Messages in this thread | | | Date | Fri, 23 May 2014 21:23:21 +0530 | From | "Srivatsa S. Bhat" <> | Subject | Re: [PATCH v6 2/3] CPU hotplug, stop-machine: Plug race-window that leads to "IPI-to-offline-CPU" |
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On 05/23/2014 09:18 PM, Peter Zijlstra wrote: > On Fri, May 23, 2014 at 09:07:18PM +0530, Srivatsa S. Bhat wrote: >> On 05/23/2014 09:03 PM, Srivatsa S. Bhat wrote: >>> On 05/23/2014 09:01 PM, Peter Zijlstra wrote: >>>> On Fri, May 23, 2014 at 08:48:07PM +0530, Srivatsa S. Bhat wrote: >>>>> On 05/23/2014 08:42 PM, Peter Zijlstra wrote: >>>>>> On Fri, May 23, 2014 at 08:15:35PM +0530, Srivatsa S. Bhat wrote: >>>>>>>>> + * During CPU offline, we don't want the other CPUs to send >>>>>>>>> + * IPIs to the active_cpu (the outgoing CPU) *after* it has >>>>>>>>> + * disabled interrupts (because, then it will notice the IPIs >>>>>>>>> + * only after it has gone offline). We can prevent this by >>>>>>>>> + * making the other CPUs disable their interrupts first - that >>>>>>>>> + * way, they will run the stop-machine code with interrupts >>>>>>>>> + * disabled, and hence won't send IPIs after that point. >>>>>> >>>>>> That's complete nonsense, you can send IPIs all you want with interrupts >>>>>> disabled. >>>>>> >>>>> >>>>> True, but that's not what the comment says. It says "you can't send IPIs >>>>> because you are running the *stop-machine* loop, because the stop-machine loop >>>>> doesn't send IPIs itself! The only possibility of sending IPIs from within >>>>> stop-machine is if that CPU can takes an interrupt and the *interrupt handler* >>>>> sends the IPI (like what the block layer used to do) - and we precisely avoid >>>>> that possibility by disabling interrupts. So no IPIs will be sent beyond >>>>> this point. >>>> >>>> but one of those CPUs is running the stop machine function, which calls >>>> CPU_DYING which runs all kinds of nonsense and therefore can send IPIs >>>> all it wants, right? >>>> >>> >>> Yes, but that CPU certainly won't IPI itself! (We are trying to avoid getting >>> IPIs on precisely that CPU - the one which is about to go offline). >>> >> >> And the comment makes that distinction between the "active-cpu" and "other CPUs" >> (where active-cpu is the one which runs the stop-machine function and eventually >> goes offline). Thus "other CPUs" won't send IPIs after that point, because they >> are running the stop-machine loop with interrupts disabled. This ensures that >> the "active-cpu" doesn't get any IPIs - which is what we want. > > OK, so clearly I'm having trouble reading today :/ Makes sense now. > > But yes, its unlikely for CPU_DYING to self-IPI, although if you really > want, I can do ;-) >
Haha :-)
> And I guess the one extra state doesn't hurt too bad for > stop_two_cpus(). >
Ok, that's good then.
Regards, Srivatsa S. Bhat
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