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 01:27:20PM -0500, Joel Fernandes wrote:
> 
> 
> > On Feb 27, 2023, at 1:20 PM, Uladzislau Rezki <urezki@gmail.com> wrote:
> > 
> > On Mon, Feb 27, 2023 at 01:15:47PM -0500, Joel Fernandes wrote:
> >> 
> >> 
> >>>> On Feb 27, 2023, at 1:06 PM, Uladzislau Rezki <urezki@gmail.com> 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@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.
> >> 
> >> 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".
> 
> I dont know if you caught up with the other threads. See replies from Paul and my reply to that.
> 
> Also what you are proposing can be more harmful. If user space has a bug and does not notify the kernel that boot completed, then the boot can stay incomplete forever. The idea with this patch is to make things better, not worse.
> 
I saw that Paul proposed to have a sysfs attribute using which you can
send a notification.

IMHO, to me this patch does not provide a clear correlation between what
is a boot complete and when it occurs. A boot complete is a synchronous
event whereas the patch thinks that after some interval a "boot" is completed.

We can imply that after, say 100 seconds an initialization of user space
is done. Maybe 100 seconds then? :)

--
Uladzislau Rezki