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

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Message-ID: <20150730154001.GM25159@twins.programming.kicks-ass.net>
Date:	Thu, 30 Jul 2015 17:40:01 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	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, rostedt@...dmis.org,
	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 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.
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