Re: regarding the x86_64 zero-based percpu patches

Rusty Russell
2009-01-15T03:23:16Z

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.