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References: <20140725233623.GA18537@linux.vnet.ibm.com> <53D2F211.4090509@gmail.com>
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From: Pranith Kumar <bobby.prani@gmail.com>
Date: Sat, 26 Jul 2014 21:51:52 -0400
Message-ID: <CAJhHMCCkS9_bO3PHc+cWSMw-eEZfnYVKdXyk6E5P5p7RbU7oLQ@mail.gmail.com>
Subject: Re: [PATCH RFC tip/core/rcu] Fix attempt to avoid offloading
 callbacks unless requested
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>,
        LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@kernel.org>,
        Lai Jiangshan <laijs@cn.fujitsu.com>,
        Dipankar Sarma <dipankar@in.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Josh Triplett <josh@joshtriplett.org>, tglx@linutronix.de,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        David Howells <dhowells@redhat.com>,
        Eric Dumazet <edumazet@google.com>, dvhart@linux.intel.com,
        Oleg Nesterov <oleg@redhat.com>, Sasha Levin <sasha.levin@oracle.com>
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Sender: linux-kernel-owner@vger.kernel.org

On Fri, Jul 25, 2014 at 10:30 PM, Frederic Weisbecker
<fweisbec@gmail.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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