linux-kernel - Re: [patch 2/3] scheduler: add full memory barriers upon task switch at runqueue lock/unlock

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Date:	Mon, 01 Feb 2010 12:13:12 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	akpm@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Nicholas Miell <nmiell@...cast.net>, laijs@...fujitsu.com,
	dipankar@...ibm.com, josh@...htriplett.org, dvhltc@...ibm.com,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	Valdis.Kletnieks@...edu, dhowells@...hat.com
Subject: Re: [patch 2/3] scheduler: add full memory barriers upon task
 switch at runqueue lock/unlock

On Mon, 2010-02-01 at 11:48 -0500, Mathieu Desnoyers wrote:

> What we have to be careful about here is that it's not enough to just
> rely on switch_mm() containing a memory barrier. What we really need to
> enforce is that switch_mm() issues memory barriers both _before_ and
> _after_ mm_cpumask modification. The "after" part is usually dealt with
> by the TLB context switch, but the "before" part usually isn't.

Then we add a smp_mb__before_clear_bit() in the switch_mm() on all archs
that do not have clear_bit imply a smp_mb().

> 
> > 
> > Btw, one reason to strongly prefer "switch_mm()" over any random context 
> > switch is that at least it won't affect inter-thread (kernel or user-land) 
> > switching, including switching to/from the idle thread.
> > 
> > So I'd be _much_ more open to a "let's guarantee that 'switch_mm()' always 
> > implies a memory barrier" model than to playing clever games with 
> > spinlocks.
> 
> If we really want to make this patch less intrusive, we can consider
> iterating on each online cpu in sys_membarrier() rather than on the
> mm_cpumask. But it comes at the cost of useless cache-line bouncing on
> large machines with few threads running in the process, as we would grab
> the rq locks one by one for all cpus.

I still think modifying the switch_mm() is better than the full
iteration.

-- Steve


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