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

[Paul E. McKenney]

On Mon, Mar 27, 2023 at 11:21:23AM +0000, Zhang, Qiang1 wrote:
> > > 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);

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.

							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