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

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Message-ID: <20140728210303.GB26017@localhost.localdomain>
Date:	Mon, 28 Jul 2014 23:03:06 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Pranith Kumar <bobby.prani@...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 Sat, Jul 26, 2014 at 09:51:52PM -0400, Pranith Kumar wrote:
> 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.

You're right, I missed the tick_nohz_full_running check :)

So if nohz_full is passed, we ignore CONFIG_NO_HZ_FULL_ALL. That looks like
the right behaviour though.

Paul what do you think? If we keep that behaviour, Maybe you could blindly do
rcu_nocb_mask |= tick_nohz_full and remove the CONFIG_NO_HZ_FULL_ALL dependency
on RCU_NOCB_ALL?
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