On Fri, Oct 23, 2015 at 07:49:51AM +0530, Viresh Kumar wrote: > On 22-10-15, 14:40, Yunhong Jiang wrote: > > A naive question is, why it's sure a tick will happen when the tickless > > processor is in idle? > > How do you get this impression? I don't think anyone has said that.
Viresh, thanks for your reply for my question. I got this impression from Frederic's comments on http://marc.info/?l=linux-kernel&m=139048415303210&w=2, "So you simply rely on the next tick to see the new timer. This should work with CONFIG_NO_HZ_IDLE but not with CONFIG_NO_HZ_FULL since the target may be running without the tick". Per my understanding of this comment, it means we can rely on the next tick for CONFIG_NO_HZ_IDLE, which means it's sure a tick will happen for CONFIG_NO_HZ_IDLE, am I right? > > We are talking about deferrable timers, which by design are only > required if the target CPU is not-idle. If it is idle, then the timer > isn't required to be serviced until the CPU wakes up. And the CPU can > take whatever time it wants to wake up again. Hmm, per http://lxr.free-electrons.com/source/include/linux/timer.h#L51, the deferreable timer will be serviced when the CPU eventually wakes up "with a subsequent non-deferrable timer". If there is no non-deferrable timer, based on Frederic's comments, we in fact depends on next tick. My confusion is, why we are sure there is next tick on CONFIG_NO_HZ_IDLE idle processor to wake it up. If there is no tick, and no other timer, will the timer get no chance to be waken up at all? I don't think "deferred for ever" is deferreable. Thanks -jyh > > > Is it because scheduler load balance is sure to send a > > tick to the processor in future? > > No. We aren't expecting the CPU to wake up any time soon. Just ignore > the deferrable timer. > > -- > viresh -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
