linux-kernel - Re: [RFC PATCH 0/4] sched: Improve cpu load accounting with nohz

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Date:	Thu, 14 Jan 2016 22:23:46 +0000
From:	Dietmar Eggemann <dietmar.eggemann@....com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Byungchul Park <byungchul.park@....com>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Luiz Capitulino <lcapitulino@...hat.com>,
	Christoph Lameter <cl@...ux.com>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mike Galbraith <efault@....de>, Rik van Riel <riel@...hat.com>
Subject: Re: [RFC PATCH 0/4] sched: Improve cpu load accounting with nohz

On 01/14/2016 09:27 PM, Peter Zijlstra wrote:
> On Thu, Jan 14, 2016 at 09:19:00PM +0000, Dietmar Eggemann wrote:
>> @@ -4346,7 +4346,10 @@ static void __update_cpu_load(struct rq *this_rq, unsigned long this_load,
>>
>>                  /* scale is effectively 1 << i now, and >> i divides by scale */
>>
>> -               old_load = this_rq->cpu_load[i] - tickless_load;
>> +               if (this_rq->cpu_load[i] > tickless_load)
>> +                       old_load = this_rq->cpu_load[i] - tickless_load;
>> +               else
>> +                       old_load = 0;
>
> Yeah, yuck. That'd go bad quick.
>

... because I set it to 0? But after the decay function we add
tickless_load to old_load. Maybe in case tickless_load >
this_rq->cpu_load[i] we decay this_rq->cpu_load[i] and do not add
tickless_load afterwards.

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