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

[Zhang, Qiang1]

> > From: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
> > 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);
+}
+
+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