linux-kernel - Re: [PATCH AUTOSEL for 4.14 015/161] printk: Add console owner and waiter logic to load balance console writes

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Message-ID: <276636c0-a62d-40b1-08d7-2ddf7b962044@mageia.org>
Date:   Thu, 19 Apr 2018 18:16:26 +0300
From:   Thomas Backlund <tmb@...eia.org>
To:     Greg KH <gregkh@...uxfoundation.org>, Jan Kara <jack@...e.cz>
CC:     Thomas Backlund <tmb@...eia.org>,
        Sasha Levin <Alexander.Levin@...rosoft.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Petr Mladek <pmladek@...e.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Byungchul Park <byungchul.park@....com>,
        Tejun Heo <tj@...nel.org>, Pavel Machek <pavel@....cz>
Subject: Re: [PATCH AUTOSEL for 4.14 015/161] printk: Add console owner and
 waiter logic to load balance console writes

Den 19.04.2018 kl. 17:22, skrev Greg KH:
> On Thu, Apr 19, 2018 at 04:05:45PM +0200, Jan Kara wrote:
>> On Thu 19-04-18 15:59:43, Greg KH wrote:
>>> On Thu, Apr 19, 2018 at 02:41:33PM +0300, Thomas Backlund wrote:
>>>> Den 16-04-2018 kl. 19:19, skrev Sasha Levin:
>>>>> On Mon, Apr 16, 2018 at 12:12:24PM -0400, Steven Rostedt wrote:
>>>>>> On Mon, 16 Apr 2018 16:02:03 +0000
>>>>>> Sasha Levin <Alexander.Levin@...rosoft.com> wrote:
>>>>>>
>>>>>>> One of the things Greg is pushing strongly for is "bug compatibility":
>>>>>>> we want the kernel to behave the same way between mainline and stable.
>>>>>>> If the code is broken, it should be broken in the same way.
>>>>>>
>>>>>> Wait! What does that mean? What's the purpose of stable if it is as
>>>>>> broken as mainline?
>>>>>
>>>>> This just means that if there is a fix that went in mainline, and the
>>>>> fix is broken somehow, we'd rather take the broken fix than not.
>>>>>
>>>>> In this scenario, *something* will be broken, it's just a matter of
>>>>> what. We'd rather have the same thing broken between mainline and
>>>>> stable.
>>>>>
>>>>
>>>> Yeah, but _intentionally_ breaking existing setups to stay "bug compatible"
>>>> _is_ a _regression_ you _really_ _dont_ want in a stable
>>>> supported distro. Because end-users dont care about upstream breaking
>>>> stuff... its the distro that takes the heat for that...
>>>>
>>>> Something "already broken" is not a regression...
>>>>
>>>> As distro maintainer that means one now have to review _every_ patch that
>>>> carries "AUTOSEL", follow all the mail threads that comes up about it, then
>>>> track if it landed in -stable queue, and read every response and possible
>>>> objection to all patches in the -stable queue a second time around... then
>>>> check if it still got included in final stable point relase and then either
>>>> revert them in distro kernel or go track down all the follow-up fixes
>>>> needed...
>>>>
>>>> Just to avoid being "bug compatible with master"
>>>
>>> I've done this "bug compatible" "breakage" more than the AUTOSEL stuff
>>> has in the past, so you had better also be reviewing all of my normal
>>> commits as well :)
>>>
>>> Anyway, we are trying not to do this, but it does, and will,
>>> occasionally happen.
>>
>> Sure, that's understood. So this was just misunderstanding. Sasha's
>> original comment really sounded like "bug compatibility" with current
>> master is desirable and that made me go "Are you serious?" as well...
> 
> As I said before in this thread, yes, sometimes I do this on purpose.
> 

And I guess this is the one that gets people the feeling that
"stable is not as stable as it used to be" ...

> As an specific example, see a recent bluetooth patch that caused a
> regression on some chromebook devices.  The chromeos developers
> rightfully complainied, and I left the commit in there to provide the
> needed "leverage" on the upstream developers to fix this properly.
> Otherwise if I had reverted the stable patch, when people move to a
> newer kernel version, things break, and no one remembers why.

I do understand what you are trying to do...

But from my distro hat on I have to treat things differently (and I dont 
think I'm alone doing it this way...)

Known breakages gets reverted even before it hits QA, so endusers wont 
see the issue at all...

So the only ones to see the issue are those building with latest 
upstream without own patches applied...

> 
> I also wrote a long response as to _why_ I do this, and even though it
> does happen, why it still is worth taking the stable updates.  Please
> see the archives for the full details.  I don't want to duplicate this
> again here.

And we do use latest stable (with some delay as I dont want to overload 
QA & endusers with a new kernel every week :))

We just revert known broken (or add known fixes) before releasing them 
to our users

--
Thomas