Messages in this thread | | | Date | Tue, 19 Jan 2016 23:33:22 +0100 | From | Frederic Weisbecker <> | Subject | Re: [PATCH 1/4] sched: Don't account tickless CPU load on tick |
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On Tue, Jan 19, 2016 at 07:56:47PM +0100, Peter Zijlstra wrote: > On Tue, Jan 19, 2016 at 05:22:11PM +0100, Frederic Weisbecker wrote: > > On Tue, Jan 19, 2016 at 02:08:57PM +0100, Peter Zijlstra wrote: > > > On Wed, Jan 13, 2016 at 05:01:28PM +0100, Frederic Weisbecker wrote: > > > > The cpu load update on tick doesn't care about dynticks and as such is > > > > buggy when occuring on nohz ticks (including idle ticks) as it resets > > > > the jiffies snapshot that was recorded on nohz entry. We eventually > > > > ignore the potentially long tickless load that happened before the > > > > tick. > > > > > > I don't get it, how can we call scheduler_tick() while > > > tick_nohz_tick_stopped() ? > > > > tick_nohz_tick_stopped() (which is ts->tick_stopped == 1) doesn't actually > > mean that the tick is really stopped. It just means that the tick fires only > > when it's really needed (timer list expired, RCU stuff, irq_work, ...). > > That's insane and broken. Fix _that_. > > If RCU, irq_work etc.. needs the tick, do not stop the tick.
This is not the first time we have this conversation :-)
RCU/irq_work/foo_needs_tick() are treated just like any timer that expire in one tick, although RCU is some more tunable there.
And timers that expire in 1 jiffy can be treated in two ways:
* If the tick is still periodic (ts->tick_stopped = 0), we don't stop the tick: we don't enter dynticks mode.
* If the tick is already stopped (or rather in dynticks mode to be more exact: ts->tick_stopped == 1) we just program the tick one jiffy ahead. This is an optimization and a simplification, if we were to restart the tick everytime we see a tick one jiffy ahead in the middle of a dynticks frame, we would have to perform all the accounting in tick_nohz_idle_exit() as well, including update_cpu_load_nohz() that locks rq->lock. Having a bunch of jiffies subsequently ticking in the middle of a dynticks frame is a common and frequent scenario and removing that optimization would have a bad visible impact.
Certainly the issue here is that "tick_stopped" can be misunderstood. ts->dynticks_active would be better but we already have ts->nohz_active which reflects something very different. I guess we need a cascading rename.
Anyway whether the next tick is one jiffy ahead or more doesn't really matter here. The issue is that ticks can fire while dynticks-idle or dyntick-buzy and update_cpu_load_active() treats them in a broken way.
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