linux-kernel - Re: [PATCH tip/core/rcu 19/19] rcu: Add fastpath bypassing funnel locking

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Message-ID: <20150803200646.GF25159@twins.programming.kicks-ass.net>
Date:	Mon, 3 Aug 2015 22:06:46 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, mingo@...nel.org,
	jiangshanlai@...il.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, dhowells@...hat.com,
	edumazet@...gle.com, dvhart@...ux.intel.com, fweisbec@...il.com,
	oleg@...hat.com, bobby.prani@...il.com, dave@...olabs.net,
	waiman.long@...com
Subject: Re: [PATCH tip/core/rcu 19/19] rcu: Add fastpath bypassing funnel
 locking

On Mon, Aug 03, 2015 at 04:05:33PM -0400, Steven Rostedt wrote:
> On Thu, 30 Jul 2015 17:40:01 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > On Thu, Jul 30, 2015 at 08:34:52AM -0700, Paul E. McKenney wrote:
> > > > If the extra read before the cmpxchg() does not hurt, we should do the
> > > > same for mutex and make the above redundant.
> > > 
> > > I am pretty sure that different hardware wants it done differently.  :-/
> > > So I agree that hard data would be good.
> > > 
> > > I could probably further optimize the RCU code by checking for a
> > > single-node tree, but I am not convinced that this is worthwhile.
> > > However, skipping three cache misses in the uncontended case is
> > > definitely worthwhile, hence this patch.  ;-)
> > 
> > I was mostly talking about the !mutex_is_locked() && mutex_try_lock()
> > thing. The fast path thing makes sense.
> 
> Note, mutex does do this for the optimistic spin. See
> mutex_try_to_aquire().

Right but that's mutex_lock(). mutex_trylock() does not.
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