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

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Date:   Tue, 21 Mar 2023 14:03:19 +0000
From:   "Zhuo, Qiuxu" <qiuxu.zhuo@...el.com>
To:     "Uladzislau Rezki (Sony)" <urezki@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>
CC:     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

> 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?

If possible, may I know the steps, commands, and related parameters to produce the results above?
Thank you!

- Qiuxu
 
> As performance results, i will provide it once we are ready with a patch.
> 
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> ---
>  kernel/rcu/tree.c | 104
> +++++++++++++++++++++++++++++++++++++++++++++-
[...]