linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	linux-kernel@vger.kernel.org, mingo@elte.hu,
	laijs@cn.fujitsu.com, dipankar@in.ibm.com,
	akpm@linux-foundation.org, josh@joshtriplett.org, niv@us.ibm.com,
	tglx@linutronix.de, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu,
	dhowells@redhat.com, eric.dumazet@gmail.com, darren@dvhart.com,
	fweisbec@gmail.com, sbw@mit.edu, patches@linaro.org,
	"Paul E. McKenney" <paul.mckenney@linaro.org>
Subject: Re: [PATCH tip/core/rcu 23/23] rcu: Simplify quiescent-state detection
Date: Thu, 6 Sep 2012 17:18:59 -0400	[thread overview]
Message-ID: <20120906211858.GA26173@Krystal> (raw)
In-Reply-To: <20120906200138.GR2448@linux.vnet.ibm.com>

* Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> On Thu, Sep 06, 2012 at 04:36:02PM +0200, Peter Zijlstra wrote:
> > On Thu, 2012-08-30 at 11:18 -0700, Paul E. McKenney wrote:
> > > From: "Paul E. McKenney" <paul.mckenney@linaro.org>
> > > 
> > > The current quiescent-state detection algorithm is needlessly
> > > complex.
> > 
> > Heh! Be careful, we might be led into believing all this RCU is actually
> > really rather simple and this complexity is a bug on your end ;-)
> 
> Actually, the smallest "toy" implementation of RCU is only about 20
> lines of code -- and on a mythical sequentially consistent system, it
> would be smaller still.  Of course, the Linux kernel implementation if
> somewhat larger.  Something about wanting scalability above a few tens of
> CPUs, real-time response (also on huge numbers of CPUs), energy-efficient
> handling of dyntick-idle mode, detection of stalled CPUs, and so on.  ;-)
> 
> > >   It records the grace-period number corresponding to
> > > the quiescent state at the time of the quiescent state, which
> > > works, but it seems better to simply erase any record of previous
> > > quiescent states at the time that the CPU notices the new grace
> > > period.  This has the further advantage of removing another piece
> > > of RCU for which lockless reasoning is required. 
> > 
> > So why didn't you do that from the start? :-)
> 
> Because I was slow and stupid!  ;-)
> 
> > That is, I'm curious to know some history, why was it so and what led
> > you to this insight?
> 
> I had historically (as in for almost 20 years now) used a counter
> to track grace periods.  Now these are in theory subject to integer
> overflow, but DYNIX/ptx was non-preemptible, so the general line of
> reasoning was that anything that might stall long enough for even a 32-bit
> grace-period counter to overflow would necessarily stall grace periods,
> thus preventing overflow.
> 
> Of course, the advent of CONFIG_PREEMPT in the Linux kernel invalidated
> this assumption, but for most uses, if the grace-period counter overflows,
> you have waited way more than a grace period, so who cares?
> 
> Then combination of TREE_RCU and dyntick-idle came along, and it became
> necessary to more carefully associate quiescent states with the corresponding
> grace period.  Now here overflow is dangerous, because it can result in
> associating an ancient quiescent state with the current grace period.
> But my attitude was that if you have a task preempted for more than one
> year, getting soft-lockup warnings every two minutes during that time,
> well, you got what you deserved.  And even then at very low probability.
> 
> However, formal validation software (such as Promela) do not take kindly
> to free-running counters.  The usual trick is to use a much narrower
> counter.  But that would mean that any attempted mechanical validation
> would give a big fat false positive on the counter used to associate
> quiescent states with grace periods.  Because I have a long-term goal
> of formally validating RCU is it sits in the Linux kernel, that counter
> had to go.

I believe this approach bring the kernel RCU implementation slightly
closer to the userspace RCU implementation we use for 32-bit QSBR and
the 32/64-bit "urcu mb" variant for libraries, of which we've indeed
been able to make a complete formal model in Promela. Simplifying the
algorithm (mainly its state-space) in order to let formal verifiers cope
with it entirely has a lot of value I think: it lets us mechanically
verify safety and progress. A nice way to lessen the number of headaches
caused by RCU! ;-)

Thanks!

Mathieu

> 
> And I do believe that the result is easier for humans to understand, so
> it is all to the good.
> 
> This time, at least.  ;-)
> 
> 							Thanx, Paul
> 

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com

  reply	other threads:[~2012-09-06 21:19 UTC|newest]

Thread overview: 85+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-08-30 18:18 [PATCH tip/core/rcu 0/23] Improvements to RT response on big systems and expedited functions Paul E. McKenney
2012-08-30 18:18 ` [PATCH tip/core/rcu 01/23] rcu: Move RCU grace-period initialization into a kthread Paul E. McKenney
2012-08-30 18:18   ` [PATCH tip/core/rcu 02/23] rcu: Allow RCU grace-period initialization to be preempted Paul E. McKenney
2012-09-02  1:09     ` Josh Triplett
2012-09-05  1:22       ` Paul E. McKenney
2012-08-30 18:18   ` [PATCH tip/core/rcu 03/23] rcu: Move RCU grace-period cleanup into kthread Paul E. McKenney
2012-09-02  1:22     ` Josh Triplett
2012-09-06 13:34     ` Peter Zijlstra
2012-09-06 17:29       ` Paul E. McKenney
2012-08-30 18:18   ` [PATCH tip/core/rcu 04/23] rcu: Allow RCU grace-period cleanup to be preempted Paul E. McKenney
2012-09-02  1:36     ` Josh Triplett
2012-08-30 18:18   ` [PATCH tip/core/rcu 05/23] rcu: Prevent offline CPUs from executing RCU core code Paul E. McKenney
2012-09-02  1:45     ` Josh Triplett
2012-08-30 18:18   ` [PATCH tip/core/rcu 06/23] rcu: Break up rcu_gp_kthread() into subfunctions Paul E. McKenney
2012-09-02  2:11     ` Josh Triplett
2012-09-06 13:39     ` Peter Zijlstra
2012-09-06 17:32       ` Paul E. McKenney
2012-09-06 18:49         ` Josh Triplett
2012-09-06 19:09           ` Peter Zijlstra
2012-09-06 20:30             ` Paul E. McKenney
2012-09-06 20:30           ` Paul E. McKenney
2012-08-30 18:18   ` [PATCH tip/core/rcu 07/23] rcu: Provide OOM handler to motivate lazy RCU callbacks Paul E. McKenney
2012-09-02  2:13     ` Josh Triplett
2012-09-03  9:08     ` Lai Jiangshan
2012-09-05 17:45       ` Paul E. McKenney
2012-09-06 13:46     ` Peter Zijlstra
2012-09-06 13:52       ` Steven Rostedt
2012-09-06 17:41         ` Paul E. McKenney
2012-09-06 17:46           ` Peter Zijlstra
2012-09-06 20:32             ` Paul E. McKenney
2012-08-30 18:18   ` [PATCH tip/core/rcu 08/23] rcu: Segregate rcu_state fields to improve cache locality Paul E. McKenney
2012-09-02  2:51     ` Josh Triplett
2012-08-30 18:18   ` [PATCH tip/core/rcu 09/23] rcu: Move quiescent-state forcing into kthread Paul E. McKenney
2012-08-30 18:18   ` [PATCH tip/core/rcu 10/23] rcu: Allow RCU quiescent-state forcing to be preempted Paul E. McKenney
2012-09-02  5:23     ` Josh Triplett
2012-08-30 18:18   ` [PATCH tip/core/rcu 11/23] rcu: Adjust debugfs tracing for kthread-based quiescent-state forcing Paul E. McKenney
2012-09-02  6:05     ` Josh Triplett
2012-08-30 18:18   ` [PATCH tip/core/rcu 12/23] rcu: Prevent force_quiescent_state() memory contention Paul E. McKenney
2012-09-02 10:47     ` Josh Triplett
2012-08-30 18:18   ` [PATCH tip/core/rcu 13/23] rcu: Control grace-period duration from sysfs Paul E. McKenney
2012-09-03  9:30     ` Josh Triplett
2012-09-03  9:31       ` Josh Triplett
2012-09-06 14:15     ` Peter Zijlstra
2012-09-06 17:53       ` Paul E. McKenney
2012-09-06 18:28         ` Peter Zijlstra
2012-09-06 20:37           ` Paul E. McKenney
2012-08-30 18:18   ` [PATCH tip/core/rcu 14/23] rcu: Remove now-unused rcu_state fields Paul E. McKenney
2012-09-03  9:31     ` Josh Triplett
2012-09-06 14:17     ` Peter Zijlstra
2012-09-06 18:02       ` Paul E. McKenney
2012-08-30 18:18   ` [PATCH tip/core/rcu 15/23] rcu: Make rcutree module parameters visible in sysfs Paul E. McKenney
2012-09-03  9:32     ` Josh Triplett
2012-08-30 18:18   ` [PATCH tip/core/rcu 16/23] rcu: Prevent initialization-time quiescent-state race Paul E. McKenney
2012-09-03  9:37     ` Josh Triplett
2012-09-05 18:19       ` Paul E. McKenney
2012-09-05 18:55         ` Josh Triplett
2012-09-05 19:49           ` Paul E. McKenney
2012-09-06 14:21         ` Peter Zijlstra
2012-09-06 16:18           ` Paul E. McKenney
2012-09-06 16:22             ` Peter Zijlstra
2012-08-30 18:18   ` [PATCH tip/core/rcu 17/23] rcu: Fix day-zero grace-period initialization/cleanup race Paul E. McKenney
2012-09-03  9:39     ` Josh Triplett
2012-09-06 14:24     ` Peter Zijlstra
2012-09-06 18:06       ` Paul E. McKenney
2012-08-30 18:18   ` [PATCH tip/core/rcu 18/23] rcu: Add random PROVE_RCU_DELAY to grace-period initialization Paul E. McKenney
2012-09-03  9:41     ` Josh Triplett
2012-09-06 14:27     ` Peter Zijlstra
2012-09-06 18:25       ` Paul E. McKenney
2012-08-30 18:18   ` [PATCH tip/core/rcu 19/23] rcu: Adjust for unconditional ->completed assignment Paul E. McKenney
2012-09-03  9:42     ` Josh Triplett
2012-08-30 18:18   ` [PATCH tip/core/rcu 20/23] rcu: Remove callback acceleration from grace-period initialization Paul E. McKenney
2012-09-03  9:42     ` Josh Triplett
2012-08-30 18:18   ` [PATCH tip/core/rcu 21/23] rcu: Eliminate signed overflow in synchronize_rcu_expedited() Paul E. McKenney
2012-09-03  9:43     ` Josh Triplett
2012-08-30 18:18   ` [PATCH tip/core/rcu 22/23] rcu: Reduce synchronize_rcu_expedited() latency Paul E. McKenney
2012-09-03  9:46     ` Josh Triplett
2012-08-30 18:18   ` [PATCH tip/core/rcu 23/23] rcu: Simplify quiescent-state detection Paul E. McKenney
2012-09-03  9:56     ` Josh Triplett
2012-09-06 14:36     ` Peter Zijlstra
2012-09-06 20:01       ` Paul E. McKenney
2012-09-06 21:18         ` Mathieu Desnoyers [this message]
2012-09-06 21:31           ` Paul E. McKenney
2012-09-02  1:04   ` [PATCH tip/core/rcu 01/23] rcu: Move RCU grace-period initialization into a kthread Josh Triplett
2012-09-06 13:32   ` Peter Zijlstra
2012-09-06 17:00     ` Paul E. McKenney

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20120906211858.GA26173@Krystal \
    --to=mathieu.desnoyers@polymtl.ca \
    --cc=Valdis.Kletnieks@vt.edu \
    --cc=akpm@linux-foundation.org \
    --cc=darren@dvhart.com \
    --cc=dhowells@redhat.com \
    --cc=dipankar@in.ibm.com \
    --cc=eric.dumazet@gmail.com \
    --cc=fweisbec@gmail.com \
    --cc=josh@joshtriplett.org \
    --cc=laijs@cn.fujitsu.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mingo@elte.hu \
    --cc=niv@us.ibm.com \
    --cc=patches@linaro.org \
    --cc=paul.mckenney@linaro.org \
    --cc=paulmck@linux.vnet.ibm.com \
    --cc=peterz@infradead.org \
    --cc=rostedt@goodmis.org \
    --cc=sbw@mit.edu \
    --cc=tglx@linutronix.de \
    --subject='Re: [PATCH tip/core/rcu 23/23] rcu: Simplify quiescent-state detection' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).