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

[E.S. Rosenberg]

Don't we rely heavily on network time these days too?
Your cellphone would also not be a real example since:
1. It is always a bit active polling the antennae in it's range and
possibly doing scheduled tasks like polling a mailserver/calendar server
etc.
2. The cellphone definitely has network time built in (though it may be
configured to not use it)

Just my uneducated 2c on time...
Eliyahu - אליהו


2014-03-26 8:41 GMT+02:00 Elazar Leibovich <elazarl at gmail.com>:

> (If you've been at the office yesterday, maybe I wouldn't have send the
> question ;-)
>
> You've followed my line of thought (thinking that there's always one CPU
> active). In some cases I think all CPUs must be asleep (otherwise, my
> Android's battery would be draining even after I press the power button).
> Still, two consecutive calls to clock_getttime should give a reasonable
> result.
>
> However, I can't find the place where the time is adjusted through the
> hardware clock.
>
> The thing is, the only architecture I can see clocksource other than
> jiffies is s390[0], so I assume hardware internal clock is not used in many
> important architectures. We can only count on the jiffies counter.
>
> The jiffies counter is incremented only by do_timer[1], which is scheduled
> to run do_timer, which runs on xtime_update, which seems to eventually runs
> inside an IRQ handler called in a certain frequency in some systems.
>
> While looking at it, it seems that I found the mechanism I was looking
> for. It looks like the no_sched mechanism, simply lowers the CPU frequency
> , and calculates the jiffies according to the new frequency while the CPU
> is idle[2].
>
> I still need to have a second look, but it looks like the CPU is always
> woken up to count jiffies, but when it's idle it woken up in lower
> frequencies, is aware to the fact it's idle, and thus is counting more
> jiffies per tick.
>
> [0] clocksource_register usage:
> http://lxr.linux.no/linux+v3.13.5/+ident=61690701
> [1] jiffies_64 usage: http://lxr.linux.no/linux+v3.13.5/+ident=59277722
> [2] tick_nohz_* -
> http://lxr.linux.no/linux+v3.13.5/kernel/time/tick-sched.c#L52
>
>
> On Wed, Mar 26, 2014 at 6:45 AM, Shachar Shemesh <shachar at shemesh.biz>wrote:
>
>>  Oops!
>>
>> The "one cpu always active" relates to the "full" tickless, not to the
>> idle tickless. Please disregard that answer, except the use of the hardware
>> clocks. I believe it is still valid, but I'll need to look at the source
>> code.
>>
>> Shachar
>>
>>
>> On 26/03/14 06:38, Shachar Shemesh wrote:
>>
>> 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
>>
>>
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