Subject: Re: powerpc: mitigate impact of decrementer reset

From: Paul Clarke <[email-masked]>
Date: Wed, 05 Nov 2014 11:06:39 -0600

Sorry it took me so long to get back to this...

On 10/07/2014 09:52 PM, Michael Ellerman wrote:
> On Tue, 2014-07-10 at 19:13:24 UTC, Paul Clarke wrote:
>> The POWER ISA defines an always-running decrementer which can be used
>> to schedule interrupts after a certain time interval has elapsed.
>> The decrementer counts down at the same frequency as the Time Base,
>> which is 512 MHz.  The maximum value of the decrementer is 0x7fffffff.
>> This works out to a maximum interval of about 4.19 seconds.
>>
>> If a larger interval is desired, the kernel will set the decrementer
>> to its maximum value and reset it after it expires (underflows)
>> a sufficient number of times until the desired interval has elapsed.
>>
>> The negative effect of this is that an unwanted latency spike will
>> impact normal processing at most every 4.19 seconds.  On an IBM
>> POWER8-based system, this spike was measured at about 25-30
>> microseconds, much of which was basic, opportunistic housekeeping
>> tasks that could otherwise have waited.
>>
>> This patch short-circuits the reset of the decrementer, exiting after
>> the decrementer reset, but before the housekeeping tasks if the only
>> need for the interrupt is simply to reset it.  After this patch,
>> the latency spike was measured at about 150 nanoseconds.

> Thanks for the excellent changelog. But this patch makes me a bit nervous :)
>
> Do you know where the latency is coming from? Is it primarily the irq work?

Yes, it is all under irq_enter (measured at ~10us) and irq_exit (~12us).

> If so I'd prefer if we could move the short circuit into __timer_interrupt()
> itself. That way we'd still have the trace points usable, and it would
> hopefully result in less duplicated logic.

But irq_enter and irq_exit are called in timer_interrupt, before 
__timer_interrupt is called.  I don't see how that helps.  The time 
spent in __timer_interrupt is minuscule by comparison.

Are you suggesting that irq_enter/exit be moved into __timer_interrupt 
as well?  (I'm not sure how that would impact the existing call to 
__timer_interrupt from tick_broadcast_ipi_handler?  And if there is no 
impact, what's the point of separating timer_interrupt and 
__timer_interrupt?)

Regards,
PC