Date: Fri, 11 Jul 2014 20:25:43 +0200
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Christoph Lameter <cl@gentwo.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        linux-kernel@vger.kernel.org, mingo@kernel.org, laijs@cn.fujitsu.com,
        dipankar@in.ibm.com, akpm@linux-foundation.org,
        mathieu.desnoyers@efficios.com, josh@joshtriplett.org, niv@us.ibm.com,
        tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org,
        dhowells@redhat.com, edumazet@google.com, dvhart@linux.intel.com,
        oleg@redhat.com, sbw@mit.edu
Subject: Re: [PATCH tip/core/rcu 11/17] rcu: Bind grace-period kthreads to
 non-NO_HZ_FULL CPUs
Message-ID: <20140711182541.GF26045@localhost.localdomain>
References: <20140707223756.GA7187@linux.vnet.ibm.com>
 <1404772701-8804-1-git-send-email-paulmck@linux.vnet.ibm.com>
 <1404772701-8804-11-git-send-email-paulmck@linux.vnet.ibm.com>
 <20140708152358.GF6571@localhost.localdomain>
 <20140708154723.GN4603@linux.vnet.ibm.com>
 <20140708183846.GJ6571@localhost.localdomain>
 <alpine.DEB.2.11.1407111309150.4777@gentwo.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <alpine.DEB.2.11.1407111309150.4777@gentwo.org>
User-Agent: Mutt/1.5.21 (2010-09-15)
Sender: linux-kernel-owner@vger.kernel.org

On Fri, Jul 11, 2014 at 01:10:41PM -0500, Christoph Lameter wrote:
> On Tue, 8 Jul 2014, Frederic Weisbecker wrote:
> 
> > > I was figuring that a fair number of the kthreads might eventually
> > > be using this, not just for the grace-period kthreads.
> >
> > Ok makes sense. But can we just rename the cpumask to housekeeping_mask?
> 
> That would imply that all no-nohz processors are housekeeping? So all
> processors with a tick are housekeeping?

Well, now that I think about it again, I would really like to keep housekeeping
to CPU 0 when nohz_full= is passed.

> 
> Could we make that set configurable? Ideally I'd like to have the ability
> restrict the housekeeping to one processor.

Ah, I'm curious about your usecase. But I think we can do that. And we should.

In fact I think that Paul could keep affining grace period kthread to CPU 0
for the sole case when we have nohz_full= parameter passed.

I think the performance issues reported to him refer to CONFIG_NO_HZ_FULL=y
config without nohz_full= parameter passed. That's the most important to address.

Optimizing the "nohz_full= passed" case is probably not very useful and worse
it complicate things a lot.

What do you think Paul? Can we simplify things that way? I'm pretty sure that
nobody cares about optimizing the nohz_full= case. That would really simplify
things to stick to CPU 0.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/