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 list > Linux-il@cs.huji.ac.il > http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il
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