On Sat, Oct 24, 2015 at 08:50:54AM +0530, Viresh Kumar wrote: > On 23-10-15, 15:10, Yunhong Jiang wrote: > > 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? > > Yeah, the CPU wouldn't like in idle for ever but the time is not known > and it can be really really long. > > > 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". > > It will be an IPI mostly.. > > > If there is no non-deferrable timer, based > > on Frederic's comments, we in fact depends on next tick. > > So, the cpu will wake up when it receives an IPI. The first thing we > do then is to restart the tick and we will then service all the > pending deferred timers. > > > 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. > > There are many kind of works we may want to do. If its really > important to be done earlier, then it should be serviced with a timer. > > deferred timers are better used for activities, which are irrelevant > once the CPU is idle. One case is doing some per-cpu load tracking for > cpufreq governors or the work that vmstat does. > > Even if the CPU wakes up after few hours (hypothetically), it > shouldn't matter.
Viresh, thanks for the clarification. So seems the original patch is correct to wakeup the full dyntick CPU even for deferred timer. Thomas/Fred, your idea? Thanks --jyh > > -- > viresh > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to [email protected] > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
