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

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Date:	Wed, 23 Feb 2011 11:34:32 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
cc:	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.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, 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);

> 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.


> > 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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