Re: rcu: performance regression

Ingo Molnar
2011-06-14T08:13:32Z

* Shaohua Li <shaohua.li@intel.com> wrote:

> Commit a26ac2455ffcf3(rcu: move TREE_RCU from softirq to kthread)
> introduced performance regression. In our AIM7 test, this commit caused
> about 40% regression.

Sigh, this commit is somewhat of a train-wreck.

> The commit runs rcu callbacks in a kthread instead of softirq. We
> observed high rate of context switch which is caused by this. Out
> test system has 64 CPUs and HZ is 1000, so we saw more than 64k
> context switch per second which is caused by the rcu thread.
>
> I also did trace and found when rcy thread is woken up, most time
> the thread doesn't handle any callbacks actually, it just
> initializes new gp or end one gp or similar.
>
> From my understanding, the purpose to make rcu runs in kthread is
> to speed up rcu callbacks run (with help of rtmutex PI), not for
> end gp and so on, which runs pretty fast actually and doesn't need
> boost. To verify my findings, I had below debug patch applied. It
> still handles rcu callbacks in kthread if there is any pending
> callbacks, but other things are still running in softirq. this
> completely solved our regression. I thought this can still boost
> callbacks run. but I'm not expert in the area, so please help.
>
> Thanks,
> Shaohua
> ---
> Documentation/filesystems/proc.txt | 1 +
> include/linux/interrupt.h | 1 +
> include/trace/events/irq.h | 3 ++-
> kernel/rcutree.c | 23 +++++++++++++++++++----
> kernel/rcutree.h | 1 +
> kernel/rcutree_plugin.h | 9 +++++++++
> kernel/softirq.c | 2 +-
> tools/perf/util/trace-event-parse.c | 1 +
> 8 files changed, 35 insertions(+), 6 deletions(-)

Paul? Unless this patch is the obviously correct solution everyone
wants to have, the other obviously correct solution is to do the
revert ...

Thanks,

Ingo