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

[Uladzislau Rezki <urezki@gmail.com>]

On Mon, Feb 27, 2023 at 10:16:51AM -0500, Joel Fernandes wrote:
> 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:
> > > >
> > > > 
> > > >>
> > > >> 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
> > > >>
> > > >> 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.
> >
> > 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?
> 
Why "rcu_boot_ended" is not enough? As i see right after that an "init"
process or shell or panic is going to be invoked by the kernel. It basically
indicates that a kernel is fully functional.

Or an idea to wait even further? Until all kernel modules are loaded by
user space.

Thanks!

--
Uladzislau Rezki