Re: [PATCH v3 2/7] sched: accumulate per-cfs_rq cpu usage

[Paul Turner]

On Thu, Oct 14, 2010 at 2:01 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Tue, 2010-10-12 at 13:21 +0530, Bharata B Rao wrote:
>> +static void account_cfs_rq_quota(struct cfs_rq *cfs_rq,
>> +               unsigned long delta_exec)
>> +{
>> +       if (cfs_rq->quota_assigned == RUNTIME_INF)
>> +               return;
>> +
>> +       cfs_rq->quota_used += delta_exec;
>> +
>> +       if (cfs_rq->quota_used < cfs_rq->quota_assigned)
>> +               return;
>> +
>> +       cfs_rq->quota_assigned += tg_request_cfs_quota(cfs_rq->tg);
>> +}
>
> That looks iffy, quota_assigned is only ever incremented and can wrap.

This can't advance at a rate faster than ~vruntime and we can't handle
wrapping there anyway (fortunately it would take something like 35k
years?)

> Why not subtract delta_exec and replenish when <0? That keeps the
> numbers small.
>

Accounting in the opposite direction allows us to catch-up in
subsequent periods when a task exceeds its bandwidth across an
interval where we are not able to immediately throttle it (e.g. costly
syscall without config_prempt).  Since we'll continue to accrue the
execution time in this case it will be effectively pre-charged against
the next slice received.
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