linux-kernel - Re: [PATCH v2 3/3] PM / EM: Skip inefficient OPPs

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Message-ID: <f8a2bbde-47c4-bd7d-96fa-228c9d9e2779@arm.com>
Date:   Tue, 25 May 2021 12:03:14 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     Vincent Donnefort <vincent.donnefort@....com>,
        Quentin Perret <qperret@...gle.com>
Cc:     peterz@...radead.org, rjw@...ysocki.net, viresh.kumar@...aro.org,
        vincent.guittot@...aro.org, linux-kernel@...r.kernel.org,
        ionela.voinescu@....com, dietmar.eggemann@....com
Subject: Re: [PATCH v2 3/3] PM / EM: Skip inefficient OPPs



On 5/25/21 10:46 AM, Vincent Donnefort wrote:
> On Tue, May 25, 2021 at 09:33:01AM +0000, Quentin Perret wrote:
>> On Friday 21 May 2021 at 17:54:24 (+0100), Vincent Donnefort wrote:
>>> @@ -161,6 +162,8 @@ static int em_create_perf_table(struct device *dev, struct em_perf_domain *pd,
>>>   		table[i].cost = div64_u64(fmax * table[i].power,
>>>   					  table[i].frequency);
>>>   		if (table[i].cost >= prev_cost) {
>>> +			table[i].flags = EM_PERF_STATE_INEFFICIENT;
>>> +			pd->flags |= EM_PERF_DOMAIN_INEFFICIENCIES;
>>
>> If we're looking for micro-optimizations, then perhaps you could store
>> the index of the next efficient OPP (which would be 'i' if the current
>> OPP is already efficient), so you can jump to it directly when doing the
>> search.
> 
> Wouldn't add any new field compared to this version so yeah might be an
> interesting improvement.
> 

That's a few more instructions to parse the 'flags' filed. I'm not sure
if that brings speed improvements vs. if we not parse and have bool
filed with a simple looping. The out-of-order core might even suffer
from this parsing and loop index manipulations...