Messages in this thread | | | Date | Wed, 22 Feb 2006 14:01:55 +0530 | From | Srivatsa Vaddagiri <> | Subject | Re: How to allocate per-cpu data for online CPUs only (and safely)? |
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On Tue, Feb 21, 2006 at 11:29:02PM +0100, Martin Peschke wrote: > I am trying to optimize the memory footprint of some code of mine. > the disadvantage of getting hold of memory for CPU's which aren't there > (yet). > > I could imagine using CPU-hotplug notifications as triggers for > additional allocations or for cleaning up unneeded memory. But > alloc_percpu() appears to conflict with that idea. > > I was briefly tempted to derive some code from alloc_percpu() more to my > liking, until I was scared off by this comment in alloc_percpu(): > > /* > * Cannot use for_each_online_cpu since a cpu may come online > * and we have no way of figuring out how to fix the array > * that we have allocated then.... > */ > > Well, and then there is kernel/profile.c, for example, which boldly > ignors alloc_percpu()'s qualms and allocates and releases per-cpu data > as needed. > > Is that the way to go? > If so, why alloc_percpu()'s reservations?
The above comment in alloc_percpu regarding use of for_each_online_cpu probably refers to the fact that once we handover 'pdata' to the caller of alloc_percpu, there is no way to go back and fix it (i.e allocate objects) for CPUs that come online later. This could have been possible if alloc_percpu kept track of all per-CPU allocations (chain togther all pdata's in a linked list?), but I guess they didn't do it to keep it simple.
I would say the code in kernel/profile.c is fine enough to emulate.
> Or, does that comment imply that the exploiter isn't expected to take > care of CPU hotplug events? > Am I missing anything? > > Thank you. > > Martin
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