On Tue, Feb 17, 2009 at 10:46:57AM +0100, Ingo Molnar wrote: > > * Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx> 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? But of course! ;-) The following patch removes the call to rcu_pending() and rcu_check_callbacks() from the x86 32-bit idle loop in order to reduce the softirq load on idle systems. Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> --- process_32.c | 3 --- 1 file changed, 3 deletions(-) diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c index a546f55..bd4da2a 100644 --- a/arch/x86/kernel/process_32.c +++ b/arch/x86/kernel/process_32.c @@ -104,9 +104,6 @@ void cpu_idle(void) check_pgt_cache(); rmb(); - if (rcu_pending(cpu)) - rcu_check_callbacks(cpu, 0); - if (cpu_is_offline(cpu)) play_dead(); -- To unsubscribe from this list: send the line "unsubscribe kernel-testers" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
