> 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. Thanks, - Joel > > -Qiuxu > >> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> >> --- >> v1->v2: >> Update some comments and description. >> ... >
