From:   "Zhuo, Qiuxu" <qiuxu.zhuo@intel.com>
To:     "Joel Fernandes (Google)" <joel@joelfernandes.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
CC:     Frederic Weisbecker <frederic@kernel.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        "rcu@vger.kernel.org" <rcu@vger.kernel.org>
Subject: RE: [PATCH RFC v2] rcu: Add a minimum time for marking boot as
 completed
Thread-Topic: [PATCH RFC v2] rcu: Add a minimum time for marking boot as
 completed
Thread-Index: AQHZSMoUan4Z/tmfYUGikm7Xfdsdaq7iacDQ
Date:   Mon, 27 Feb 2023 07:53:35 +0000
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References: <20230225033421.3323164-1-joel@joelfernandes.org>
In-Reply-To: <20230225033421.3323164-1-joel@joelfernandes.org>
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> From: Joel Fernandes (Google) <joel@joelfernandes.org>
> Sent: Saturday, February 25, 2023 11:34 AM
> To: linux-kernel@vger.kernel.org
> Cc: Joel Fernandes (Google) <joel@joelfernandes.org>; Frederic Weisbecker
> <frederic@kernel.org>; Lai Jiangshan <jiangshanlai@gmail.com>; linux-
> doc@vger.kernel.org; Paul E. McKenney <paulmck@kernel.org>;
> rcu@vger.kernel.org
> Subject: [PATCH RFC v2] rcu: Add a minimum time for marking boot as
> completed
>=20
> 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.
>=20
> 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.
>=20
> For these reasons, this commit adds a config option
> 'CONFIG_RCU_BOOT_END_DELAY' and a boot parameter
> rcupdate.boot_end_delay.
>=20
> 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 no=
t
> 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=3Dy kernel concern more about the=20
real-time latency than the overall OS boot time?

If so, we might make rcupdate.boot_end_delay =3D 0 as the default value=20
(NOT the default 20s) for CONFIG_PREEMPT_RT=3Dy kernels?=20

-Qiuxu

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