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From:   Joel Fernandes <joel@joelfernandes.org>
Mime-Version: 1.0 (1.0)
Subject: Re: [PATCH RFC v2] rcu: Add a minimum time for marking boot as completed
Date:   Mon, 27 Feb 2023 08:22:06 -0500
Message-Id: <764CA486-6FB2-4667-B8CB-56E3AC31FD58@joelfernandes.org>
References: <IA1PR11MB617175CA398D425B7489CE4889AF9@IA1PR11MB6171.namprd11.prod.outlook.com>
Cc:     linux-kernel@vger.kernel.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
In-Reply-To: <IA1PR11MB617175CA398D425B7489CE4889AF9@IA1PR11MB6171.namprd11.prod.outlook.com>
To:     "Zhuo, Qiuxu" <qiuxu.zhuo@intel.com>
Precedence: bulk


> On Feb 27, 2023, at 2:53 AM, Zhuo, Qiuxu <qiuxu.zhuo@intel.com> wrote:
>=20
> =EF=BB=BF
>>=20
>> 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' t=
o
>> 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 v=
alue
>> 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.
>=20
> Hi Joel,
>=20
> Just some thoughts on the default value of 20s, correct me if I'm wrong :-=
).
>=20
> Does the OS with CONFIG_PREEMPT_RT=3Dy kernel concern more about the=20
> real-time latency than the overall OS boot time?

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

>=20
> 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

Could you measure how much time your RT system takes to boot before the appl=
ication 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 f=
or their system.

Thanks,

 - Joel


>=20
> -Qiuxu
>=20
>> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
>> ---
>> v1->v2:
>>    Update some comments and description.
>> ...
>=20