From: Josh Triplett <josh@joshtriplett.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
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,
niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org,
rostedt@goodmis.org, Valdis.Kletnieks@vt.edu,
dhowells@redhat.com, eric.dumazet@gmail.com, darren@dvhart.com,
fweisbec@gmail.com, patches@linaro.org,
"Paul E. McKenney" <paul.mckenney@linaro.org>
Subject: Re: [PATCH RFC tip/core/rcu 14/41] rcu: Limit lazy-callback duration
Date: Thu, 2 Feb 2012 20:07:51 -0800 [thread overview]
Message-ID: <20120203040751.GA3008@leaf> (raw)
In-Reply-To: <20120202171342.GP2518@linux.vnet.ibm.com>
On Thu, Feb 02, 2012 at 09:13:42AM -0800, Paul E. McKenney wrote:
> On Wed, Feb 01, 2012 at 06:03:56PM -0800, Josh Triplett wrote:
> > On Wed, Feb 01, 2012 at 11:41:32AM -0800, Paul E. McKenney wrote:
> > > Currently, a given CPU is permitted to remain in dyntick-idle mode
> > > indefinitely if it has only lazy RCU callbacks queued. This is vulnerable
> > > to corner cases in NUMA systems, so limit the time to six seconds by
> > > default. (Currently controlled by a cpp macro.)
> >
> > I wonder: should this scale with the number of callbacks, or do we not
> > want to make estimates about memory usage based on that?
>
> Interesting. Which way would you scale it? ;-)
Heh, I'd figured "don't wait too long if you have a giant pile of
callbacks", but I can see how the other direction could make sense as
well. :)
> > Interestingly, with kfree_rcu, we actually know at callback queuing time
> > *exactly* how much memory we'll get back by calling the callback, and we
> > could sum up those numbers.
>
> We can indeed calculate for kfree_rcu(), but we won't be able to for
> call_rcu_lazy(), which is my current approach for cases where you cannot
> use kfree_rcu() due to (for example) freeing up a linked structure.
> A very large fraction of the call_rcu()s in the kernel could become
> call_rcu_lazy().
So, doing anything other than freeing memory makes a callback non-lazy?
Based on that, I'd find it at least somewhat surprising if any of the
current callers of call_rcu (other than synchronize_rcu() and similar)
had non-lazy callbacks.
> At some point in the future, it might make sense to tie into the
> low-memory notifier, which could potentially allow the longer timeout
> to be omitted.
Exactly the kind of thing that made me wonder about tracking the actual
amount of memory to free. Still seems like a potentially useful
statistic to track on its own.
> My current guess is that the recent change allowing idle CPUs to
> exhaust their callback lists will make this kind of fine-tuning
> unnecessary, but we will see!
Good point; given that fix, idle CPUs should never need to wake up for
callbacks at all.
- Josh Triplett
next prev parent reply other threads:[~2012-02-03 4:08 UTC|newest]
Thread overview: 104+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-02-01 19:41 [PATCH RFC 0/41] RCU commits for 3.4 Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 01/41] rcu: Bring RTFP.txt up to date Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 02/41] rcu: Improve synchronize_rcu() diagnostics Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 03/41] rcu: Add lockdep-RCU checks for simple self-deadlock Paul E. McKenney
2012-02-02 0:55 ` Josh Triplett
2012-02-02 16:20 ` Paul E. McKenney
2012-02-02 19:56 ` Josh Triplett
2012-02-02 20:42 ` Paul E. McKenney
2012-02-03 9:04 ` Josh Triplett
2012-02-03 18:05 ` Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 04/41] rcu: Add diagnostic for misaligned rcu_head structures Paul E. McKenney
2012-02-02 1:00 ` Josh Triplett
2012-02-02 16:22 ` Paul E. McKenney
2012-02-02 20:11 ` Josh Triplett
2012-02-02 1:01 ` Josh Triplett
2012-02-02 16:27 ` Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 05/41] rcu: Avoid waking up CPUs having only kfree_rcu() callbacks Paul E. McKenney
2012-02-02 1:15 ` Josh Triplett
2012-02-02 16:34 ` Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 06/41] rcu: Move RCU_TRACE to lib/Kconfig.debug Paul E. McKenney
2012-02-02 1:39 ` Josh Triplett
2012-02-02 17:05 ` Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 07/41] s390: Convert call_rcu() to kfree_rcu() Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 08/41] tcm_fc: " Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 09/41] ipv4: " Paul E. McKenney
2012-02-01 19:49 ` David Miller
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 10/41] " Paul E. McKenney
2012-02-01 19:50 ` David Miller
2012-02-02 0:24 ` Josh Triplett
2012-02-02 15:56 ` Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 11/41] mac80211: " Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 12/41] rcu: Simplify offline processing Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 13/41] rcu: Make rcutorture flag online/offline failures Paul E. McKenney
2012-02-02 1:46 ` Josh Triplett
2012-02-02 17:08 ` Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 14/41] rcu: Limit lazy-callback duration Paul E. McKenney
2012-02-02 2:03 ` Josh Triplett
2012-02-02 17:13 ` Paul E. McKenney
2012-02-03 4:07 ` Josh Triplett [this message]
2012-02-03 5:54 ` Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 15/41] rcu: Check for callback invocation from offline CPUs Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 16/41] rcu: Don't make callbacks go through second full grace period Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 17/41] rcu: Remove single-rcu_node optimization in rcu_start_gp() Paul E. McKenney
2012-02-02 2:13 ` Josh Triplett
2012-02-02 17:16 ` Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 18/41] rcu: Protect __rcu_read_unlock() against scheduler-using irq handlers Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 19/41] rcu: Streamline code produced by __rcu_read_unlock() Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 20/41] rcu: Prevent RCU callbacks from executing before scheduler initialized Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 21/41] rcu: Inform RCU of irq_exit() activity Paul E. McKenney
2012-02-02 2:30 ` Josh Triplett
2012-02-02 17:30 ` Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 22/41] rcu: Simplify unboosting checks Paul E. McKenney
2012-02-02 2:38 ` Josh Triplett
2012-02-02 17:48 ` Paul E. McKenney
2012-02-03 4:23 ` Josh Triplett
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 23/41] rcu: Clean up straggling rcu_preempt_needs_cpu() name Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 24/41] rcu: Check for idle-loop entry while in RCU read-side critical section Paul E. McKenney
2012-02-02 5:13 ` Josh Triplett
2012-02-02 17:50 ` Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 25/41] rcu: Make rcu_sleep_check() also check rcu_lock_map Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 26/41] rcu: Note that rcu_access_pointer() can be used for teardown Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 27/41] rcu: Remove #ifdef CONFIG_SMP from TREE_RCU Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 28/41] rcu: Set RCU CPU stall times via sysfs Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 29/41] rcu: Print scheduling-clock information on RCU CPU stall-warning messages Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 30/41] rcutorture: Permit holding off CPU-hotplug operations during boot Paul E. McKenney
2012-02-02 5:43 ` Josh Triplett
2012-02-02 17:56 ` Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 31/41] rcu: Add CPU-stall capability to rcutorture Paul E. McKenney
2012-02-02 5:53 ` Josh Triplett
2012-02-02 9:15 ` Julia Lawall
2012-02-02 18:03 ` Paul E. McKenney
2012-02-02 18:00 ` Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 32/41] rcu: Update stall-warning documentation Paul E. McKenney
2012-02-02 5:56 ` Josh Triplett
2012-02-02 18:18 ` Paul E. McKenney
2012-02-03 5:42 ` Josh Triplett
2012-02-03 5:58 ` Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 33/41] rcu: Make boolean rcutorture parameters be of type "bool" Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 34/41] rcu: Check for illegal use of RCU from offlined CPUs Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 35/41] rcu: Move synchronize_sched_expedited() to rcutree.c Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 36/41] rcu: No interrupt disabling for rcu_prepare_for_idle() Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 37/41] lockdep: Add CPU-idle/offline warning to lockdep-RCU splat Paul E. McKenney
2012-02-02 6:07 ` Josh Triplett
2012-02-02 18:30 ` Paul E. McKenney
2012-02-03 6:12 ` Josh Triplett
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 38/41] rcu: Rework detection of use of RCU by offline CPUs Paul E. McKenney
2012-02-02 6:11 ` Josh Triplett
2012-02-02 18:31 ` Paul E. McKenney
2012-02-03 9:17 ` Josh Triplett
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 39/41] rcu: Wait at least a jiffy before declaring a CPU to be offline Paul E. McKenney
2012-02-02 6:12 ` Josh Triplett
2012-02-02 18:27 ` Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 40/41] rcu: Call out dangers of expedited RCU primitives Paul E. McKenney
2012-02-01 19:41 ` [PATCH RFC tip/core/rcu 41/41] rcu: Trace only after NULL-pointer check Paul E. McKenney
2012-02-02 0:18 ` [PATCH RFC tip/core/rcu 01/41] rcu: Bring RTFP.txt up to date Josh Triplett
2012-02-02 1:33 ` Paul E. McKenney
2012-02-02 2:01 ` Josh Triplett
2012-02-02 16:47 ` Paul E. McKenney
2012-02-02 22:32 ` Josh Triplett
2012-02-03 18:00 ` Paul E. McKenney
2012-02-02 22:47 ` [PATCH RFC 0/41] RCU commits for 3.4 Kevin Hilman
2012-02-02 23:58 ` Paul E. McKenney
2012-02-03 19:54 ` Kevin Hilman
2012-02-06 7:04 ` Paul E. McKenney
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