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

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Message-ID: <CAHk-=whK=Pb9QZhYkqFGUjK1Sas620qjO1Y_EzSBiHSXxJfTYQ@mail.gmail.com>
Date:   Fri, 22 Feb 2019 13:55:20 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Will Deacon <will.deacon@....com>
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 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?

                 Linus