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? thanks, - Joel
