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

[Uladzislau Rezki]

On Mon, Feb 27, 2023 at 01:15:47PM -0500, Joel Fernandes wrote:
> 
> 
> > On Feb 27, 2023, at 1:06 PM, Uladzislau Rezki <urezki@xxxxxxxxx> wrote:
> > 
> > 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@xxxxxxxxxx> 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@xxxxxxxxx> wrote:
> >>>>> 
> >>>>> 
> >>>>>> 
> >>>>>> 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?
> >>>> 
> >>>> 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.
> 
> I mentioned in commit message it is daemons, userspace initialization etc. There is a lot of userspace booting up as well and using the kernel while doing so.
> 
> So, It does not make sense to me to mark kernel as booted too early. And no harm in adding some builtin kernel hysteresis. What am I missing?
> 
Than it is up to user space to decide when it is ready in terms of "boot completed".

--
Uladzislau Rezki