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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
