Re: [Xen-devel] Re: [PATCH] CPUIDLE: shorten hpet spin_lock holding time

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 22/04/2010 08:22, "Keir Fraser" <keir.fraser@xxxxxxxxxxxxx> wrote:

>> If a CPU cleared itself from cpuidle_mwait_flags, then this CPU didn't need a
>> IPI to be waken. And one useless write to softirq_pending doesn't have any
>> side effect. So this case should be acceptable.
> 
> That's not totally convincing. The write to softirq_pending has one extra
> side effect: it is possible that the next time TIMER_SOFTIRQ really needs to
> be raised on that CPU, it will not receive notification via IPI, because the
> flag is already set in its softirq_pending mask.
> 
> Hm, let me see if I can come up with a patch for this and post it for you.

How about the attached patch? It MWAITs on a completely new flag field,
avoiding the IPI-avoidance semantics of softirq_pending. It also folds in
your patch. It also does wakeup-waiting checks on timer_deadline_start, that
being the field that initiates wakeup via the MONITORed memory region.

 -- Keir

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