Re: [PATCH 0/9] Per-cgroup /proc/stat

[Glauber Costa]

On 09/19/2011 08:07 PM, Paul Turner wrote:
> On 09/15/11 01:56, Peter Zijlstra wrote:
> > On Wed, 2011-09-14 at 13:23 -0700, Andi Kleen wrote:
> > > Peter Zijlstra<a.p.zijlstra@xxxxxxxxx> writes:
> > > > Guys we should seriously trim back a lot of that code, not grow ever
> > > > more and more. The sad fact is that if you build a kernel with
> > > > cpu-cgroup support the context switch cost is more than double that
> > > > of a
> > > > kernel without, and then you haven't even started creating cgroups yet.
> > >
> > > That sounds indeed quite bad. Is it known why it is so costly?
> >
> > Mostly because all data structures grow and all code paths grow, some by
> > quite a bit, its spread all over the place, lots of little cuts etc..
> >
> > pjt and I tried trimming some of the code paths with static_branch() but
> > didn't really get anywhere.. need to get back to looking at this stuff
> > sometime soon.
>
> When I get some time I think I'm just going to post a patch[*] that
> merges the useful _field_ (usage, usage_percpu) from cpuacct into cpu
> since we are *already* doing the accounting on the entity level making
> this addition free.
agree.

> At that point we could !CONFIG_CGROUP_CPUACCT by default and deprecate
> the beast without breaking ABI for those who really need it (either
> because their applications have hard-coded paths or because they really
> like cgroup user/sys time -- which we COULD duplicate into cpu but I'm
> inclined not to).

Well, why ? Now that I look into it, one of the nice ways to achieve 
what I am proposing in this patchset is:
 1) get rid of cpuacct.
 2) do all accounting per-cpu cgroup, and then merge it to fs/proc/stat.c

> [*]: the only real caveat is how loudly people scream about the code
> duplication; I think it's worth it if it let's us kill cpuacct in the
> long run.

One way to deprecate it, is probably disallowing cpuacct to have any 
tasks written to its task file. We then expose whatever information 
there is in cpu/.

It may get ugly since we'll need to touch core cgroup code, but it is 
nice from a user PoV.

> Another unrelated optimization on this path I have sitting around in
> patches/ to push at some point is keeping the left-most entity out of
> tree; since the worst case is an entity with a lower-vruntime comes
> along and we insert the previous left-most and the best case is we get
> to pick it without futzing with the rb-tree. I think this was good for a
> percent or two when I hacked it together before.
>
> Another idea I have kicking around for this path is the introduction of
> a link_entity which bridges over nr_running=1 chains (break it
> opportunistically when an element in the chain goes to nr_running=2).
> This one requires some pretty careful accounting around the breaking of
> a chain though so I'm not touching it until I get the new load tracking
> code out. (Incidentally when I benchmarked it before LPC I had it
> working out to be a little more efficient than the current math good for
> ~2-3% on pipe_test.)
>
> - Paul

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