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Re: [Xen-devel] Re: [PATCH] xen: always handle VIRQ_TIMER first.

To: Jan Beulich <JBeulich@xxxxxxxxxx>
Subject: Re: [Xen-devel] Re: [PATCH] xen: always handle VIRQ_TIMER first.
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Mon, 18 Oct 2010 09:52:05 -0700
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, Keir Fraser <keir@xxxxxxx>, Ian Campbell <ian.campbell@xxxxxxxxxx>
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 On 10/18/2010 06:20 AM, Jan Beulich wrote:
>>>> On 17.10.10 at 08:11, Jeremy Fitzhardinge <jeremy@xxxxxxxx> wrote:
>> I think (but I haven't spelunked into that code lately) that after a
>> tickless idle period it will update jiffies N ticks based on the
>> clocksource, and then run any other interrupt handler code, so jiffies
>> will always appear to be up to date.
>> Ah, yes, here it is:
>> /**
>>  * tick_nohz_update_jiffies - update jiffies when idle was interrupted
>>  *
>>  * Called from interrupt entry when the CPU was idle
>>  *
>>  * In case the sched_tick was stopped on this CPU, we have to check if 
>> jiffies
>>  * must be updated. Otherwise an interrupt handler could use a stale jiffy
>>  * value. We do this unconditionally on any cpu, as we don't know whether 
>> the
>>  * cpu, which has the update task assigned is in a long sleep.
>>  */
>> static void tick_nohz_update_jiffies(ktime_t now)
>> {
>>      ...
>> }
> But this is available only with CONFIG_NO_HZ, which is a freely
> selectable option. So perhaps the code should still be added
> inside an #ifndef CONFIG_NO_HZ?

Non-NO_HZ is a pretty pessimal configuration for a VM, or indeed any
system which cares about power.  Are there any use cases for which its a
good idea?

However, you could change it to always update jiffies on any interrupt
entrypoint, regardless of whether its coming from an idle state.  Or
even just change "jiffies" into a macro which calls a function to just
compute the current value without needing to rely on interrupts at all.


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