calls are more lightweight than schedule() ...

Hopefully, although if you enable nohz_full there's a fair chance to
reverse that :-)