Re: [PATCH 1/1] Reduce synchronize_rcu() waiting time

[Joel Fernandes <joel@joelfernandes.org>]

Hello,

> On Mar 27, 2023, at 9:06 PM, Paul E. McKenney <paulmck@kernel.org> wrote:
> 
> On Mon, Mar 27, 2023 at 11:21:23AM +0000, Zhang, Qiang1 wrote:
>>>> From: Uladzislau Rezki (Sony) <urezki@gmail.com>
>>>> Sent: Tuesday, March 21, 2023 6:28 PM
>>>> [...]
>>>> Subject: [PATCH 1/1] Reduce synchronize_rcu() waiting time
>>>> 
>>>> A call to a synchronize_rcu() can be expensive from time point of view.
>>>> Different workloads can be affected by this especially the ones which use this
>>>> API in its time critical sections.
>>>> 
>>> 
>>> This is interesting and meaningful research. ;-)
>>> 
>>>> For example in case of NOCB scenario the wakeme_after_rcu() callback
>>>> invocation depends on where in a nocb-list it is located. Below is an example
>>>> when it was the last out of ~3600 callbacks:
>>> 
>> 
>> 
>> 
>> Can it be implemented separately as follows?  it seems that the code is simpler
>> (only personal opinion)  😊.
>> 
>> But I didn't test whether this reduce synchronize_rcu() waiting time
>> 
>> +static void rcu_poll_wait_gp(struct rcu_tasks *rtp)
>> +{
>> +       unsigned long gp_snap;
>> +
>> +       gp_snap = start_poll_synchronize_rcu();
>> +       while (!poll_state_synchronize_rcu(gp_snap))
>> +               schedule_timeout_idle(1);
> 
> I could be wrong, but my guess is that the guys working with
> battery-powered devices are not going to be very happy with this loop.
> 
> All those wakeups by all tasks waiting for a grace period end up
> consuming a surprisingly large amount of energy.

Is that really the common case? On the general topic of wake-ups:
Most of the time there should be only one
task waiting synchronously on a GP to end. If that is
true, then it feels like waking
up nocb Kthreads which indirectly wake other threads is doing more work than usual?

I am curious to measure how much does Vlad patch reduce wakeups in the common case.

I was also wondering how Vlad patch effects RCU-barrier ordering. I guess
we want the wake up to happen in the order of
other callbacks also waiting.

One last note, most battery powered systems are perhaps already using expedited RCU ;-)

Thoughts?

 - Joel 

> 
>                            Thanx, Paul
> 
>> +}
>> +
>> +void call_rcu_poll(struct rcu_head *rhp, rcu_callback_t func);
>> +DEFINE_RCU_TASKS(rcu_poll, rcu_poll_wait_gp, call_rcu_poll,
>> +                 "RCU Poll");
>> +void call_rcu_poll(struct rcu_head *rhp, rcu_callback_t func)
>> +{
>> +       call_rcu_tasks_generic(rhp, func, &rcu_poll);
>> +}
>> +EXPORT_SYMBOL_GPL(call_rcu_poll);
>> +
>> +void synchronize_rcu_poll(void)
>> +{
>> +       synchronize_rcu_tasks_generic(&rcu_poll);
>> +}
>> +EXPORT_SYMBOL_GPL(synchronize_rcu_poll);
>> +
>> +static int __init rcu_spawn_poll_kthread(void)
>> +{
>> +       cblist_init_generic(&rcu_poll);
>> +       rcu_poll.gp_sleep = HZ / 10;
>> +       rcu_spawn_tasks_kthread_generic(&rcu_poll);
>> +       return 0;
>> +}
>> 
>> Thanks
>> Zqiang
>> 
>> 
>>>> 
>>>> <snip>
>>>>  <...>-29      [001] d..1. 21950.145313: rcu_batch_start: rcu_preempt
>>>> CBs=3613 bl=28
>>>> ...
>>>>  <...>-29      [001] ..... 21950.152578: rcu_invoke_callback: rcu_preempt
>>>> rhp=00000000b2d6dee8 func=__free_vm_area_struct.cfi_jt
>>>>  <...>-29      [001] ..... 21950.152579: rcu_invoke_callback: rcu_preempt
>>>> rhp=00000000a446f607 func=__free_vm_area_struct.cfi_jt
>>>>  <...>-29      [001] ..... 21950.152580: rcu_invoke_callback: rcu_preempt
>>>> rhp=00000000a5cab03b func=__free_vm_area_struct.cfi_jt
>>>>  <...>-29      [001] ..... 21950.152581: rcu_invoke_callback: rcu_preempt
>>>> rhp=0000000013b7e5ee func=__free_vm_area_struct.cfi_jt
>>>>  <...>-29      [001] ..... 21950.152582: rcu_invoke_callback: rcu_preempt
>>>> rhp=000000000a8ca6f9 func=__free_vm_area_struct.cfi_jt
>>>>  <...>-29      [001] ..... 21950.152583: rcu_invoke_callback: rcu_preempt
>>>> rhp=000000008f162ca8 func=wakeme_after_rcu.cfi_jt
>>>>  <...>-29      [001] d..1. 21950.152625: rcu_batch_end: rcu_preempt CBs-
>>>> invoked=3612 idle=....
>>>> <snip>
>>>> 
>>> 
>>> Did the results above tell us that CBs-invoked=3612 during the time 21950.145313 ~ 21950.152625?
>>> 
>>> Yes.
>>> 
>>> 
>>> If possible, may I know the steps, commands, and related parameters to produce the results above?
>>> Thank you!
>>> 
>>> Build the kernel with CONFIG_RCU_TRACE configuration. Update your "set_event"
>>> file with appropriate traces:
>>> 
>>> <snip>
>>> XQ-DQ54:/sys/kernel/tracing # echo rcu:rcu_batch_start rcu:rcu_batch_end rcu:rcu_invoke_callback > set_event
>>> 
>>> XQ-DQ54:/sys/kernel/tracing # cat set_event
>>> rcu:rcu_batch_start
>>> rcu:rcu_invoke_callback
>>> rcu:rcu_batch_end
>>> XQ-DQ54:/sys/kernel/tracing #
>>> <snip>
>>> 
>>> Collect traces as much as you want: XQ-DQ54:/sys/kernel/tracing # echo 1 > tracing_on; sleep 10; echo 0 > tracing_on
>>> Next problem is how to parse it. Of course you will not be able to parse
>>> megabytes of traces. For that purpose i use a special C trace parser.
>>> If you need an example please let me know i can show here.
>>> 
>>> --
>>> Uladzislau Rezki