linux-kernel - Re: [PATCH RFC tip/core/rcu] Fix attempt to avoid offloading callbacks unless requested

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Date:	Sat, 26 Jul 2014 21:51:52 -0400
From:	Pranith Kumar <bobby.prani@...il.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Josh Triplett <josh@...htriplett.org>, tglx@...utronix.de,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	David Howells <dhowells@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>, dvhart@...ux.intel.com,
	Oleg Nesterov <oleg@...hat.com>,
	Sasha Levin <sasha.levin@...cle.com>
Subject: Re: [PATCH RFC tip/core/rcu] Fix attempt to avoid offloading
 callbacks unless requested

On Fri, Jul 25, 2014 at 10:30 PM, Frederic Weisbecker
<fweisbec@...il.com> wrote:

>> I understand that if CONFIG_NO_HZ_FULL_ALL is set then CONFIG_NOCB_CPU_ALL
>> will also be set and there is no need for this cpumask_or().
>>
>> Is there any reason for the coupling between CONFIG_NO_HZ_FULL_ALL
>> and CONFIG_NOCB_CPU_ALL?
>
> Yeah, for any nohz full CPU, we need the corresponding CPU to be rcu_nocb.
> So if all CPUs are full dynticks, all CPUs must be rcunocb.
>
> That said with this patch, the dependency is perhaps not needed anymore.
>
>>
>> I ask because a user can override CONFIG_NO_HZ_FULL_ALL=y at boot time
>> using the nohz_full= boot time parameter.
>
> No, the content of nohz_full= is ignored with CONFIG_NO_HZ_FULL_ALL=y.
>

Please correct me if I am wrong but that does not seem to be the case.
If a boot parameter is passed, we are setting up tick_nohz_full_mask
from tick_nohz_full_setup() and marking tick_nohz_full_running as true.
Later on we check this flag and skip the CONFIG_NO_HZ_FULL_ALL
initialization.

> That said you made me check and I realize that when that happens, we alloc
> the mask two times and we leak the first. I need to fix that.

This does not actually happen as we do the initialization only once.
Am I missing something?

>
> Thanks.


-- 
Pranith
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