From: Frederic Weisbecker <fweisbec@gmail.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
linaro-sched-sig@lists.linaro.org,
Alessio Igor Bogani <abogani@kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
Avi Kivity <avi@redhat.com>, Chris Metcalf <cmetcalf@tilera.com>,
Christoph Lameter <cl@linux.com>,
Daniel Lezcano <daniel.lezcano@linaro.org>,
Geoff Levand <geoff@infradead.org>,
Gilad Ben Yossef <gilad@benyossef.com>,
Hakan Akkan <hakanakkan@gmail.com>,
Ingo Molnar <mingo@kernel.org>, Kevin Hilman <khilman@ti.com>,
Max Krasnyansky <maxk@qualcomm.com>,
Peter Zijlstra <peterz@infradead.org>,
Stephen Hemminger <shemminger@vyatta.com>,
Steven Rostedt <rostedt@goodmis.org>,
Sven-Thorsten Dietrich <thebigcorporation@gmail.com>,
Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH 17/41] rcu: Restart tick if we enqueue a callback in a nohz/cpuset CPU
Date: Thu, 31 May 2012 17:56:30 +0200 [thread overview]
Message-ID: <20120531155626.GC27841@somewhere.redhat.com> (raw)
In-Reply-To: <20120523161514.GD2402@linux.vnet.ibm.com>
On Wed, May 23, 2012 at 09:15:14AM -0700, Paul E. McKenney wrote:
> On Wed, May 23, 2012 at 04:03:36PM +0200, Frederic Weisbecker wrote:
> > On Tue, May 22, 2012 at 10:30:47AM -0700, Paul E. McKenney wrote:
> > > On Tue, May 22, 2012 at 10:27:14AM -0700, Paul E. McKenney wrote:
> > > > On Tue, May 01, 2012 at 01:54:51AM +0200, Frederic Weisbecker wrote:
> > > > > If we enqueue an rcu callback, we need the CPU tick to stay
> > > > > alive until we take care of those by completing the appropriate
> > > > > grace period.
> > > > >
> > > > > Thus, when we call_rcu(), send a self IPI that checks rcu_needs_cpu()
> > > > > so that we restore a periodic tick behaviour that can take care of
> > > > > everything.
> > > >
> > > > Ouch, I hadn't considered RCU callbacks being posted from within an
> > > > extended quiescent state. I guess I need to make __call_rcu() either
> > > > complain about this or handle it correctly... It would -usually- be
> > > > harmless, but there is getting to be quite a bit of active machinery
> > > > in the various idle loops, so just assuming that it cannot happen is
> > > > probably getting to be an obsolete assumption.
> > >
> > > Adaptive ticks does restart the tick upon entering the kernel, correct?
> >
> > No, it keeps the tick down. The tick is restarted only if it's needed:
> > when more than one task are on the runqueue, a posix cpu timer is running,
> > a CPU needs the current one to report a quiescent state, etc...
>
> Ah, I didn't realize that you didn't restart the tick upon entry to the
> kernel. So this is why you need the IPI -- because there is no tick, if
> the system call runs for a long time, RCU is not guaranteed to make any
> progress on that CPU.
>
> In the common case, this will not be a problem because system calls
> normally spend a short amount of time in the kernel, so normally RCU's
> dyntick-idle detection will handle this case. The exception to this
> rule is when there is a long CPU-bound code path in the kernel, where
> "long" means many milliseconds. In this exception case, this CPU needs
> to be interrupted or whatever is needed to force the CPU to progress
> through RCU.
Exactly!
next prev parent reply other threads:[~2012-05-31 15:56 UTC|newest]
Thread overview: 96+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-04-30 23:54 [RFC][PATCH 00/41] Nohz cpusets v3 (adaptive tickless kernel) Frederic Weisbecker
2012-04-30 23:54 ` [PATCH 01/41] nohz: Separate idle sleeping time accounting from nohz logic Frederic Weisbecker
2012-04-30 23:54 ` [PATCH 02/41] nohz: Make nohz API agnostic against idle ticks cputime accounting Frederic Weisbecker
2012-04-30 23:54 ` [PATCH 03/41] nohz: Rename ts->idle_tick to ts->last_tick Frederic Weisbecker
2012-04-30 23:54 ` [PATCH 04/41] nohz: Move nohz load balancer selection into idle logic Frederic Weisbecker
2012-05-07 15:51 ` Christoph Lameter
2012-04-30 23:54 ` [PATCH 05/41] nohz: Move ts->idle_calls incrementation into strict " Frederic Weisbecker
2012-04-30 23:54 ` [PATCH 06/41] nohz: Move next idle expiry time record into idle logic area Frederic Weisbecker
2012-04-30 23:54 ` [PATCH 07/41] cpuset: Set up interface for nohz flag Frederic Weisbecker
2012-05-07 15:55 ` Christoph Lameter
2012-05-08 14:20 ` Frederic Weisbecker
2012-05-08 14:50 ` Peter Zijlstra
2012-05-08 15:18 ` Christoph Lameter
2012-05-08 15:27 ` Peter Zijlstra
2012-05-08 15:38 ` Christoph Lameter
2012-05-08 15:48 ` Peter Zijlstra
2012-05-08 15:57 ` Christoph Lameter
2012-05-08 16:16 ` Peter Zijlstra
2012-05-08 16:25 ` Peter Zijlstra
2012-05-08 19:50 ` Mike Galbraith
2012-05-08 20:45 ` Christoph Lameter
2012-05-09 4:21 ` Mike Galbraith
2012-05-09 11:02 ` Frederic Weisbecker
2012-05-09 11:07 ` Frederic Weisbecker
2012-05-09 14:23 ` Christoph Lameter
2012-05-09 14:22 ` Christoph Lameter
2012-05-09 14:47 ` Mike Galbraith
2012-05-09 15:05 ` Christoph Lameter
2012-05-09 15:33 ` Mike Galbraith
2012-05-09 15:40 ` Christoph Lameter
2012-05-08 15:16 ` Christoph Lameter
2012-04-30 23:54 ` [PATCH 08/41] nohz: Try not to give the timekeeping duty to an adaptive tickless cpu Frederic Weisbecker
2012-05-07 16:02 ` Christoph Lameter
2012-05-08 17:35 ` Frederic Weisbecker
2012-04-30 23:54 ` [PATCH 09/41] x86: New cpuset nohz irq vector Frederic Weisbecker
2012-04-30 23:54 ` [PATCH 10/41] nohz: Adaptive tick stop and restart on nohz cpuset Frederic Weisbecker
2012-04-30 23:54 ` [PATCH 11/41] nohz/cpuset: Don't turn off the tick if rcu needs it Frederic Weisbecker
2012-05-22 17:16 ` Paul E. McKenney
2012-05-23 13:52 ` Frederic Weisbecker
2012-05-23 15:15 ` Paul E. McKenney
2012-05-23 16:06 ` Frederic Weisbecker
2012-05-23 16:27 ` Paul E. McKenney
2012-05-31 16:01 ` Frederic Weisbecker
2012-05-31 22:02 ` Paul E. McKenney
2012-04-30 23:54 ` [PATCH 12/41] nohz/cpuset: Wake up adaptive nohz CPU when a timer gets enqueued Frederic Weisbecker
2012-04-30 23:54 ` [PATCH 13/41] nohz/cpuset: Don't stop the tick if posix cpu timers are running Frederic Weisbecker
2012-04-30 23:54 ` [PATCH 14/41] nohz/cpuset: Restart tick when nohz flag is cleared on cpuset Frederic Weisbecker
2012-04-30 23:54 ` [PATCH 15/41] nohz/cpuset: Restart the tick if printk needs it Frederic Weisbecker
2012-04-30 23:54 ` [PATCH 16/41] rcu: Restart the tick on non-responding adaptive nohz CPUs Frederic Weisbecker
2012-05-22 17:20 ` Paul E. McKenney
2012-05-23 13:57 ` Frederic Weisbecker
2012-05-23 15:20 ` Paul E. McKenney
2012-05-23 15:57 ` Frederic Weisbecker
2012-04-30 23:54 ` [PATCH 17/41] rcu: Restart tick if we enqueue a callback in a nohz/cpuset CPU Frederic Weisbecker
2012-05-22 17:27 ` Paul E. McKenney
2012-05-22 17:30 ` Paul E. McKenney
2012-05-23 14:03 ` Frederic Weisbecker
2012-05-23 16:15 ` Paul E. McKenney
2012-05-31 15:56 ` Frederic Weisbecker [this message]
2012-05-23 14:00 ` Frederic Weisbecker
2012-05-23 16:01 ` Paul E. McKenney
2012-04-30 23:54 ` [PATCH 18/41] nohz: Generalize tickless cpu time accounting Frederic Weisbecker
2012-04-30 23:54 ` [PATCH 19/41] nohz/cpuset: Account user and system times in adaptive nohz mode Frederic Weisbecker
2012-04-30 23:54 ` [PATCH 20/41] nohz/cpuset: New API to flush cputimes on nohz cpusets Frederic Weisbecker
2012-04-30 23:54 ` [PATCH 21/41] nohz/cpuset: Flush cputime on threads in nohz cpusets when waiting leader Frederic Weisbecker
2012-04-30 23:54 ` [PATCH 22/41] nohz/cpuset: Flush cputimes on procfs stat file read Frederic Weisbecker
2012-04-30 23:54 ` [PATCH 23/41] nohz/cpuset: Flush cputimes for getrusage() and times() syscalls Frederic Weisbecker
2012-04-30 23:54 ` [PATCH 24/41] x86: Syscall hooks for nohz cpusets Frederic Weisbecker
2012-04-30 23:54 ` [PATCH 25/41] x86: Exception " Frederic Weisbecker
2012-04-30 23:55 ` [PATCH 26/41] x86: Add adaptive tickless hooks on do_notify_resume() Frederic Weisbecker
2012-04-30 23:55 ` [PATCH 27/41] nohz/cpuset: enable addition&removal of cpus while in adaptive nohz mode Frederic Weisbecker
2012-04-30 23:55 ` [PATCH 28/41] nohz: Don't restart the tick before scheduling to idle Frederic Weisbecker
2012-04-30 23:55 ` [PATCH 29/41] sched: Comment on rq->clock correctness in ttwu_do_wakeup() in nohz Frederic Weisbecker
2012-04-30 23:55 ` [PATCH 30/41] sched: Update rq clock on nohz CPU before migrating tasks Frederic Weisbecker
2012-04-30 23:55 ` [PATCH 31/41] sched: Update rq clock on nohz CPU before setting fair group shares Frederic Weisbecker
2012-04-30 23:55 ` [PATCH 32/41] sched: Update rq clock on tickless CPUs before calling check_preempt_curr() Frederic Weisbecker
2012-04-30 23:55 ` [PATCH 33/41] sched: Update rq clock earlier in unthrottle_cfs_rq Frederic Weisbecker
2012-04-30 23:55 ` [PATCH 34/41] sched: Update clock of nohz busiest rq before balancing Frederic Weisbecker
2012-04-30 23:55 ` [PATCH 35/41] sched: Update rq clock before idle balancing Frederic Weisbecker
2012-05-02 3:36 ` Michael Wang
2012-05-02 10:55 ` Frederic Weisbecker
2012-04-30 23:55 ` [PATCH 36/41] sched: Update nohz rq clock before searching busiest group on load balancing Frederic Weisbecker
2012-04-30 23:55 ` [PATCH 37/41] rcu: New rcu_user_enter() and rcu_user_exit() APIs Frederic Weisbecker
2012-05-22 18:23 ` Paul E. McKenney
2012-05-23 14:22 ` Frederic Weisbecker
2012-05-23 16:28 ` Paul E. McKenney
2012-04-30 23:55 ` [PATCH 38/41] rcu: New rcu_user_enter_irq() and rcu_user_exit_irq() APIs Frederic Weisbecker
2012-05-22 18:33 ` Paul E. McKenney
2012-05-23 14:31 ` Frederic Weisbecker
2012-04-30 23:55 ` [PATCH 39/41] rcu: Switch to extended quiescent state in userspace from nohz cpuset Frederic Weisbecker
2012-05-22 18:36 ` Paul E. McKenney
2012-05-22 23:04 ` Paul E. McKenney
2012-05-23 14:33 ` Frederic Weisbecker
2012-04-30 23:55 ` [PATCH 40/41] nohz: Exit RCU idle mode when we schedule before resuming userspace Frederic Weisbecker
2012-04-30 23:55 ` [PATCH 41/41] nohz/cpuset: Disable under some configs Frederic Weisbecker
2012-05-07 22:10 ` [RFC][PATCH 00/41] Nohz cpusets v3 (adaptive tickless kernel) Geoff Levand
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