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Date:   Mon, 27 Mar 2023 18:59:00 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     "Zhang, Qiang1" <qiang1.zhang@intel.com>
Cc:     Uladzislau Rezki <urezki@gmail.com>,
        "Zhuo, Qiuxu" <qiuxu.zhuo@intel.com>, RCU <rcu@vger.kernel.org>,
        "quic_neeraju@quicinc.com" <quic_neeraju@quicinc.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@sony.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Frederic Weisbecker <frederic@kernel.org>
Subject: Re: [PATCH 1/1] Reduce synchronize_rcu() waiting time
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On Tue, Mar 28, 2023 at 01:32:56AM +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.
> 
> Agree,  maybe Uladzislau 's patch will have similar problems.

We will soon find out.  ;-)

							Thanx, Paul

> Thanks
> Zqiang
> 
> 
> >
> >							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