Messages in this thread | | | Date | Tue, 30 Aug 2011 16:45:02 +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 08:33:23PM +0200, Peter Zijlstra wrote: > On Mon, 2011-08-29 at 20:23 +0200, Frederic Weisbecker wrote: > > > > 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 sounds like an optimization, and should thus be done later.
The optimization is already there upstream. I can split the logic for non-idle case but I'm not sure about the point of that.
> > > > 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... > > softirqs shouldn't be pending when you go into nohz mode..
You mean it can't happen or we don't want that to happen?
> > That is, I'm really not seeing what's wrong with the very simple: > > > if (tick_nohz_can_stop_tick()) > tick_nohz_stop_tick(); > > > and relying on everybody who invalidates tick_nohz_can_stop_tick(), to > do: > > tick_nohz_start_tick();
May be for the non-idle case. But for the idle case I need to ensure this is necessary somewhere.
> > I'm also not quite sure why you always IPI, is that to avoid lock > inversions?
Exactly! I think I wrote that to some changelog but I'm not sure. I'll check that.
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