Who's counting jiffies when all CPUs idle in NO_HZ mode?

[Shachar Shemesh]

So I answer this here, and then I get a visit in the office with the
same question... :-)

On 25/03/14 23:04, Elazar Leibovich wrote:
> (I'm talking now about MONOTONIC_CLOCK_RAW, not taking NTP adjustment
> into account)
>
> To my understanding, the basic time counting mechanism at the Linux
> kernel, is the jiffies counter. The way it counts time, is by
> leveraging a CPU interrupt happening at a certain known frequency.
> Every time this interrupt occurs, the interrupt handler would
> increment a counter. By multiplying this counter with the IRQ's
> frequency, we can estimate how much time passed.
>
> Now, to my understanding, the NO_HZ_IDLE=y configuration, would
> prevent any interrupt on idle CPUs.
At least for the time being, this does not mean what you think it means.
See http://lwn.net/Articles/549580/

Even in full tickless mode (properly referred to as "full" tickless
mode), the boot CPU is still on a counter. Full ticklessness is still
some way away.

Even when we do achieve that lofty goal, most CPUs have a hardware
counter that counts the time. Just like NAPI for network moved from an
interrupt mode to polling mode for performance's sake, so we can do here.
>
> So to my understanding, if all CPUs are idle, nothing is going to run
> on any CPU.
>
No.
> Who would count jiffies then? How can we be sure how much time passed
> with no hardware clock (as it is the case in some systems), and all
> CPUs asleep?
When we finally move into the utopian full tickless mode, jiffies will
lose their original meaning. In full tickless mode, there is no base
frequency for the clock. As such, jiffies become an arbitrary number
based on which you can decide how much time has passed, and can be set
to whatever. How low to set them becomes a question of power management,
then.

Shachar
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.cs.huji.ac.il/pipermail/linux-il/attachments/20140326/19833ae9/attachment-0001.html>