Messages in this thread | | | From | bsegall@google ... | Subject | Re: [PATCH 2/2 v4] sched: Rewrite per entity runnable load average tracking | Date | Mon, 28 Jul 2014 09:58:19 -0700 |
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Peter Zijlstra <peterz@infradead.org> writes:
>> @@ -4551,18 +4382,34 @@ migrate_task_rq_fair(struct task_struct *p, int next_cpu) >> { >> struct sched_entity *se = &p->se; >> struct cfs_rq *cfs_rq = cfs_rq_of(se); >> + u64 last_update_time; >> >> /* >> + * Task on old CPU catches up with its old cfs_rq, and subtract itself from >> + * the cfs_rq (task must be off the queue now). >> */ >> +#ifndef CONFIG_64BIT >> + u64 last_update_time_copy; >> + >> + do { >> + last_update_time_copy = cfs_rq->load_last_update_time_copy; >> + smp_rmb(); >> + last_update_time = cfs_rq->avg.last_update_time; >> + } while (last_update_time != last_update_time_copy); >> +#else >> + last_update_time = cfs_rq->avg.last_update_time; >> +#endif >> + __update_load_avg(last_update_time, &se->avg, 0); >> + atomic_long_add(se->avg.load_avg, &cfs_rq->removed_load_avg); >> + >> + /* >> + * We are supposed to update the task to "current" time, then its up to date >> + * and ready to go to new CPU/cfs_rq. But we have difficulty in getting >> + * what current time is, so simply throw away the out-of-date time. This >> + * will result in the wakee task is less decayed, but giving the wakee more >> + * load sounds not bad. >> + */ >> + se->avg.last_update_time = 0; >> >> /* We have migrated, no longer consider this task hot */ >> se->exec_start = 0; > > > And here we try and make good on that assumption. The thing I worry > about is what happens if the machine is entirely idle... > > What guarantees an semi up-to-date cfs_rq->avg.last_update_time.
update_blocked_averages I think should do just as good a job as the old code, which isn't perfect but is about as good as you can get worst case.
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