linux-kernel - Re: [PATCH 3/3] rcu: Drive configuration directly from SMP and PREEMPT

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Message-ID: <CAONaPpGf0Xw80mStLtx4pMfvOxKoBH0MNyrHwUyp9d4=EdAFPw@mail.gmail.com>
Date:	Tue, 22 Nov 2011 00:51:51 +0100
From:	John Kacur <jkacur@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	rt-users <linux-rt-users@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 3/3] rcu: Drive configuration directly from SMP and PREEMPT

On Tue, Nov 22, 2011 at 12:46 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Tue, 2011-11-22 at 00:18 +0100, John Kacur wrote:
>
>> Imagine you have a uni-processor system and want to do real-time.
>> According to the descriptions in init/Kconfig, you would want to
>> select TINY_PREEMPT_RCU. The description is
>>
>>         This option selects the RCU implementation that is designed
>>         for real-time UP systems.  This option greatly reduces the
>>         memory footprint of RCU.
>>
>> Without this patch, you cannot choose this option because of the &&
>> !PREEMPT_RT_FULL
>> So, that is a bug, and makes this patch appropriate for stable.
>
>
> 3.0-rt never supported rcutiny, which means adding it now is not a bug
> fix but a new feature. Sure, it may have been a mistake that Thomas kept
> rcutiny out for 3.0-rt, but because 3.0-rt never supported it, and
> 3.0-rt can fully support rcutree with !SMP, there is no bug to be fixed
> here. You're adding a new feature to stable, not fixing someones
> problem.
>
>>
>> I suppose if you want to be really conservative, you can say we only
>> need that third hunk.
>> However, this upstream patch, makes these options in v3.0-rt match the
>> options in v3.2-rc2-rt3.
>>
>> Furthermore, it doesn't seem like a good idea to me to support
>> configurations in a stable branch that are not supported upstream.
>
> No, upstream changed the game, in which the configuration can't be
> supported. But the stable release can fully support this configuration.
> there's no reason to this change just because the latest tree can't
> support it. It's like we have to deprecate everything in stable that has
> been deprecated in upstream.
>
> Stable is not a mirror of upstream, it's a snapshot in time. Only if we
> discover something that crashes, or causes huge latencies do we want to
> update stable. Not when we realized that a feature wasn't supported by
> -rt.
>

Okay, I get that and I agree with you - will try to be more careful
about that in the future. However, in this particular case, I don't
think there is a viable rcu choice for UP real-time. Perhaps we can
revisit this again in the future if anyone cares about it.

Thanks
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