On Wed, Mar 01, 2023 at 08:08:54PM -0500, Joel Fernandes wrote: > On Wed, Mar 1, 2023 at 7:49 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > > > On Wed, Mar 01, 2023 at 04:31:01PM -0500, Joel Fernandes wrote: > > > On Wed, Mar 1, 2023 at 12:11 PM Frederic Weisbecker <frederic@xxxxxxxxxx> wrote: > > > [...] > > > > > Hmmm I see what you mean, so a conservative and configurable "fail-safe" > > > > > timeout followed by sysctl to end the boot earlier than the timeout, should > > > > > do it (something like 30 seconds IMHO sounds reasonable)? In any case, > > > > > whatever way we go, we would not end the kernel boot before > > > > > rcu_end_inkernel_boot() is called at least once (which is the current > > > > > behavior). > > > > > > > > > > So it would be: > > > > > > > > > > low level boot + initcalls > > > > > 20 sec 30 second timeout > > > > > |------------------------------|-------------------------- > > > > > | | > > > > > old rcu_end_inkernel_boot() new rcu_end_inkernel_boot() > > > > > > > > > > But it could be, if user decides: > > > > > low level boot + initcalls > > > > > 20 sec 10 second timeout > > > > > |------------------------------|-------------------------- > > > > > | | > > > > > old rcu_end_inkernel_boot() new rcu_end_inkernel_boot() > > > > > via /sys/ entry. > > > > > > > > The problem I have with a random default timeout is that it may break sensitive > > > > workloads. If the default is 30 and say the boot only takes 5 seconds and > > > > immediately launches a latency sensitive task, this may break things in a > > > > subtle way during these 25 seconds when it usually didn't. Because expedited > > > > RCU is a hammer interrupting all non-idle CPUs. > > > > > > > > Until now forcing expedited RCU was only performed before any user code. Now it > > > > crosses the boundary so better be careful. I'd personally advocate for keeping > > > > the current call before init is launched. Then provide an end_boot_sysctl kernel > > > > boot parameter that will ignore the current call before init and let the user > > > > signal that through sysctl. > > > > > > Considering that the PREEMPT-RT system benefits from it within the 8 > > > seconds, I will go ahead make the default 15 seconds or so and make it > > > tunable. Hopefully that will be an acceptable compromise, with > > > sufficient documentation, changelog, and so forth... If you agree I'd > > > appreciate your Ack on the next posting. > > > > Just checking on the sysfs portion of this. > > Yes, the current plan is to add a sysfs node (likely sysctl) with the > 15-second failsafe. Whew!!! Very good, then! > > After all, tuning kernel > > boot parameters to specific systems is not all that much fun, especially > > when you have lots of systems. > > Are you suggesting to drop "end the boot" sysfs and just have a > minimum-time approach as I initially did? Not at all. I was just concerned that the sysfs was getting lost in the shuffle. All good now, thank you! Thanx, Paul
