linux-kernel - RE: [PATCH RFC v2] rcu: Add a minimum time for marking boot as completed

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Message-ID: <IA1PR11MB617175CA398D425B7489CE4889AF9@IA1PR11MB6171.namprd11.prod.outlook.com>
Date:   Mon, 27 Feb 2023 07:53:35 +0000
From:   "Zhuo, Qiuxu" <qiuxu.zhuo@...el.com>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     Frederic Weisbecker <frederic@...nel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>
Subject: RE: [PATCH RFC v2] rcu: Add a minimum time for marking boot as
 completed

> From: Joel Fernandes (Google) <joel@...lfernandes.org>
> Sent: Saturday, February 25, 2023 11:34 AM
> To: linux-kernel@...r.kernel.org
> Cc: Joel Fernandes (Google) <joel@...lfernandes.org>; Frederic Weisbecker
> <frederic@...nel.org>; Lai Jiangshan <jiangshanlai@...il.com>; linux-
> doc@...r.kernel.org; Paul E. McKenney <paulmck@...nel.org>;
> rcu@...r.kernel.org
> Subject: [PATCH RFC v2] rcu: Add a minimum time for marking boot as
> completed
> 
> On many systems, a great deal of boot happens after the kernel thinks the
> boot has completed. It is difficult to determine if the system has really
> booted from the kernel side. Some features like lazy-RCU can risk slowing
> down boot time if, say, a callback has been added that the boot
> synchronously depends on.
> 
> Further, it is better to boot systems which pass 'rcu_normal_after_boot' to
> stay expedited for as long as the system is still booting.
> 
> For these reasons, this commit adds a config option
> 'CONFIG_RCU_BOOT_END_DELAY' and a boot parameter
> rcupdate.boot_end_delay.
> 
> By default, this value is 20s. A system designer can choose to specify a value
> here to keep RCU from marking boot completion.  The boot sequence will not
> be marked ended until at least boot_end_delay milliseconds have passed.

Hi Joel,

Just some thoughts on the default value of 20s, correct me if I'm wrong :-).

Does the OS with CONFIG_PREEMPT_RT=y kernel concern more about the 
real-time latency than the overall OS boot time?

If so, we might make rcupdate.boot_end_delay = 0 as the default value 
(NOT the default 20s) for CONFIG_PREEMPT_RT=y kernels? 

-Qiuxu

> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> ---
> v1->v2:
> 	Update some comments and description.
>  ...