Messages in this thread | | | Date | Mon, 29 Aug 2011 20:23:56 +0200 | From | Frederic Weisbecker <> | Subject | Re: [PATCH 09/32] nohz: Move ts->idle_calls into strict idle logic |
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On Mon, Aug 29, 2011 at 07:59:49PM +0200, Peter Zijlstra wrote: > On Mon, 2011-08-29 at 19:34 +0200, Frederic Weisbecker wrote: > > On Mon, Aug 29, 2011 at 04:47:47PM +0200, Peter Zijlstra wrote: > > > On Mon, 2011-08-15 at 17:52 +0200, Frederic Weisbecker wrote: > > > > +static bool tick_nohz_can_stop_tick(int cpu, struct tick_sched *ts) > > > > +{ > > > > + /* > > > > + * If this cpu is offline and it is the one which updates > > > > + * jiffies, then give up the assignment and let it be taken by > > > > + * the cpu which runs the tick timer next. If we don't drop > > > > + * this here the jiffies might be stale and do_timer() never > > > > + * invoked. > > > > + */ > > > > + if (unlikely(!cpu_online(cpu))) { > > > > + if (cpu == tick_do_timer_cpu) > > > > + tick_do_timer_cpu = TICK_DO_TIMER_NONE; > > > > + } > > > > + > > > > + if (unlikely(ts->nohz_mode == NOHZ_MODE_INACTIVE)) > > > > + return false; > > > > + > > > > + if (need_resched()) > > > > + return false; > > > > + > > > > + if (unlikely(local_softirq_pending() && cpu_online(cpu))) { > > > > + static int ratelimit; > > > > + > > > > + if (ratelimit < 10) { > > > > + printk(KERN_ERR "NOHZ: local_softirq_pending %02x\n", > > > > + (unsigned int) local_softirq_pending()); > > > > + ratelimit++; > > > > + } > > > > + return false; > > > > + } > > > > + > > > > + return true; > > > > +} > > > > > > Why aren't rcu_needs_cpu(), printk_needs_cpu() and arch_needs_cpu() not > > > in there? > > > > > > That are typical 'can we go sleep now?' functions. > > > > Because when one of these functions are positive, the ts->next_jiffies and > > ts->last_jiffies stats are updated. Not with the above. > > Also I start to think the above checks are only useful in the idle case. > > Then call it tick_nohz_can_stop_tick_idle() or so, and create > tick_nohz_can_stop_tick() to deal with all stuff.
Yeah I need to have a deeper look into these checks.
> > > We still want tick_nohz_stop_sched_tick() to have the *needs_cpu() checks > > so that they can restore a HZ periodic behaviour on interrupt return if > > needed. > > Well, no, on interrupt return you shouldn't do anything. If you've > stopped the tick it stays stopped until you do something that needs it, > then that action will re-enable it.
Sure, when something needs the tick in this mode, we usually receive an IPI and restart the tick from there but then tick_nohz_stop_sched_tick() handles the cases with *needs_cpu() very well on interrupt return (our IPI return) by doing a kind of "light" HZ mode by logically switching to nohz mode but with the next timer happening in HZ, assuming it's a matter of one tick and we will switch to a real nohz behaviour soon.
I don't see a good reason to duplicate that logic with a pure restart from the IPI.
> > That said I wonder if some of the above conditions should restore a periodic > > behaviour on interrupt return... > > I would expect the tick not to be stopped when tick_nohz_can_stop_tick() > returns false. If it returns true, then I expect anything that needs it > to re-enable it. >
Yeah. In the case of need_resched() in idle I believe the CPU doesn't really go to sleep later so it should be fine. But for the case of softirq pending or nohz_mode, I'm not sure...
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