Messages in this thread | | | Date | Thu, 10 Oct 2013 18:54:12 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2 |
| |
On 10/10, Andrew Morton wrote: > > On Thu, 10 Oct 2013 17:26:12 +0200 Oleg Nesterov <oleg@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().
> 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?
Well, then we actually need to park all tasks in system, I guess. IOW, freezer.
Oleg.
| |