Re: rcu: performance regression

[Alex,Shi]

On Tue, 2011-06-14 at 16:33 +0800, Alex,Shi wrote:
> On Tue, 2011-06-14 at 13:26 +0800, Li, Shaohua wrote:
> > Commit a26ac2455ffcf3(rcu: move TREE_RCU from softirq to kthread)
> > introduced performance regression. In our AIM7 test, this commit caused
> > about 40% regression.
> > 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, 

I am interested how you do trace and get 64k CS more? 
> 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.
> 
> This commit also cause hackbench process mode performance dropping, and
> Shaohua's patch do recovered this. But in hackbench testing, the vmstat
> show context switch have some reduce. And perf tool show
> root_domain->cpupri->prio_to_cpu[]->lock has contention with the commit.
> 
> 
>    11.53%        hackbench  [kernel]                   [k] 
>                   |
>                   --- _raw_spin_lock_irqsave
>                       cpupri_set
>                       __enqueue_rt_entity
>                       enqueue_rt_entity
>                       enqueue_task_rt
>                       enqueue_task
>                       activate_task
>                       ttwu_activate
>                       ttwu_do_activate.clone.3
>                       try_to_wake_up
>                       wake_up_process
>                       invoke_rcu_cpu_kthread
>                       rcu_check_callbacks
>                       update_process_times
>                       tick_sched_timer
>                       __run_hrtimer


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