[Devel] Re: [PATCH v3 3/6] expose fine-grained per-cpu data for cpuacct stats

[Glauber Costa]

On 05/30/2012 03:24 PM, Paul Turner wrote:
>> +static int cpuacct_stats_percpu_show(struct cgroup *cgrp, struct cftype *cft,
>> >  +                                    struct cgroup_map_cb *cb)
>> >  +{
>> >  +       struct cpuacct *ca = cgroup_ca(cgrp);
>> >  +       int cpu;
>> >  +
>> >  +       for_each_online_cpu(cpu) {
>> >  +               do_fill_cb(cb, ca, "user", cpu, CPUTIME_USER);
>> >  +               do_fill_cb(cb, ca, "nice", cpu, CPUTIME_NICE);
>> >  +               do_fill_cb(cb, ca, "system", cpu, CPUTIME_SYSTEM);
>> >  +               do_fill_cb(cb, ca, "irq", cpu, CPUTIME_IRQ);
>> >  +               do_fill_cb(cb, ca, "softirq", cpu, CPUTIME_SOFTIRQ);
>> >  +               do_fill_cb(cb, ca, "guest", cpu, CPUTIME_GUEST);
>> >  +               do_fill_cb(cb, ca, "guest_nice", cpu, CPUTIME_GUEST_NICE);
>> >  +       }
>> >  +
> I don't know if there's much that can be trivially done about it but I
> suspect these are a bit of a memory allocation time-bomb on a many-CPU
> machine.  The cgroup:seq_file mating (via read_map) treats everything
> as/one/  record.  This means that seq_printf is going to end up
> eventually allocating a buffer that can fit_everything_  (as well as
> every power-of-2 on the way there).  Adding insult to injury is that
> that the backing buffer is kmalloc() not vmalloc().
>
> 200+ bytes per-cpu above really is not unreasonable (46 bytes just for
> the text, plus a byte per base 10 digit we end up reporting), but that
> then  leaves us looking at order-12/13 allocations just to print this
> thing when there are O(many) cpus.
>

And how's /proc/stat different ?
It will suffer from the very same problems, since it also have this very 
same information (actually more, since I am skipping some), per-cpu.

Now, if you guys are okay with a file per-cpu, I can do it as well.
It pollutes the filesystem, but at least protects against the fact that 
this is kmalloc-backed.