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

From: Paul E. McKenney
Date: Wed Oct 22 2008 - 17:03:13 EST

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


Thanx, Paul
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