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
