linux-kernel - Re: [PATCH] cpufreq, store_scaling_governor requires policy->rwsem to be held for duration of changing governors [v2]

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Message-ID: <53DAA749.5080506@redhat.com>
Date:	Thu, 31 Jul 2014 16:30:01 -0400
From:	Prarit Bhargava <prarit@...hat.com>
To:	Saravana Kannan <skannan@...eaurora.org>
CC:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	linux-kernel@...r.kernel.org,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Lenny Szubowicz <lszubowi@...hat.com>, linux-pm@...r.kernel.org
Subject: Re: [PATCH] cpufreq, store_scaling_governor requires policy->rwsem
 to be held for duration of changing governors [v2]



On 07/31/2014 04:24 PM, Saravana Kannan wrote:
> On 07/31/2014 11:26 AM, Prarit Bhargava wrote:
>>
>>
>> On 07/31/2014 02:38 PM, Rafael J. Wysocki wrote:
>>> On Thursday, July 31, 2014 01:57:29 PM Prarit Bhargava wrote:
>>>>
>>>> On 07/31/2014 12:36 PM, Rafael J. Wysocki wrote:
>>>>> On Thursday, July 31, 2014 06:23:18 AM Prarit Bhargava wrote:
>>>>>>
>>>>>> On 07/30/2014 10:16 PM, Rafael J. Wysocki wrote:
>>>>>>> On Wednesday, July 30, 2014 06:36:00 PM Saravana Kannan wrote:
>>>>>>>> On 07/30/2014 02:40 PM, Rafael J. Wysocki wrote:
>>>>>>>>> On Wednesday, July 30, 2014 10:18:25 AM Prarit Bhargava wrote:
>>>>>>>>>>
>>>>>>>>>> On 07/29/2014 08:03 PM, Rafael J. Wysocki wrote:
>>>>>>>>>>> On Tuesday, July 29, 2014 07:46:02 AM Prarit Bhargava wrote:
>>>>>>>>>
>>>>>>>>> [cut]
>>>>>>>>>
>>>>>>>>>>>> This patch effectively reverts commit 955ef483.
>>>>>>>>
>>>>>>>> The issue reported in this patch is valid. We are seeing that internally
>>>>>>>> too. I believe I reported it in another thread (within the past month).
>>>>>>>>
>>>>>>>> However, the original patch fixes a real deadlock issue (I'm too tired
>>>>>>>> to look it up now). We can revet the original, but it's going to bring
>>>>>>>> back the original issue. I just want to make sure Prarit and Raphael
>>>>>>>> realize this before proceeding.
>>>>>>>>
>>>>>>>> I do have plans for a proper fix for the mainline (not stable branches),
>>>>>>>> but plan to do that after the current set of suspend/hotplug patches go
>>>>>>>> through. The fix would be easier to make after that.
>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> OK, I'm convinced by this.
>>>>>>>>>>>
>>>>>>>>>>> I suppose we should push it for -stable from 3.10 through 3.15.x, right?
>>>>>>>>>>
>>>>>>>>>> Rafael, I think that is a good idea.  I'm not sure what the protocol
>>>>>>>>>> is for
>>>>>>>>>> adding stable@...nel.org though ...
>>>>>>
>>>>>> Rafael, let me (again) re-write the patch description.  I think Saravana has
>>>>>> raised an important issue that I have not clearly identified why it is
>>>>>> safe to
>>>>>> remove this code in my patch description.  Also, I want to clearly
>>>>>> identify the
>>>>>> appropriate -stable releases to push this out to.
>>>>>>
>>>>>> I'll submit a [v3] later today or tomorrow.
>>>>>
>>>>> In any case that's too late for 3.16 final, unless there's an -rc8.
>>>>>
>>>>> Thanks for doing that work!
>>>>
>>>> Ugh ... I tried this (yet another) large system and hit another panic :(.
>>>>
>>>> I'm investigating now, and I'm hoping this is just something "new".
>>>
>>> Well, I've applied your patch as is and I can push it to Linus.
>>>
>>> However, if you want to update the changelog, I'll not do that, but in that
>>> case the patch will have to wait for the next week.
>>
>> Rafael, please let it wait for next week.  I _need_ to make sure this is correct
>> and I'd rather not pushed something half-done.
>>
> 
> Prarit,
> 
> I'm not an expert on sysfs locking, but I would think the specific sysfs lock
> would depend on the file/attribute group. So, can you please try to hotplug a
> core in/out (to trigger the POLICY_EXIT) and then read a sysfs file exported by
> the governor? scaling_governor doesn't cut it since that file is not removed on
> policy exit event to governor. If it's ondemand, try reading/write it's sampling
> rate file.

Thanks Saravana -- will do.  I will get back to you shortly on this.

P.

> 
> -Saravana
> 
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