On Mon, Feb 27, 2023 at 01:27:20PM -0500, Joel Fernandes wrote: > > > > On Feb 27, 2023, at 1:20 PM, Uladzislau Rezki <urezki@xxxxxxxxx> 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@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". > > 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
