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

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Message-ID: <20151019162423.GP3816@twins.programming.kicks-ass.net>
Date:	Mon, 19 Oct 2015 18:24:23 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	"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 04:21:08PM +0100, Catalin Marinas wrote:
> 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?
> 
> FWIW, the arm64 __switch_to() has a heavy barrier (DSB) but the reason
> for this was to cope with potentially interrupted cache or TLB
> maintenance (which require a DSB on the same CPU) and thread migration
> to another CPU.

Right, but there's a path through schedule() that does not pass through
__switch_to(); when we pick the current task as the most eligible task
and next == prev.

In that case there really only is the wmb, a spin lock, an atomic op and
a spin unlock (and a whole bunch of 'normal' code of course).
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