linux-kernel - Re: [RFC PATCH 01/20] asm-generic/mmiowb: Add generic implementation of mmiowb() tracking

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Date:   Tue, 26 Feb 2019 18:25:39 +0000
From:   Will Deacon <will.deacon@....com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     linux-arch <linux-arch@...r.kernel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Arnd Bergmann <arnd@...db.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Palmer Dabbelt <palmer@...ive.com>,
        Daniel Lustig <dlustig@...dia.com>,
        David Howells <dhowells@...hat.com>,
        Alan Stern <stern@...land.harvard.edu>,
        "Maciej W. Rozycki" <macro@...ux-mips.org>,
        Paul Burton <paul.burton@...s.com>,
        Ingo Molnar <mingo@...nel.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>, Tony Luck <tony.luck@...el.com>
Subject: Re: [RFC PATCH 01/20] asm-generic/mmiowb: Add generic implementation
 of mmiowb() tracking

On Fri, Feb 22, 2019 at 01:55:20PM -0800, Linus Torvalds wrote:
> On Fri, Feb 22, 2019 at 1:49 PM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > The case we want to go fast is the spin-lock and unlock case, not the
> > "set pending" case.
> >
> > And the way you implemented this, it's exactly the wrong way around.
> 
> Oh, one more comment: couldn't we make that mmiowb flag be right next
> to the preemption count?
> 
> Because that's the common case anyway, where a spinlock increments the
> preemption count too. If we put the mmiowb state in the same
> cacheline, we don't cause extra cache effects, which is what really
> matters, I guess.
> 
> I realize this is somewhat inconvenient, because some architectures
> put preempt count in the thread structure, and others do it as a
> percpu variable. But maybe the architecture could just declare where
> the mmiowb state is?

I think that should be doable... I'll have a play.

Will