On 10/10/2013 10:24 PM, Oleg Nesterov wrote: > On 10/10, Andrew Morton wrote: >> >> On Thu, 10 Oct 2013 17:26:12 +0200 Oleg Nesterov <o...@redhat.com> wrote: >> >>> On 10/10, Ingo Molnar wrote: >>>> >>>> So ... why not make it _really_ cheap, i.e. the read lock costing nothing, >>>> and tie CPU hotplug to freezing all tasks in the system? >>>> >>>> Actual CPU hot unplugging and repluggin is _ridiculously_ rare in a >>>> system, I don't understand how we tolerate _any_ overhead from this utter >>>> slowpath. >>> >>> Well, iirc Srivatsa (cc'ed) pointed out that some systems do cpu_down/up >>> quite often to save the power. >> >> cpu hotremove already uses stop_machine, > > And Srivatsa wants to remove it from cpu_down(). >
Yes, I have worked on several designs to remove stop_machine() from the cpu_down path. http://lwn.net/Articles/538819/ http://lwn.net/Articles/556727/ >> so such an approach shouldn't >> actually worsen things (a lot) for them? > > this depends on what this "freezing all tasks" actually means. > I understood it as try_to_freeze_tasks/etc, looks too heavy... > > But my only point was, I am not sure we can assume that cpu_down/up > is extremly rare and its cost does not matter. > >> use stop_machine() on the add/remove >> (ie, "writer") side and nothing at all on the "reader" side. Is there >> anything which fundamentally prevents cpu hotplug from doing the same? > Its certainly possible to remove stop_machine() from CPU hotplug, as I've demonstrated in the patchsets mentioned above. And there were pretty good performance improvements too, arising out of that change, as described here: http://article.gmane.org/gmane.linux.ports.ppc.embedded/56122 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/