linux-kernel - Re: [PATCH] memory/samsung: reduce unnecessary mutex lock area

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Message-ID: <fd463b97-6db7-f7ba-c4bd-5c709a4898c0@arm.com>
Date:   Tue, 12 May 2020 10:23:03 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     Krzysztof Kozlowski <krzk@...nel.org>
Cc:     Bernard Zhao <bernard@...o.com>, Kukjin Kim <kgene@...nel.org>,
        linux-pm@...r.kernel.org,
        "linux-samsung-soc@...r.kernel.org" 
        <linux-samsung-soc@...r.kernel.org>,
        linux-arm-kernel@...ts.infradead.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        opensource.kernel@...o.com
Subject: Re: [PATCH] memory/samsung: reduce unnecessary mutex lock area



On 5/12/20 10:05 AM, Krzysztof Kozlowski wrote:
> On Tue, 12 May 2020 at 10:47, Lukasz Luba <lukasz.luba@....com> wrote:
>>
>> Hi Krzysztof,
>>
>> I am sorry, I was a bit busy recently.
>>
>> On 5/12/20 7:50 AM, Krzysztof Kozlowski wrote:
>>> On Fri, May 08, 2020 at 06:13:38AM -0700, Bernard Zhao wrote:
>>>> Maybe dmc->df->lock is unnecessary to protect function
>>>> exynos5_dmc_perf_events_check(dmc). If we have to protect,
>>>> dmc->lock is more better and more effective.
>>>> Also, it seems not needed to protect "if (ret) & dev_warn"
>>>> branch.
>>>>
>>>> Signed-off-by: Bernard Zhao <bernard@...o.com>
>>>> ---
>>>>    drivers/memory/samsung/exynos5422-dmc.c | 6 ++----
>>>>    1 file changed, 2 insertions(+), 4 deletions(-)
>>>
>>> I checked the concurrent accesses and it looks correct.
>>>
>>> Lukasz, any review from your side?
>>
>> The lock from devfreq lock protects from a scenario when
>> concurrent access from devfreq framework uses internal dmc fields 'load'
>> and 'total' (which are set to 'busy_time', 'total_time').
>> The .get_dev_status can be called at any time (even due to thermal
>> devfreq cooling action) and reads above fields.
>> That's why the calculation of the new values inside dmc is protected.
> 
> I looked at this path (get_dev_status) and currently in devfreq it
> will be only called from update_devfreq() -> get_target_freq()... at
> least when looking at devfreq core and governors. On the other hand
> you are right that this is public function and this call scenario
> might change. It could be called directly from other paths sooner or
> later.

Indeed, I am currently changing this while I am adding devfreq devices
to the Energy Model.

> 
>> This patch should not be taken IMO. Maybe we can release lock before the
>> if statement, just to speed-up.
> 
> Yep.
> 
> Bernard, you can send just this part of the patch.

Thank you Bernard and please submit the patch v2.

> 
> Best regards,
> Krzysztof
> 

Thank you Krzysztof for your time spent on this.


Regards,
Lukasz