linux-kernel - Re: [Bug #12650] Strange load average and ksoftirqd behavior with 2.6.29-rc2-git1

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Message-ID: <20090216160613.GA6785@linux.vnet.ibm.com>
Date:	Mon, 16 Feb 2009 08:06:13 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Damien Wyart <damien.wyart@...e.fr>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mike Galbraith <efault@....de>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kernel Testers List <kernel-testers@...r.kernel.org>
Subject: Re: [Bug #12650] Strange load average and ksoftirqd behavior with
	2.6.29-rc2-git1

On Mon, Feb 16, 2009 at 02:21:51PM +0100, Ingo Molnar wrote:
> 
> * Damien Wyart <damien.wyart@...e.fr> wrote:
> 
> > * Ingo Molnar <mingo@...e.hu> [090216 13:26]:
> > > We do get 0x100 which is 1 << RCU_SOFTIRQ, i.e. the RCU softirq. Paul, 
> > > this indeed seems to be a CONFIG_TREE_RCU=y bug.
> > 
> > > What is weird is that RCU_SOFTIRQ gets set again and again - but there's 
> > > no raise_softirq() calls. Could you please do a two-CPU trace too via:
> > 
> > >    echo 3 > /debug/tracing/tracing_cpumask
> > 
> > > So that we can see what's happening on the other CPU?
> > 
> > > Also, could you please apply the debug patch below (or update to the 
> > > very latest -tip tree), so that we get trace entries of softirq triggers 
> > > too?
> > 
> > Ok, the new trace with these additional modifications is here:
> > http://damien.wyart.free.fr/ksoftirqd_pb/trace_tip_2009.02.16_1300_ksoftirqd_pb_abstime_proc_mask3.txt.gz
> 
> thanks.
> 
> This confirms that SOFTIRQ_RCU gets raised here in the timer IRQ:
> 
>   136.255963 |   0)   sleep-2345   |               |          update_process_times() {
>   136.255964 |   0)   sleep-2345   |               |            account_process_tick() {
>   136.255965 |   0)   sleep-2345   |   0.779 us    |              account_system_time();
>   136.255966 |   0)   sleep-2345   |   2.262 us    |            }
>   136.255967 |   0)   sleep-2345   |               |            run_local_timers() {
>   136.255968 |   0)   sleep-2345   |   0.802 us    |              hrtimer_run_queues();
>   136.255969 |   0)   sleep-2345   |               |              raise_softirq() {
>   136.255970 |   0)   sleep-2345   |               |                raise_softirq_irqoff() {
>   136.255971 |   0)   sleep-2345   |               |                  __raise_softirq_irqoff() {
>   136.255972 |   0)   sleep-2345   |               |                    /* nr: 1 */
>   136.255973 |   0)   sleep-2345   |   2.194 us    |                  }
>   136.255974 |   0)   sleep-2345   |   3.832 us    |                }
>   136.255975 |   0)   sleep-2345   |   5.491 us    |              }
>   136.255976 |   0)   sleep-2345   |   8.667 us    |            }
>   136.255976 |   0)   sleep-2345   |   0.792 us    |            rcu_pending();
>   136.255978 |   0)   sleep-2345   |               |            rcu_check_callbacks() {
>   136.255979 |   0)   sleep-2345   |   0.781 us    |              idle_cpu();
>   136.255981 |   0)   sleep-2345   |               |              raise_softirq() {
>   136.255981 |   0)   sleep-2345   |               |                raise_softirq_irqoff() {
>   136.255982 |   0)   sleep-2345   |               |                  __raise_softirq_irqoff() {
>   136.255983 |   0)   sleep-2345   |               |                    /* nr: 8 */
>   136.255984 |   0)   sleep-2345   |   1.555 us    |                  }
>   136.255984 |   0)   sleep-2345   |   3.059 us    |                }
>   136.255985 |   0)   sleep-2345   |   4.594 us    |              }
>   136.255986 |   0)   sleep-2345   |   7.800 us    |            }
>   136.255987 |   0)   sleep-2345   |   0.737 us    |            printk_tick();
> 
> again and again.

Interesting...

I will take a look!

							Thanx, Paul
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