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

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Message-Id: <764CA486-6FB2-4667-B8CB-56E3AC31FD58@joelfernandes.org>
Date:   Mon, 27 Feb 2023 08:22:06 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Zhuo, Qiuxu" <qiuxu.zhuo@...el.com>
Cc:     linux-kernel@...r.kernel.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: Re: [PATCH RFC v2] rcu: Add a minimum time for marking boot as completed



> On Feb 27, 2023, at 2:53 AM, Zhuo, Qiuxu <qiuxu.zhuo@...el.com> wrote:
> 
> 
>> 
>> 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?

But every system has to boot, even an RT system.

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

Could you measure how much time your RT system takes to boot before the application runs?

I can change it to default 0 essentially NOOPing it, but I would rather have a saner default (10 seconds even), than having someone forget to tune this for their system.

Thanks,

 - Joel


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