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 <elaz...@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 <shac...@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 >> >> >> _______________________________________________ >> Linux-il mailing >> listlinux...@cs.huji.ac.ilhttp://mailman.cs.huji.ac.il/mailman/listinfo/linux-il >> >> >> >> _______________________________________________ >> Linux-il mailing list >> Linux-il@cs.huji.ac.il >> http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il >> >> > > _______________________________________________ > Linux-il mailing list > Linux-il@cs.huji.ac.il > http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il > >
_______________________________________________ Linux-il mailing list Linux-il@cs.huji.ac.il http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il
