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 ... [...]