linux-kernel - Re: [PATCH RFC tip/core/rcu 11/11] rcu: move TREE

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Message-ID: <20110223191043.GU2163@linux.vnet.ibm.com>
Date:	Wed, 23 Feb 2011 11:10:43 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com,
	"Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH RFC tip/core/rcu 11/11] rcu: move TREE_RCU from softirq
 to kthread

On Wed, Feb 23, 2011 at 11:34:32AM -0600, Christoph Lameter wrote:
> On Wed, 23 Feb 2011, Mathieu Desnoyers wrote:
> 
> > > > > +
> > > > > +	local_irq_save(flags);
> > > > > +	cpu = smp_processor_id();
> 
> Drop this line.
> 
> > > > > +	if (per_cpu(rcu_cpu_kthread_task, cpu) == NULL) {
> 
> use this_cpu_read(rcu_cpu_kthread_task)
> 
> > > > > +		local_irq_restore(flags);
> > > > > +		return;
> > > > > +	}
> > > > > +	per_cpu(rcu_cpu_has_work, cpu) = 1;
> 
> this_cpu_write(rcu_cpu_has_work, 1);

I have made these changes, thank you!

These do introduce redundant preempt_disable()/preempt_enable() calls, but
this is not on a fastpath, so should be OK, and the improved readability
is certainly nice.  The read and the write do need to happen on the same
CPU, FWIW.

> > This is not quite true on x86_64 and s390 anymore. __get_cpu_var() now
> > uses a segment selector override to get the local CPU variable on x86.
> > See x86's percpu.h for details.
> 
> __get_cpu_var cannot use a segment override since there are places where
> the address of the variable is taken. One needs to use this_cpu_ops for
> that.

Thanks for the info!

							Thanx, Paul

> > > True, but we could also argue that the multiple checks for being preempt
> > > can also be an issue.
> >
> > At least on x86 preemption don't actually need to be disabled: selection
> > of the right per-cpu memory location is done atomically with the rest of
> > the instruction by the segment selector.
> 
> Right.
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