Re: [CFS Bandwidth Control v4 5/7] sched: add exports tracking cfs bandwidth control statistics

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Thu, 2011-02-24 at 19:26 -0800, Paul Turner wrote:
> On Wed, Feb 23, 2011 at 5:32 AM, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> > On Tue, 2011-02-15 at 19:18 -0800, Paul Turner wrote:
> >> +       raw_spin_lock(&cfs_b->lock);
> >> +       cfs_b->throttled_time += (rq->clock - cfs_rq->throttled_timestamp);
> >> +       raw_spin_unlock(&cfs_b->lock);
> >
> > That seems to put the cost of things on the wrong side. Read is rare,
> > update is frequent, and you made the frequent thing the most expensive
> > one.
> 
> Hum.. the trade-off here is non-trivial I think
> 
> - This update is only once per-quota period (*if* we throttled within
> that period).  This places the frequency in the 10s-100s of ms range.
> - Sampling would probably occur on an order of once a second (assuming
> some enterprise management system that cares about these statistics).

Ugh,. people are really polling state like that? If the event is rare
pushing state is much saner.