linux-kernel - Re: [PATCH tip/core/rcu 04/18] rcu: Use single-stage IPI algorithm for RCU expedited grace period

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Message-ID: <20151008090012.GH3604@twins.programming.kicks-ass.net>
Date:	Thu, 8 Oct 2015 11:00:12 +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
Subject: Re: [PATCH tip/core/rcu 04/18] rcu: Use single-stage IPI algorithm
 for RCU expedited grace period

On Wed, Oct 07, 2015 at 09:14:45AM -0700, Paul E. McKenney wrote:
> On Wed, Oct 07, 2015 at 03:49:29PM +0200, Peter Zijlstra wrote:
> > On Wed, Oct 07, 2015 at 03:43:11PM +0200, Peter Zijlstra wrote:
> > > On Tue, Oct 06, 2015 at 09:29:23AM -0700, Paul E. McKenney wrote:
> > > > @@ -167,42 +307,18 @@ static void rcu_preempt_note_context_switch(void)
> > > 
> > > > -		raw_spin_unlock_irqrestore(&rnp->lock, flags);
> > > 
> > > This again reminds me that we should move rcu_note_context_switch()
> > > under the IRQ disable section the scheduler already has or remove the
> > > IRQ disable from rcu_note_context_switch().
> > 
> > Ah, this is the unlikely path where we actually need to do work. The
> > normal fast paths no longer require IRQs disabled, you already fixed
> > that last time, good!
> 
> But why not make the less-likely paths a bit cheaper while we are at it?  ;-)

True, less critical, but if we're almost there we might as well fix that
up too.
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