From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Pranith Kumar <bobby.prani@gmail.com>
Cc: Josh Triplett <josh@joshtriplett.org>,
Steven Rostedt <rostedt@goodmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Lai Jiangshan <laijs@cn.fujitsu.com>,
"open list:READ-COPY UPDATE..." <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 11/16] rcu: Check for spurious wakeup using return value
Date: Thu, 24 Jul 2014 11:12:23 -0700 [thread overview]
Message-ID: <20140724181223.GO11241@linux.vnet.ibm.com> (raw)
In-Reply-To: <CAJhHMCDhwnKF5tiWvHyGrz3KhCab+QUnAfEmkzjsBrNn5O3gyQ@mail.gmail.com>
On Thu, Jul 24, 2014 at 12:03:34AM -0400, Pranith Kumar wrote:
> On Wed, Jul 23, 2014 at 11:43 PM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
> > On Wed, Jul 23, 2014 at 10:36:19PM -0400, Pranith Kumar wrote:
> >> On Wed, Jul 23, 2014 at 8:26 AM, Paul E. McKenney
> >> <paulmck@linux.vnet.ibm.com> wrote:
> >> > On Wed, Jul 23, 2014 at 01:09:48AM -0400, Pranith Kumar wrote:
> >> >> When the gp_kthread wakes up from the wait event, it returns 0 if the wake up is
> >> >> due to the condition having been met. This commit checks this return value
> >> >> for a spurious wake up before calling rcu_gp_init().
> >> >>
> >> >> Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
> >> >
> >> > How does this added check help? I don't see that it does. If the flag
> >> > is set, we want to wake up. If we get a spurious wakeup, but then the
> >> > flag gets set before we actually wake up, we still want to wake up.
> >>
> >> So I took a look at the docs again, and using the return value is the
> >> recommended way to check for spurious wakeups.
> >>
> >> The condition in wait_event_interruptible() is checked when the task
> >> is woken up (either due to stray signals or explicitly) and it returns
> >> true if condition evaluates to true.
>
> this should be returns '0' if the condition evaluates to true.
Ah, but if the condition changes while wait_event_interruptible() is in
the process of returning, it is quite possible that the answer will be
different afterwards. For example, consider this scenario:
o wait_event_interruptible() decides to return spuriously for
whatever reason, and thus returns a non-zero value.
o An invocation of (say) rcu_start_gp_advanced() sets ->gp_flags
to RCU_GP_FLAG_INIT, thus requesting that a new grace period
start.
o At this point, the return value says that we should not start
a new grace period, but the ->gp_flags value says that we
should.
Because it is the ->gp_flags value that really knows, the current code
ignores wait_event_interruptible()'s return value and just checks
the ->gp_flags value.
> >> In the current scenario, if we get a spurious wakeup, we take the
> >> costly path of checking this condition again (with a barrier and lock)
> >> before going back to wait.
> >>
> >> The scenario of getting an actual wakeup after getting a spurious
> >> wakeup exists even today, this is the window after detecting a
> >> spurious wakeup and before going back to wait. I am not sure if using
> >> the return value enlarges that window as we are going back to sleep
> >> immediately.
> >>
> >> Thoughts?
> >
> > If the flag is set, why should we care whether or not the wakeup was
> > spurious? If the flag is not set, why should we care whether or not
> > wait_event_interruptible() thought that the wakeup was not spurious?
>
> A correction about the return value above: return will be 0 if the
> condition is true, in this case if the flag is set.
>
> If the flag is set, ret will be 0 and we will go ahead with
> rcu_gp_init(). (no change wrt current behavior)
Sorry, this is not always correct. RCU is highly concurrent, so you do
need to start thinking in terms of concurrency. Your reasoning above
is a symptom of single-threaded thinking. Please see my scenario above
showing how the return can be non-zero even though ->gp_flags is set.
> If the flag is not set, currently we go ahead and call rcu_gp_init()
> from where we check if the flag is set (after a lock+barrier) and
> return.
True enough. Is that really a problem? If so, exactly why is it a
problem?
> If we care about what wait_event_interruptible() returns, we can go
> back and wait for an actual wakeup much earlier without the additional
> overhead of calling rcu_gp_init().
The key phrase here is "If we care". Should we care? If so, why?
I suggest running some random benchmark and counting how many times
rcu_gp_init() is called and how many times rcu_gp_init() returns
because ->gp_flags is not set. If rcu_gp_init() returns because
->gp_flags is not set a significant fraction of the time, then this
-might- be worth worrying about. (Extra credit: Under what conditions
would it be worth worrying about, and how would you go about checking
to see whether those conditions hold?)
Thanx, Paul
next prev parent reply other threads:[~2014-07-24 18:12 UTC|newest]
Thread overview: 58+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-07-23 5:09 [PATCH 00/16] rcu: Some minor fixes and cleanups Pranith Kumar
2014-07-23 5:09 ` [PATCH 01/16] rcu: Use rcu_num_nodes instead of NUM_RCU_NODES Pranith Kumar
2014-07-23 5:09 ` [PATCH 02/16] rcu: Check return value for cpumask allocation Pranith Kumar
2014-07-23 12:06 ` Paul E. McKenney
2014-07-23 12:49 ` Pranith Kumar
2014-07-23 17:14 ` Pranith Kumar
2014-07-23 18:01 ` Paul E. McKenney
2014-07-23 5:09 ` [PATCH 03/16] rcu: Fix comment for gp_state field values Pranith Kumar
2014-07-23 5:09 ` [PATCH 04/16] rcu: Remove redundant check for an online CPU Pranith Kumar
2014-07-23 12:09 ` Paul E. McKenney
2014-07-23 13:23 ` Pranith Kumar
2014-07-23 13:41 ` Paul E. McKenney
2014-07-23 14:01 ` Pranith Kumar
2014-07-23 14:14 ` Paul E. McKenney
2014-07-23 15:07 ` Pranith Kumar
2014-07-23 15:21 ` Pranith Kumar
2014-07-23 5:09 ` [PATCH 05/16] rcu: Add noreturn attribute to boost kthread Pranith Kumar
2014-07-23 5:09 ` [PATCH 06/16] rcu: Clear gp_flags only when actually starting new gp Pranith Kumar
2014-07-23 12:13 ` Paul E. McKenney
2014-07-23 5:09 ` [PATCH 07/16] rcu: Save and restore irq flags in rcu_gp_cleanup() Pranith Kumar
2014-07-23 12:16 ` Paul E. McKenney
2014-07-23 5:09 ` [PATCH 08/16] rcu: Clean up rcu_spawn_one_boost_kthread() Pranith Kumar
2014-07-23 5:09 ` [PATCH 09/16] rcu: Remove redundant check for online cpu Pranith Kumar
2014-07-23 12:21 ` Paul E. McKenney
2014-07-23 12:59 ` Pranith Kumar
2014-07-23 13:50 ` Paul E. McKenney
2014-07-23 14:12 ` Pranith Kumar
2014-07-23 14:23 ` Paul E. McKenney
2014-07-23 15:11 ` Pranith Kumar
2014-07-23 15:30 ` Paul E. McKenney
2014-07-23 15:44 ` Pranith Kumar
2014-07-23 19:15 ` Paul E. McKenney
2014-07-23 20:01 ` Pranith Kumar
2014-07-23 20:16 ` Pranith Kumar
2014-07-23 20:23 ` Paul E. McKenney
2014-07-23 5:09 ` [PATCH 10/16] rcu: Check for RCU_FLAG_GP_INIT bit in gp_flags for spurious wakeup Pranith Kumar
2014-07-23 12:23 ` Paul E. McKenney
2014-07-23 5:09 ` [PATCH 11/16] rcu: Check for spurious wakeup using return value Pranith Kumar
2014-07-23 12:26 ` Paul E. McKenney
2014-07-24 2:36 ` Pranith Kumar
2014-07-24 3:43 ` Paul E. McKenney
2014-07-24 4:03 ` Pranith Kumar
2014-07-24 18:12 ` Paul E. McKenney [this message]
2014-07-24 19:59 ` Pranith Kumar
2014-07-24 20:27 ` Paul E. McKenney
2014-07-23 5:09 ` [PATCH 12/16] rcu: Rename rcu_spawn_gp_kthread() to rcu_spawn_kthreads() Pranith Kumar
2014-07-23 5:09 ` [PATCH 13/16] rcu: Spawn nocb kthreads from rcu_prepare_kthreads() Pranith Kumar
2014-07-23 5:09 ` [PATCH 14/16] rcu: Remove redundant checks for rcu_scheduler_fully_active Pranith Kumar
2014-07-23 12:27 ` Paul E. McKenney
2014-07-23 5:09 ` [PATCH 15/16] rcu: Check for a nocb cpu before trying to spawn nocb threads Pranith Kumar
2014-07-23 12:28 ` Paul E. McKenney
2014-07-23 13:14 ` Pranith Kumar
2014-07-23 13:42 ` Paul E. McKenney
2014-07-23 5:09 ` [PATCH 16/16] rcu: kvm.sh: Fix error when you pass --cpus argument Pranith Kumar
2014-07-23 12:31 ` Paul E. McKenney
2014-07-23 14:45 ` [PATCH 00/16] rcu: Some minor fixes and cleanups Paul E. McKenney
2014-08-27 1:10 ` Pranith Kumar
2014-08-27 3:20 ` Paul E. McKenney
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