Messages in this thread | | | Date | Tue, 8 Dec 2015 16:57:05 +0100 | From | Frederic Weisbecker <> | Subject | Re: [PATCH 2/7] nohz: New tick dependency mask |
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On Wed, Dec 02, 2015 at 04:09:08PM +0100, Peter Zijlstra wrote: > On Wed, Dec 02, 2015 at 03:08:16PM +0100, Frederic Weisbecker wrote: > > On Wed, Dec 02, 2015 at 11:56:33AM +0100, Peter Zijlstra wrote: > > > On Tue, Dec 01, 2015 at 11:20:28PM +0100, Frederic Weisbecker wrote: > > > > > > + prev = fetch_or(dep, BIT_MASK(bit)); > > > > > > + if (!prev) { > > > > > > + /* > > > > > > + * We need the IPIs to be sent from sane process context. > > > > > > > > > > Why ? > > > > > > > > Because posix timers code is all called with interrupts disabled and we can't > > > > send IPIs then. > > > > > > > > > > > > > > > + * The posix cpu timers are always set with irqs disabled. > > > > > > + */ > > > > > > + schedule_work(&kick_all_work); > > > > > > + } > > > > > > +} > > > > > > + > > > > > > +/* > > > > > > + * Set a global tick dependency. Lets do the wide IPI kick asynchronously > > > > > > + * for callers with irqs disabled. > > > > > > > > > > This seems to suggest you can call this with IRQs disabled > > > > > > > > Ah right, that's a misleading comment. We need to use the _delayed() version > > > > when interrupts are disabled. > > > > > > Why can't you use tick_nohz_full_kick_cpu() for all that, which is > > > usable from IRQ context and avoid all that delayed muck? > > > > Because I need to kick all the CPUs where the task/signal is running on. That's > > a bit difficult to do though. I think we had something to try to send an IPI to a task, > > but I can't retrieve it. Looks easy to do anyway. But in the signal case I'd need to do > > that for all tasks in the group. That sounds like a costly loop. > > > > So I simplify that with a global IPI. > > Sure, but what I meant was that: > > for_each_cpu(cpu, mask) > tick_nohz_full_kick_cpu(cpu); > > is IRQ safe, whereas the current thingy is not. Sure, its a wee bit less > efficient, but do we really care?
Right, this overhead probably doesn't matter much. I'll do that and we'll see if people complain.
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