On 15/09/16 16:14, Peter Zijlstra wrote:
On Thu, Sep 15, 2016 at 02:11:49PM +0100, Dietmar Eggemann wrote:
On 12/09/16 08:47, Vincent Guittot wrote:
+/* Take into account change of load of a child task group */ +static inline void +update_tg_cfs_load(struct cfs_rq *cfs_rq, struct sched_entity *se) +{
- struct cfs_rq *gcfs_rq = group_cfs_rq(se);
- long delta, load = gcfs_rq->avg.load_avg;
- /* If the load of group cfs_rq is null, the load of the
* sched_entity will also be null so we can skip the formula*/- if (load) {
long tg_load;/* Get tg's load and ensure tg_load > 0 */tg_load = atomic_long_read(&gcfs_rq->tg->load_avg) + 1;/* Ensure tg_load >= load and updated with current load*/tg_load -= gcfs_rq->tg_load_avg_contrib;tg_load += load;/* scale gcfs_rq's load into tg's shares*/load *= scale_load_down(gcfs_rq->tg->shares);load /= tg_load;/** we need to compute a correction term in the case that the* task group is consuming <1 cpu so that we would contribute* the same load as a task of equal weight.Wasn't 'consuming <1' related to 'NICE_0_LOAD' and not scale_load_down(gcfs_rq->tg->shares) before the rewrite of PELT (v4.2, __update_group_entity_contrib())?
So the approximation was: min(1, runnable_avg) * shares;
And it just so happened that we tracked runnable_avg in 10 bit fixed point, which then happened to be NICE_0_LOAD.
But here we have load_avg, which already includes a '* shares' factor. So that then becomes min(shares, load_avg).
Makes sense, understand it now.
We did however loose a lot on why and how min(1, runnable_avg) is a sensible thing to do...
Do you refer to the big comment on top of this if condition in the old code in __update_group_entity_contrib()? The last two subsections of it I never understood ...
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