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From:   Joel Fernandes <joel@joelfernandes.org>
Date:   Mon, 27 Feb 2023 10:16:51 -0500
Message-ID: <CAEXW_YSVm7fjJaX=AT-Yg70wXL-_1RxoVPfzN8M8zJAkc0um3g@mail.gmail.com>
Subject: Re: [PATCH RFC v2] rcu: Add a minimum time for marking boot as completed
To:     paulmck@kernel.org
Cc:     "Zhuo, Qiuxu" <qiuxu.zhuo@intel.com>, linux-kernel@vger.kernel.org,
        Frederic Weisbecker <frederic@kernel.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        linux-doc@vger.kernel.org, rcu@vger.kernel.org
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On Mon, Feb 27, 2023 at 9:55 AM Paul E. McKenney <paulmck@kernel.org> wrote=
:
>
> On Mon, Feb 27, 2023 at 08:22:06AM -0500, Joel Fernandes wrote:
> >
> >
> > > On Feb 27, 2023, at 2:53 AM, Zhuo, Qiuxu <qiuxu.zhuo@intel.com> wrote=
:
> > >
> > > =EF=BB=BF
> > >>
> > >> 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 Weisb=
ecker
> > >> <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
> > >>
> > >> On many systems, a great deal of boot happens after the kernel think=
s the
> > >> boot has completed. It is difficult to determine if the system has r=
eally
> > >> booted from the kernel side. Some features like lazy-RCU can risk sl=
owing
> > >> 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_b=
oot' 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 speci=
fy a value
> > >> here to keep RCU from marking boot completion.  The boot sequence wi=
ll not
> > >> be marked ended until at least boot_end_delay milliseconds have pass=
ed.
> > >
> > > Hi Joel,
> > >
> > > Just some thoughts on the default value of 20s, correct me if I'm wro=
ng :-).
> > >
> > > Does the OS with CONFIG_PREEMPT_RT=3Dy 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 =3D 0 as the default val=
ue
> > > (NOT the default 20s) for CONFIG_PREEMPT_RT=3Dy 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.
>
> Provide a /sys location that the userspace code writes to when it
> is ready?  Different systems with different hardware and software
> configurations are going to take different amounts of time to boot,
> correct?

I could add a sysfs node, but I still wanted this patch as well
because I am wary of systems where yet more userspace changes are
required. I feel the kernel should itself be able to do this. Yes, it
is possible the system completes "booting" at a different time than
what the kernel thinks. But it does that anyway (even without this
patch), so I am not seeing a good reason to not do this in the kernel.
It is also only a minimum cap, so if the in-kernel boot takes too
long, then the patch will have no effect.

Thoughts?

thanks,

 - Joel