linux-kernel - Re: [PATCH, RFC] v7 scalable classic RCU implementation

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Date:	Wed, 22 Oct 2008 14:02:54 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Manfred Spraul <manfred@...orfullife.com>
Cc:	linux-kernel@...r.kernel.org, cl@...ux-foundation.org,
	mingo@...e.hu, akpm@...ux-foundation.org, dipankar@...ibm.com,
	josht@...ux.vnet.ibm.com, schamp@....com, niv@...ibm.com,
	dvhltc@...ibm.com, ego@...ibm.com, laijs@...fujitsu.com,
	rostedt@...dmis.org, peterz@...radead.org, penberg@...helsinki.fi,
	andi@...stfloor.org, tglx@...utronix.de
Subject: Re: [PATCH, RFC] v7 scalable classic RCU implementation

On Wed, Oct 22, 2008 at 08:41:11PM +0200, Manfred Spraul wrote:
> Paul E. McKenney wrote:
>> Only once per such CPU every grace period -- seems in the noise to me.
>> But I should revisit, as I have changed things quite a bit since I
>> made that decision many weeks ago.  ;-)
>>
>>   
> Another small point:
> Does your implementation support rcu_check_callbacks() with cpu != 
> smp_processor_id()?
> I don't think my locking would support it properly.
> Thus:
> - cpu != smp_processor_id() doesn't work.
> - stack space for a useless parameter.
> - the explicit cpu parameter prevents the rcu code from using 
> get_cpu_var().
>
> What about modifying the rcu_check_callbacks() prototype? I'd propose to 
> remove the cpu parameter.

That would work fine for rcutree.c.  If I were to invoke
rcu_check_callbacks() remotely, I would use something like
smp_call_function() to make it happen.

Hmmm...  Looks like rcu_pending is also always called with its cpu
parameter set to the current CPU, and same for rcu_needs_cpu().
And given that all the external uses of rcu_check_callbacks() are
of the following form:

	if (rcu_pending(cpu))
		rcu_check_callbacks(cpu, whatever);


perhaps rcu_pending() should be an internal-to-RCU API invoked from
rcu_check_callbacks().

Thoughts?

							Thanx, Paul
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