linux-kernel - Re: [PATCH] sched: Call update_group_power only for local

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Date:	Fri, 2 Jul 2010 09:20:02 -0700
From:	Venkatesh Pallipadi <venki@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Gautham R Shenoy <ego@...ibm.com>,
	Joel Schopp <jschopp@...tin.ibm.com>,
	Suresh B Siddha <suresh.b.siddha@...el.com>
Subject: Re: [PATCH] sched: Call update_group_power only for local_group

On Fri, Jul 2, 2010 at 1:05 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, 2010-07-01 at 16:12 -0700, Venkatesh Pallipadi wrote:
>> commit 871e35b moved update_group_power() call in update_sg_lb_stats(),
>> resulting in it being called for each group, even though it only updates
>> the power of local group. As a result we have frequent redundant
>> update_group_power() calls.
>>
>> Move it back under "if (local_group)" condition.
>>
>> This reduces the number of calls to update_group_power by a factor of 4
>> on my 4 core in 4 NUMA nodes test system.
>
> Hrm,.. so Gautham removed that because for things like the NO_HZ
> balancer the initial balance_cpu == this_cpu constraint doesn't hold.
>
> Not I don't think the local_group constraint holds for that either, so
> the below would again break that..
>
> Should we perhaps have a conditional on this_rq->nohz_balance_kick or
> so?

The thing is that update_group_power is only updating the power of
local group (sd->groups).
It is getting called multiple times however for each group as
update_sd_lb_stats loops
through groups->next calling update_sg_lb_stats.
If we really want to update the power of non-local groups,
update_cpu_power has to change
to take a groups parameter and non this_cpu as arguments and may have
to access non-local
rq etc.

Thanks,
Venki
>
>> --- a/kernel/sched_fair.c
>> +++ b/kernel/sched_fair.c
>> @@ -2359,8 +2359,11 @@ static inline void update_sg_lb_stats(struct sched_domain *sd,
>>       unsigned int balance_cpu = -1, first_idle_cpu = 0;
>>       unsigned long avg_load_per_task = 0;
>>
>> -     if (local_group)
>> +     if (local_group) {
>>               balance_cpu = group_first_cpu(group);
>> +             update_group_power(sd, this_cpu);
>> +     }
>> +
>>
>>       /* Tally up the load of all CPUs in the group */
>>       max_cpu_load = 0;
>
>
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