On 10/01/2013 11:26 PM, Peter Zijlstra wrote: > On Tue, Oct 01, 2013 at 07:45:08PM +0200, Oleg Nesterov wrote: >> On 10/01, Peter Zijlstra wrote: >>> >>> On Tue, Oct 01, 2013 at 10:41:15PM +0530, Srivatsa S. Bhat wrote: >>>> However, as Oleg said, its definitely worth considering whether this proposed >>>> change in semantics is going to hurt us in the future. CPU_POST_DEAD has certainly >>>> proved to be very useful in certain challenging situations (commit 1aee40ac9c >>>> explains one such example), so IMHO we should be very careful not to undermine >>>> its utility. >>> >>> Urgh.. crazy things. I've always understood POST_DEAD to mean 'will be >>> called at some time after the unplug' with no further guarantees. And my >>> patch preserves that. >> >> I tend to agree with Srivatsa... Without a strong reason it would be better >> to preserve the current logic: "some time after" should not be after the >> next CPU_DOWN/UP*. But I won't argue too much. > > Nah, I think breaking it is the right thing :-) > >> But note that you do not strictly need this change. Just kill cpuhp_waitcount, >> then we can change cpu_hotplug_begin/end to use xxx_enter/exit we discuss in >> another thread, this should likely "join" all synchronize_sched's. > > That would still be 4k * sync_sched() == terribly long. > >> Or split cpu_hotplug_begin() into 2 helpers which handle FAST -> SLOW and >> SLOW -> BLOCK transitions, then move the first "FAST -> SLOW" handler outside >> of for_each_online_cpu(). > > Right, that's more messy but would work if we cannot teach cpufreq (and > possibly others) to not rely on state you shouldn't rely on anyway. > > I tihnk the only guarnatee POST_DEAD should have is that it should be > called before UP_PREPARE of the same cpu ;-) Nothing more, nothing less. > Conceptually, that hints at a totally per-cpu implementation of CPU hotplug, in which what happens to one CPU doesn't affect the others in the hotplug path.. and yeah, that sounds very tempting! ;-) but I guess that will need to be preceded by a massive rework of many of the existing hotplug callbacks ;-) Regards, Srivatsa S. Bhat -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>