Re: [RFC 2/2] blockcg: export latency info for each cgroup

[Tejun Heo <tj@kernel.org>]

Hello,

On Wed, Oct 04, 2017 at 10:41:20AM -0700, Shaohua Li wrote:
> Export the latency info to user. The latency is a good sign to indicate
> if IO is congested or not. User can use the info to make decisions like
> adjust cgroup settings.

Nice, yeah, this can be really useful.

> Existing io.stat shows accumulated IO bytes and requests, but
> accumulated value for latency doesn't make much sense. This patch
> exports the latency info in a 100ms interval.

We probably want running avg of a few intervals.

> A micro benchmark running fio test against null_blk in a third level
> cgroup shows around 4% regression. If I only do the latency accouting

Heh, that's quite a bit.

> for leaf cgroup, the regression seems to disappear. So not quite sure if
> we should do the accounting for intermediate nodes or if the whole thing
> should be enabled optionally.

I suspect that if we make the calculations and propagations lazy, the
overhead will likely become negligible.  Can you please take a look at
how the basic resource accounting code in kernel/cgroup/stat.c?  It's
an infra code for collecting stats without burdening hot path.  It's
currently only used for CPU but we can easily expand it to cover other
resources.  Having to do running avgs might interfere with the lazy
propagation a bit but I think we can make it work if we make the right
trade-offs.

Thanks.

-- 
tejun