Re: [RFC tg_shares_up improvements - v1 05/12] sched: fix update_cfs_load synchronization

Paul Turner
2010-10-21T18:25:39Z

On Thu, Oct 21, 2010 at 2:52 AM, Bharata B Rao
<bharata@linux.vnet.ibm.com> wrote:
> On Fri, Oct 15, 2010 at 09:43:54PM -0700, pjt@google.com wrote:
>> Using cfs_rq->nr_running is not sufficient to synchronize update_cfs_load with
>> the put path since nr_running accounting occurs at deactivation.
>>
>> It's also not safe to make the removal decision based on load_avg as this fails
>> with both high periods and low shares.  Resolve this by clipping history at 8
>> foldings.
>>
>
> Did you mean 4 foldings (and not 8) above since I see you are truncating
> the load history at 4 idle periods ?

We end up folding every period/2 (after the first fold) since after
that we start at period/2 and not 0.

Thus, if we were issuing 'continuous' load updates triggering this
condition would mean that we had folded idle periods into our load_avg
8 times.

Granted, it's possible for there to be less folds given sporadic load
updates so I'll just update the comment to reference periods.

Thanks

>
>
>> @@ -685,9 +686,19 @@ static void update_cfs_load(struct cfs_r
>>       now = rq_of(cfs_rq)->clock;
>>       delta = now - cfs_rq->load_stamp;
>>
>> +     /* truncate load history at 4 idle periods */
>> +     if (cfs_rq->load_stamp > cfs_rq->load_last &&
>> +         now - cfs_rq->load_last > 4 * period) {
>> +             cfs_rq->load_period = 0;
>> +             cfs_rq->load_avg = 0;
>> +     }
>> +
>>       cfs_rq->load_stamp = now;
>>       cfs_rq->load_period += delta;
>> -     cfs_rq->load_avg += delta * cfs_rq->load.weight;
>> +     if (load) {
>> +             cfs_rq->load_last = now;
>> +             cfs_rq->load_avg += delta * load;
>> +     }
>>
>>       while (cfs_rq->load_period > period) {
>>               /*
>> @@ -700,10 +711,8 @@ static void update_cfs_load(struct cfs_r
>>               cfs_rq->load_avg /= 2;
>>       }
>>
>> -     if (lb && !cfs_rq->nr_running) {
>> -             if (cfs_rq->load_avg < (period / 8))
>> -                     list_del_leaf_cfs_rq(cfs_rq);
>> -     }
>> +     if (!cfs_rq->curr && !cfs_rq->nr_running && !cfs_rq->load_avg)
>> +             list_del_leaf_cfs_rq(cfs_rq);
>>  }
>>
>