Date: Tue, 9 Jul 2013 06:23:59 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com,
        dipankar@in.ibm.com, akpm@linux-foundation.org,
        mathieu.desnoyers@polymtl.ca, josh@joshtriplett.org, niv@us.ibm.com,
        tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com,
        edumazet@google.com, darren@dvhart.com, fweisbec@gmail.com,
        sbw@mit.edu
Subject: Re: [PATCH RFC nohz_full 2/7] nohz_full: Add rcu_dyntick data for
 scalable detection of all-idle state
Message-ID: <20130709132359.GF16780@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
References: <20130709012934.GA26058@linux.vnet.ibm.com>
 <1373333406-26979-1-git-send-email-paulmck@linux.vnet.ibm.com>
 <1373333406-26979-2-git-send-email-paulmck@linux.vnet.ibm.com>
 <20130709093728.GB17211@twins.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20130709093728.GB17211@twins.programming.kicks-ass.net>
User-Agent: Mutt/1.5.21 (2010-09-15)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2543
Lines: 48

On Tue, Jul 09, 2013 at 11:37:28AM +0200, Peter Zijlstra wrote:
> On Mon, Jul 08, 2013 at 06:30:01PM -0700, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> > 
> > This commit adds fields to the rcu_dyntick structure that are used to
> > detect idle CPUs.  These new fields differ from the existing ones in
> > that the existing ones consider a CPU executing in user mode to be idle,
> > where the new ones consider CPUs executing in user mode to be busy.
> > The handling of these new fields is otherwise quite similar to that for
> > the exiting fields.  This commit also adds the initialization required
> > for these fields.
> > 
> > So, why is usermode execution treated differently, with RCU considering
> > it a quiescent state equivalent to idle, while in contrast the new
> > full-system idle state detection considers usermode execution to be
> > non-idle?
> > 
> > It turns out that although one of RCU's quiescent states is usermode
> > execution, it is not a full-system idle state.  This is because the
> > purpose of the full-system idle state is not RCU, but rather determining
> > when accurate timekeeping can safely be disabled.  Whenever accurate
> > timekeeping is required in a CONFIG_NO_HZ_FULL kernel, at least one
> > CPU must keep the scheduling-clock tick going.  If even one CPU is
> > executing in user mode, accurate timekeeping is requires, particularly for
> > architectures where gettimeofday() and friends do not enter the kernel.
> > Only when all CPUs are really and truly idle can accurate timekeeping be
> > disabled, allowing all CPUs to turn off the scheduling clock interrupt,
> > thus greatly improving energy efficiency.
> > 
> > This naturally raises the question "Why is this code in RCU rather than in
> > timekeeping?", and the answer is that RCU has the data and infrastructure
> > to efficiently make this determination.
> 
> but but but but... why doesn't the regular nohz code qualify? I'd think
> that too would be tracking pretty much the same things, no?

The regular nohz code is identifying which CPUs are idle, but is doing
so on a CPU-by-CPU basis.  Before turning off system-wide timekeeping,
we need to know that -all- of the CPUs are idle.  The regular nohz code
does not do this.

							Thanx, Paul

--
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/