Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [Patch v9 7/8] sched/fair: Enable tuning of decay period
To:     Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Peter Zijlstra <peterz@infradead.org>
References: <1580250967-4386-1-git-send-email-thara.gopinath@linaro.org>
 <1580250967-4386-8-git-send-email-thara.gopinath@linaro.org>
 <4eb10687-1a62-cee3-7285-3f50cc023071@infradead.org>
 <5E380D1D.7020500@linaro.org>
 <20200203155549.GL14914@hirez.programming.kicks-ass.net>
 <cc83634f-b3af-6024-7f89-9b231b153070@arm.com>
Cc:     Randy Dunlap <rdunlap@infradead.org>, mingo@redhat.com,
        ionela.voinescu@arm.com, vincent.guittot@linaro.org,
        rui.zhang@intel.com, qperret@google.com, daniel.lezcano@linaro.org,
        viresh.kumar@linaro.org, rostedt@goodmis.org, will@kernel.org,
        catalin.marinas@arm.com, sudeep.holla@arm.com,
        juri.lelli@redhat.com, corbet@lwn.net,
        linux-kernel@vger.kernel.org, amit.kachhap@gmail.com,
        javi.merino@kernel.org, amit.kucheria@verdurent.com
From:   Thara Gopinath <thara.gopinath@linaro.org>
Message-ID: <5E3DE7CC.3060300@linaro.org>
Date:   Fri, 7 Feb 2020 17:42:20 -0500
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101
 Thunderbird/38.5.1
MIME-Version: 1.0
In-Reply-To: <cc83634f-b3af-6024-7f89-9b231b153070@arm.com>
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 02/04/2020 03:39 AM, Dietmar Eggemann wrote:
> On 03/02/2020 16:55, Peter Zijlstra wrote:
>> On Mon, Feb 03, 2020 at 07:07:57AM -0500, Thara Gopinath wrote:
>>> On 01/28/2020 06:56 PM, Randy Dunlap wrote:
>>>> Hi,
>>>>
>>>> On 1/28/20 2:36 PM, Thara Gopinath wrote:
>>>>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>>>>> index e35b28e..be4147b 100644
>>>>> --- a/Documentation/admin-guide/kernel-parameters.txt
>>>>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>>>>> @@ -4376,6 +4376,11 @@
>>>>>  			incurs a small amount of overhead in the scheduler
>>>>>  			but is useful for debugging and performance tuning.
>>>>>  
>>>>> +	sched_thermal_decay_shift=
>>>>> +			[KNL, SMP] Set decay shift for thermal pressure signal.
>>>>> +			Format: integer between 0 and 10
>>>>> +			Default is 0.
>>>>> +
>>>>
>>>> That tells an admin [or any reader] almost nothing about this kernel parameter
>>>> or what it does.  And nothing about what unit the value is in.
>>>> Does the value 0 disable this feature?
>>>
>>> Thanks for the review. 0 does not disable "thermal pressure" feature. 0
>>> means the default decay period for averaging PELT signals (which is
>>> usually 32 but configurable) will also be applied for thermal pressure
>>> signal. A shift will shift the default decay period.
>>>
>>> You are right. It needs more explanation here. I will fix it and send v10.
>>
>> Or just send an update for this patch? I'm thinking most of this is
>> looking good.
> 
> I do agree. IMHO, there are just two little things outstanding:
> 
> (1) arch_scale_thermal_pressure() instead  of
>     arch_cpu_thermal_pressure() in v8 4/7

The "scale_" part was discussed in v6. Ionela had suggested that having
"scale" is not suited for this function because "thermal pressure" is
not exactly scaled but subtracted. I actually agree with that.

https://lore.kernel.org/lkml/20191223175005.GA31446@arm.com/

Having said that if everyone feel the same about naming of this
function, I can change it one last time.

> 
> (2) guarding of thermal pressure code in Arm's arch_topology driver  w/
>     CONFIG_HAVE_SCHED_THERMAL_PRESSURE plus disabling it by default for
>     Arm64.
It was enabled by default as per your suggestion in v9.

The patch can be dropped.

I don't understand the need to guard arch_topology with
CONFIG_HAVE_SCHED_THERMAL_PRESSURE. CONFIG_HAVE_SCHED_THERMAL_PRESSURE
is for scheduler to enable/disable averaging of thermal pressure. We
wanted to separate updating and retrieving of instantaneous thermal
pressure from scheduler. Guarding it with
CONFIG_HAVE_SCHED_THERMAL_PRESSURE is to me equivalent to putting back
this whole code in the scheduler framework. I am against it. I also do
not see other arch_ functions guarded similarly.

> 


-- 
Warm Regards
Thara