linux-kernel - Re: Q: schedule() and implied barriers on arm64

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Message-ID: <20151019090405.GG3816@twins.programming.kicks-ass.net>
Date:	Mon, 19 Oct 2015 11:04:05 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Will Deacon <will.deacon@....com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Q: schedule() and implied barriers on arm64

On Mon, Oct 19, 2015 at 09:06:05AM +0200, Ingo Molnar wrote:
> 
> * Peter Zijlstra <peterz@...radead.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 :-)
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