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

From: Peter Zijlstra <peterz@infradead.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: linux-kernel@vger.kernel.org, mingo@kernel.org,
	jiangshanlai@gmail.com, dipankar@in.ibm.com,
	akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
	josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org,
	dhowells@redhat.com, edumazet@google.com, dvhart@linux.intel.com,
	fweisbec@gmail.com, oleg@redhat.com, bobby.prani@gmail.com
Subject: Re: [PATCH tip/core/rcu 04/18] rcu: Use single-stage IPI algorithm for RCU expedited grace period
Date: Thu, 8 Oct 2015 11:00:12 +0200	[thread overview]
Message-ID: <20151008090012.GH3604@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <20151007161445.GN3910@linux.vnet.ibm.com>

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.