linux-kernel - Re: [PATCH 1/1] Reduce synchronize

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Date:   Tue, 21 Mar 2023 16:15:53 +0100
From:   Uladzislau Rezki <urezki@...il.com>
To:     "Zhuo, Qiuxu" <qiuxu.zhuo@...el.com>
Cc:     "Uladzislau Rezki (Sony)" <urezki@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        RCU <rcu@...r.kernel.org>,
        "quic_neeraju@...cinc.com" <quic_neeraju@...cinc.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [PATCH 1/1] Reduce synchronize_rcu() waiting time

On Tue, Mar 21, 2023 at 02:03:19PM +0000, Zhuo, Qiuxu wrote:
> > From: Uladzislau Rezki (Sony) <urezki@...il.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:
> > 
> > <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