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

Ingo Molnar
2009-02-17T09:47:25Z

* Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:

> On Mon, Feb 16, 2009 at 02:39:44PM -0800, Paul E. McKenney wrote:
> > On Mon, Feb 16, 2009 at 09:09:23PM +0100, Ingo Molnar wrote:
> > >
> > > * Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> > >
> > > > Here the calls to rcu_process_callbacks() are only 75
> > > > microseconds apart, so that this function is consuming more
> > > > than 10% of a CPU. The strange thing is that I don't see a
> > > > raise_softirq() in between, though perhaps it gets inlined or
> > > > something that makes it invisible to ftrace.
> > >
> > > look at the latest trace please, that has even the most inline
> > > raise-softirq method instrumented, so all the raising is
> > > visible.
> >
> > Ah, my apologies! This time looking at:
> >
> > http://damien.wyart.free.fr/ksoftirqd_pb/trace_tip_2009.02.16_ksoftirqd_pb_abstime_proc.txt.gz
> >
> >
> > 799.521187 | 1) <idle>-0 | | rcu_check_callbacks() {
> > 799.521371 | 1) <idle>-0 | | rcu_check_callbacks() {
> > 799.521555 | 1) <idle>-0 | | rcu_check_callbacks() {
> > 799.521738 | 1) <idle>-0 | | rcu_check_callbacks() {
> > 799.521934 | 1) <idle>-0 | | rcu_check_callbacks() {
> > 799.522068 | 1) ksoftir-2324 | | rcu_check_callbacks() {
> > 799.522208 | 1) <idle>-0 | | rcu_check_callbacks() {
> > 799.522392 | 1) <idle>-0 | | rcu_check_callbacks() {
> > 799.522575 | 1) <idle>-0 | | rcu_check_callbacks() {
> > 799.522759 | 1) <idle>-0 | | rcu_check_callbacks() {
> > 799.522956 | 1) <idle>-0 | | rcu_check_callbacks() {
> > 799.523074 | 1) ksoftir-2324 | | rcu_check_callbacks() {
> > 799.523214 | 1) <idle>-0 | | rcu_check_callbacks() {
> > 799.523397 | 1) <idle>-0 | | rcu_check_callbacks() {
> > 799.523579 | 1) <idle>-0 | | rcu_check_callbacks() {
> > 799.523762 | 1) <idle>-0 | | rcu_check_callbacks() {
> > 799.523960 | 1) <idle>-0 | | rcu_check_callbacks() {
> > 799.524079 | 1) ksoftir-2324 | | rcu_check_callbacks() {
> > 799.524220 | 1) <idle>-0 | | rcu_check_callbacks() {
> > 799.524403 | 1) <idle>-0 | | rcu_check_callbacks() {
> > 799.524587 | 1) <idle>-0 | | rcu_check_callbacks() {
> > 799.524770 | 1) <idle>-0 | | rcu_check_callbacks() {
> > [ . . . ]
> >
> > Yikes!!!
> >
> > Why is rcu_check_callbacks() being invoked so often? It should be called
> > but once per jiffy, and here it is called no less than 22 times in about
> > 3.5 milliseconds, meaning one call every 160 microseconds or so.
>
> BTW, the other question I have is "why do we need to call
> rcu_pending() and rcu_check_callbacks() from the idle loop of
> 32-bit x86, especially given that no other architecture does
> this?". Don't get me wrong, it would be good to get rcutree's
> rcu_pending() to avoid spuriously saying that
> rcu_check_callbacks() should be invoked, so I would still like
> the trace with my patch, but...

There's no strong reason - we've been back and forth about RCU
in the dynticks code. Mind sending a test patch for Damien to
try?

Ingo