linux-kernel - Re: [PATCH v2] barriers: introduce smp_mb__release

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Message-ID: <20151009072931.GT3816@twins.programming.kicks-ass.net>
Date:	Fri, 9 Oct 2015 09:29:31 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Michael Ellerman <mpe@...erman.id.au>,
	Will Deacon <will.deacon@....com>, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org, Boqun Feng <boqun.feng@...il.com>,
	Anton Blanchard <anton@...ba.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2] barriers: introduce smp_mb__release_acquire and
 update documentation

On Thu, Oct 08, 2015 at 02:44:39PM -0700, Paul E. McKenney wrote:

> > > > I am with Peter -- we do need the benchmark results for PPC.
> > > 
> > > Urgh, sorry guys. I have been slowly doing some benchmarks, but time is not
> > > plentiful at the moment.
> > > 
> > > If we do a straight lwsync -> sync conversion for unlock it looks like that
> > > will cost us ~4.2% on Anton's standard context switch benchmark.
> > 
> > And that does not seem to agree with Paul's smp_mb__after_unlock_lock()
> > usage and would not be sufficient for the same (as of yet unexplained)
> > reason.
> > 
> > Why does it matter which of the LOCK or UNLOCK gets promoted to full
> > barrier on PPC in order to become RCsc?
> 
> You could do either.  However, as I understand it, there is hardware for
> which bc;isync is faster than lwsync.  For such hardware, it is cheaper
> to upgrade the unlock from lwsync to sync than to upgrade the lock from
> bc;isync to sync.  If I recall correctly, the kernel rewrites itself at
> boot to select whichever of lwsync or bc;isync is better for the hardware
> at hand.

Fair enough. I'll go wake up and think about the other issue ;-)
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