Re: RCU related performance regression in 3.3

[Pascal Chapperon <pascal.chapperon@wanadoo.fr>]

Le 16/04/2012 23:02, Paul E. McKenney a écrit :
.
>> 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@linux.vnet.ibm.com>
>
> 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 ]---