On 28 September 2016 at 04:31, Dietmar Eggemann <dietmar.eggem...@arm.com> wrote: > On 28/09/16 12:19, Peter Zijlstra wrote: >> On Wed, Sep 28, 2016 at 12:06:43PM +0100, Dietmar Eggemann wrote: >>> On 28/09/16 11:14, Peter Zijlstra wrote: >>>> On Fri, Sep 23, 2016 at 12:58:08PM +0100, Matt Fleming wrote: > > [...] > >>> Not sure what you mean by 'after fixing' but the se is initialized with >>> a possibly stale 'now' value in post_init_entity_util_avg()-> >>> attach_entity_load_avg() before the clock is updated in >>> activate_task()->enqueue_task(). >> >> I meant that after I fix the above issue of calling post_init with a >> stale clock. So the + update_rq_clock(rq) in the patch. > > OK. > > [...] > >>>> While staring at this, I don't think we can still hit >>>> vruntime_normalized() with a new task, so I _think_ we can remove that >>>> !se->sum_exec_runtime clause there (and rejoice), no? >>> >>> I'm afraid that with accurate timing we will get the same situation that >>> we add and subtract the same amount of load (probably 1024 now and not >>> 1002 (or less)) to/from cfs_rq->runnable_load_avg for the initial (fork) >>> hackbench run. >>> After all, it's 'runnable' based. >> >> The idea was that since we now update rq clock before post_init and then >> leave it be, both post_init and enqueue see the exact same timestamp, >> and the delta is 0, resulting in no aging. >> >> Or did I fail to make that happen? > > No, but IMHO what Matt wants is ageing for the hackench tasks at the end > of their fork phase so there is a tiny amount of > cfs_rq->runnable_load_avg left on cpuX after the fork related dequeue so > the (load-based) fork-balancer chooses cpuY for the next hackbench task. > That's why he wanted to avoid the __update_load_avg(se) on enqueue (thus > adding 1024 to cfs_rq->runnable_load_avg) and do the ageing only on > dequeue (removing <1024 from cfs_rq->runnable_load_avg).
ok so i'm a bit confused there my understand of your explanation above is that now we left a small amount of load in runnable_load_avg after the dequeue so another cpu will be chosen. But this explanation seems to be the opposite of what Matt said in a previous email that: "The performance drop comes from the fact that enqueueing/dequeueing a task with load 1002 during fork() results in a zero runnable_load_avg, which signals to the load balancer that the CPU is idle, so the next time we fork() we'll pick the same CPU to enqueue on -- and the cycle continues." > >
