linux-kernel - Re: [PATCH 2/2 v3] sched: Rewrite per entity runnable load average tracking

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Date:	Wed, 16 Jul 2014 11:53:23 -0700
From:	bsegall@...gle.com
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	Yuyang Du <yuyang.du@...el.com>,
	"mingo\@redhat.com" <mingo@...hat.com>,
	"peterz\@infradead.org" <peterz@...radead.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"pjt\@google.com" <pjt@...gle.com>,
	"arjan.van.de.ven\@intel.com" <arjan.van.de.ven@...el.com>,
	"len.brown\@intel.com" <len.brown@...el.com>,
	"rafael.j.wysocki\@intel.com" <rafael.j.wysocki@...el.com>,
	"alan.cox\@intel.com" <alan.cox@...el.com>,
	"mark.gross\@intel.com" <mark.gross@...el.com>,
	"fengguang.wu\@intel.com" <fengguang.wu@...el.com>,
	"umgwanakikbuti\@gmail.com" <umgwanakikbuti@...il.com>
Subject: Re: [PATCH 2/2 v3] sched: Rewrite per entity runnable load average tracking

Morten Rasmussen <morten.rasmussen@....com> writes:

> On Wed, Jul 16, 2014 at 02:50:47AM +0100, Yuyang Du wrote:
>
> [...]
>
>> +/*
>> + * Update load_avg of the cfs_rq along with its own se. They should get
>> + * synchronized: group se's load_avg is used for task_h_load calc, and
>> + * 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)
>>  {
>> -       long old_contrib = se->avg.load_avg_contrib;
>> +       int decayed;
>> 
>> -       if (entity_is_task(se)) {
>> -               __update_task_entity_contrib(se);
>> -       } else {
>> -               __update_tg_runnable_avg(&se->avg, group_cfs_rq(se));
>> -               __update_group_entity_contrib(se);
>> +       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);
>>         }
>> 
>> -       return se->avg.load_avg_contrib - old_contrib;
>> -}
>> +       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)
>> +               sa_q->last_update_time_copy = sa_q->last_update_time;
>
> This doesn't build on 32 bit. You need:
>
> -               sa_q->last_update_time_copy = sa_q->last_update_time;
> +               cfs_rq->load_last_update_time_copy = cfs_rq->avg.last_update_time;
>
> to make it build. But I'm not convinced that this synchronization is
> right.
>
> First let me say that I'm not an expert on synchronization. It seems to
> me that there is nothing preventing reordering of the writes in
> __update_load_avg() which sets cfs_rq->avg.last_update_time and the
> update of cfs_rq->avg.load_last_update_time_copy.

You're correct, this needs to be if(...) { smp_wmb(); copy = time; },
the same as update_min_vruntime.
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