Re: RCU related performance regression in 3.3
From: Pascal Chapperon
Date: Wed Apr 18 2012 - 05:37:29 EST
Le 16/04/2012 23:02, Paul E. McKenney a Ãcrit :
.
So it seems that mount and unmount operations are often slower with
RCU_FAST_NO_HZ during boot and shutdown. Are these operations also
slower during runtime? If so, the RCU event tracing across both a fast
and a slow mount or unmount operation would likely be quite helpful.
-
Mount and umount operations are not slower with RCU_FAST_NO_HZ during
runtime; systemctl start and stop operations are also not slower. In
fact, i couldn't find a single operation slower during runtime with
RCU_FAST_NO_HZ.
-
Actually, one other possibility is that RCU_FAST_NO_HZ's timer is
being migrated. If you get a chance, could you please try out the
diagnostic patch below?
Thanx, Paul
------------------------------------------------------------------------
rcu: Check for timer migration for RCU_FAST_NO_HZ
If RCU_FAST_NO_HZ's timer is migrated, then the CPU that went dyntick-idle
with callbacks might never wake up, which could indefinitely postpone
invocation of its callbacks, which could in turn result in a system hang.
But if the timer is migrated, then it might actually fire. In contrast,
if it remains on the CPU that posted it, it is guaranteed to be cancelled.
This patch therefore adds a WARN_ON_ONCE() to this timer's handler as
a diagnostic test.
Signed-off-by: Paul E. McKenney<paulmck@xxxxxxxxxxxxxxxxxx>
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index c023464..67ee640 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -2053,6 +2053,7 @@ static bool rcu_cpu_has_nonlazy_callbacks(int cpu)
*/
static enum hrtimer_restart rcu_idle_gp_timer_func(struct hrtimer *hrtp)
{
+ WARN_ON_ONCE(1);
trace_rcu_prep_idle("Timer");
return HRTIMER_NORESTART;
}
-
The result below.
Pascal
[ 0.758325] ------------[ cut here ]------------
[ 0.758330] WARNING: at kernel/rcutree_plugin.h:2056
rcu_idle_gp_timer_func+0x27/0x30()
[ 0.758332] Hardware name: GX780R/GT780R/GT780DXR/GT783R
[ 0.758334] Modules linked in:
[ 0.758337] Pid: 0, comm: swapper/0 Not tainted 3.4.0-rc2-rcu+ #38
[ 0.758338] Call Trace:
[ 0.758340] <IRQ>
[<ffffffff81057a1f>]warn_slowpath_common+0x7f/0xc0
[ 0.758348] [<ffffffff81057a7a>] warn_slowpath_null+0x1a/0x20
[ 0.758350] [<ffffffff810e61e7>] rcu_idle_gp_timer_func+0x27/0x30
[ 0.758354] [<ffffffff8107d5f1>] __run_hrtimer+0x71/0x1e0
[ 0.758357] [<ffffffff810e61c0>] ? rcu_batches_completed+0x20/0x20
[ 0.758360] [<ffffffff8107df3b>] hrtimer_interrupt+0xeb/0x210
[ 0.758365] [<ffffffff81601dd9>] smp_apic_timer_interrupt+0x69/0x99
[ 0.758368] [<ffffffff81600b4a>] apic_timer_interrupt+0x6a/0x70
[ 0.758369] <EOI> [<ffffffff8101b979>] ? sched_clock+0x9/0x10
[ 0.758374] [<ffffffff8101cc05>] ? mwait_idle+0x95/0x230
[ 0.758377] [<ffffffff8101d629>] cpu_idle+0xd9/0x120
[ 0.758380] [<ffffffff815d4f0e>] rest_init+0x72/0x74
[ 0.758384] [<ffffffff81cf6c12>] start_kernel+0x3c1/0x3ce
[ 0.758386] [<ffffffff81cf6582>] ? loglevel+0x31/0x31
[ 0.758389] [<ffffffff81cf6346>
x86_64_start_reservations+0x131/0x135
[ 0.758392] [<ffffffff81cf6140>] ? early_idt_handlers+0x140/0x140
[ 0.758394] [<ffffffff81cf644c>] x86_64_start_kernel+0x102/0x111
[ 0.758398] ---[ end trace 82bc736bb33fe366 ]---
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