Re: [Xen-devel] [PATCH v3 4/6] xen: RCU: don't let a CPU with a callback go idle.

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 08/18/2017 07:04 PM, Dario Faggioli wrote:
> If a CPU has a callback queued, it must be ready to invoke
> it, as soon as all the other CPUs involved in the grace period
> has gone through a quiescent state.
> 
> But if we let such CPU go idle, we can't really tell when (if!)
> it will realize that it is actually time to invoke the callback.
> To solve this problem, a CPU that has a callback queued (and has
> already gone through a quiescent state itself) will stay online,
> until the grace period ends, and the callback can be invoked.
> 
> This is similar to what Linux does, and is the second and last
> step for fixing the overly long (or infinite!) grace periods.
> The problem, though, is that, within Linux, we have the tick,
> so, all that is necessary is to not stop the tick for the CPU
> (even if it has gone idle). In Xen, there's no tick, so we must
> avoid for the CPU to go idle entirely, and let it spin on
> rcu_pending(), consuming power and causing overhead.
> 
> In this commit, we implement the above, using rcu_needs_cpu(),
> in a way similar to how it is used in Linux. This it correct,
> useful and not wasteful for CPUs that participate in grace
> period, but have not a callback queued. For the ones that
> has callbacks, an optimization that avoids having to spin is
> introduced in a subsequent change.
> 
> Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

Acked-by: George Dunlap <george.dunlap@xxxxxxxxxx>

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