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

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> From: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> Sent: Saturday, February 25, 2023 11:34 AM
> To: linux-kernel@xxxxxxxxxxxxxxx
> Cc: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>; Frederic Weisbecker
> <frederic@xxxxxxxxxx>; Lai Jiangshan <jiangshanlai@xxxxxxxxx>; linux-
> doc@xxxxxxxxxxxxxxx; Paul E. McKenney <paulmck@xxxxxxxxxx>;
> rcu@xxxxxxxxxxxxxxx
> 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@xxxxxxxxxxxxxxxxx>
> ---
> v1->v2:
> 	Update some comments and description.
>  ...





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