Messages in this thread | | | From | Rusty Russell <> | Subject | Re: regarding the x86_64 zero-based percpu patches | Date | Thu, 15 Jan 2009 12:04:21 +1030 |
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On Tuesday 13 January 2009 04:14:58 Eric W. Biederman wrote: > 2M of per cpu data doesn't make sense, and likely indicates a design > flaw somewhere. It just doesn't make sense to have large amounts of > data allocated per cpu. > > The most common user of per cpu data I am aware of is allocating one > word per cpu for counters.
This is why I did a brief audit. Here it is:
With x86/32 allyesconfig (trimmed a little, until it booted under kvm) we have 37148 bytes of static percpu data, and 117228 bytes of dynamic percpu data.
File and line Number Size Total net/ipv4/af_inet.c:1287 21 2048 43008 net/ipv4/af_inet.c:1290 21 2048 43008 kernel/workqueue.c:819 72 128 9126 net/ipv4/af_inet.c:1287 48 128 6144 net/ipv4/af_inet.c:1290 48 128 6144 net/ipv4/route.c:3258 1 4096 4096 include/linux/genhd.h:271 72 40 2880 lib/percpu_counter.c:77 194 4 776 net/ipv4/af_inet.c:1287 1 288 288 net/ipv4/af_inet.c:1290 1 288 288 net/ipv4/af_inet.c:1287 1 256 256 net/ipv4/af_inet.c:1290 1 256 256 net/core/neighbour.c:1424 4 44 176 kernel/kexec.c:1143 1 176 176 net/ipv4/af_inet.c:1287 1 104 104 net/ipv4/af_inet.c:1290 1 104 104 arch/x86/.../acpi-cpufreq.c:528 96 1 96 arch/x86/acpi/cstate.c:153 1 64 64 net/.../nf_conntrack_core.c:1209 1 60 60
Others: 178
This is why my patch series adds "big_percpu_alloc" (basically identical to current code) for the bigger/unbounded users.
I don't think moving per-cpu areas is going to fly. We do put complex datastructures in there. And you're going to need preempt_disable() on all per-cpu ops on many archs to make it work (assuming you use stop_machine to do the realloc. Even a rough audit quickly becomes overwhelming: 20 of the first 1/4 of DECLARE_PER_CPUs are non-movable datastructures.
Cheers, Rusty.
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