linux-kernel - Re: [RFC][PATCH 0/3] locking/mutex: Rewrite basic mutex

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Message-ID: <20160823203417.GV10153@twins.programming.kicks-ass.net>
Date:   Tue, 23 Aug 2016 22:34:17 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Davidlohr Bueso <dave@...olabs.net>,
        Waiman Long <waiman.long@....com>,
        Jason Low <jason.low2@....com>,
        Ding Tianhong <dingtianhong@...wei.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <Will.Deacon@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Imre Deak <imre.deak@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Terry Rudd <terry.rudd@....com>,
        "Paul E. McKenney" <paulmck@...ibm.com>,
        Jason Low <jason.low2@...com>
Subject: Re: [RFC][PATCH 0/3] locking/mutex: Rewrite basic mutex

On Tue, Aug 23, 2016 at 02:53:07PM -0400, Linus Torvalds wrote:
> On Tue, Aug 23, 2016 at 12:17 PM, Davidlohr Bueso <dave@...olabs.net> wrote:
> >
> >> 46 files changed, 160 insertions(+), 1298 deletions(-)
> >
> > Oh my.
> 
> Yeah, that looks like a pretty compelling argument right there, if
> there isn't any other really major downside to this...
> 
> Peter, is there some downside that isn't obvious? Like "Well, this
> does regress performance because it now always does X"?

The biggest difference is the mutex fast paths, where they were a single
atomic and branch they're now a bit bigger. How much that matters in
practise is something that we'll have to benchmark a bit.

Esp. the mutex_lock() fast-path now also needs to load current, which
at least should be fairly hot but can still be a number of dependent
loads on some archs.