On Mon, Jul 28, 2014 at 01:39:39PM +0200, Peter Zijlstra wrote: > > -static inline void __update_group_entity_contrib(struct sched_entity *se) > > +static inline void update_tg_load_avg(struct cfs_rq *cfs_rq) > > { > > + long delta = cfs_rq->avg.load_avg - cfs_rq->tg_load_avg_contrib; > > > > + if (delta) { > > + atomic_long_add(delta, &cfs_rq->tg->load_avg); > > + cfs_rq->tg_load_avg_contrib = cfs_rq->avg.load_avg; > > } > > } > > We talked about this before, you made that an unconditional atomic op on > an already hot line. > > You need some words on why this isn't a problem. Either in a comment or > in the Changelog. You cannot leave such changes undocumented. I am all for not updating trivial delta, e.g., 1 or 2. I just had no theory in selecting a "good" threshold.
The current code uses 1/8 or 1/64 of contrib. Though it is not fair comparison, because how current tg load is calculated is a big story (no offense), I choose 1/64 as the threshold. > > +#define subtract_until_zero(minuend, subtrahend) \ > > + (subtrahend < minuend ? minuend - subtrahend : 0) > > WTH is a minuend or subtrahend? Are you a wordsmith in your spare time > and like to make up your own words? > > Also, isn't writing: x = max(0, x-y), far more readable to begin with? > Ok. IIUC, max() does not handle minus number super good, and we don't need the type overhead in max(), so still use my macro, but won't be wordsmith again, :) > > +/* > > + * Group cfs_rq's load_avg is used for task_h_load and update_cfs_share > > + * calc. > > + */ > > +static inline int update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq) > > { > > + int decayed; > > > > + if (atomic_long_read(&cfs_rq->removed_load_avg)) { > > + long r = atomic_long_xchg(&cfs_rq->removed_load_avg, 0); > > + cfs_rq->avg.load_avg = > > subtract_until_zero(cfs_rq->avg.load_avg, r); > > + r *= LOAD_AVG_MAX; > > + cfs_rq->avg.load_sum = > > subtract_until_zero(cfs_rq->avg.load_sum, r); > > } > > > > + decayed = __update_load_avg(now, &cfs_rq->avg, cfs_rq->load.weight); > > > > +#ifndef CONFIG_64BIT > > + if (cfs_rq->avg.last_update_time != cfs_rq->load_last_update_time_copy) > > { > > + smp_wmb(); > > + cfs_rq->load_last_update_time_copy = > > cfs_rq->avg.last_update_time; > > + } > > +#endif > > > > + return decayed; > > +} > > Its a bit unfortunate that we update the copy in another function than > the original, but I think I see why you did that. But is it at all > likely that we do not need to update? That is, does that compare make > any sense? I think we can assume last_update_time will mostly be changed, because it won't be changed only in two cases: 1) minus delta time, 2) within a period, 1ms, these two cases seemingly are minority. So yes, we can save the compare. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/