On Tue, 16 Nov 2010, Andrew Morton wrote: > > Following patch use (2) beucase removing get_online_cpus() makes good > > side effect. It reduce potentially cpu-hotplug vs memory-shortage deadlock > > risk. > > Well. Being able to run for_each_online_cpu() is a pretty low-level > and fundamental thing. It's something we're likely to want to do more > and more of as time passes. It seems a bad thing to tell ourselves > that we cannot use it in reclaim context. That blots out large chunks > of filesystem and IO-layer code as well! The online map can change if no locks were taken. Thus it becomes something difficult to do in some code paths and overhead increases significantly. > > threshold = (*calculate_pressure)(zone); > > - for_each_online_cpu(cpu) > > + for_each_possible_cpu(cpu) > > per_cpu_ptr(zone->pageset, cpu)->stat_threshold > > = threshold; > > } > > - put_online_cpus(); > > } > > That's a pretty sad change IMO, especially of num_possible_cpus is much > larger than num_online_cpus. num_possible_cpus should only be higher if the arch code has detected that the system has the ability to physically online and offline cpus. I have never actually seen such a system. Heard rumors from Fujitsu that they have something. Maybe the virtualization people also need this? Otherwise cpu online/offline is useful mainly to debug the cpu offline/online handling in various subsystems which is unsurprisingly often buggy given the rarity of encountering such hardware. > What do we need to do to make get_online_cpus() safe to use in reclaim > context? (And in kswapd context, if that's really equivalent to > "reclaim context"). I think its not worth the effort. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom policy in Canada: sign http://dissolvethecrtc.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>