Date: Tue, 29 Jul 2014 19:33:32 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: 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,
        tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com,
        edumazet@google.com, dvhart@linux.intel.com, fweisbec@gmail.com,
        oleg@redhat.com, bobby.prani@gmail.com
Subject: Re: [PATCH RFC tip/core/rcu 1/9] rcu: Add call_rcu_tasks()
Message-ID: <20140729173332.GM3935@laptop>
References: <20140728225556.GA19493@linux.vnet.ibm.com>
 <1406588180-21933-1-git-send-email-paulmck@linux.vnet.ibm.com>
 <20140729081416.GR20603@laptop.programming.kicks-ass.net>
 <20140729172304.GA16073@linux.vnet.ibm.com>
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On Tue, Jul 29, 2014 at 10:23:04AM -0700, Paul E. McKenney wrote:
> On Tue, Jul 29, 2014 at 10:14:16AM +0200, Peter Zijlstra wrote:
> > On Mon, Jul 28, 2014 at 03:56:12PM -0700, Paul E. McKenney wrote:
> > > @@ -254,6 +254,8 @@ void rcu_check_callbacks(int cpu, int user)
> > >  		rcu_sched_qs(cpu);
> > >  	else if (!in_softirq())
> > >  		rcu_bh_qs(cpu);
> > > +	if (user)
> > > +		rcu_note_voluntary_context_switch(current);
> > >  }
> > 
> > There's nothing like sending email you can't find something... :-)
> 
> Well, this is unfortunately only a partial solution.  It does not handle
> the NO_HZ_FULL scheduling-clock-free usermode execution.  I have ink on
> paper indicating a couple of ways to do that, but figured I should get
> feedback on this stuff before going too much farther.

Yah, so the nohz_full already has the horrid overhead of user<->kernel
switches, so you can 'trivially' hook into those.

FWIW its _the_ thing that makes nohz_full uninteresting for me. The
required overhead is insane. But yes there are people willing to pay
that etc..
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