On Mon, Feb 27, 2023 at 03:05:02PM -0800, Paul E. McKenney wrote:

On Mon, Feb 27, 2023 at 02:10:30PM -0500, Joel Fernandes wrote:

The combination of sysfs manipulated by userspace and a kernel failsafe
makes sense to me.  Especially if by default triggering the failsafe
splats.  That way, bugs where userspace fails to update the sysfs file
get caught.

The non-default silent-failsafe mode is also useful to allow some power
savings in advance of userspace getting the sysfs updating in place.
And of course the default splatting setup can be used in internal testing
with the release software being more tolerant of userspace foibles.

I'm wondering, this is all about CONFIG_RCU_LAZY, right? Or does also expedited
GP turned off a bit early or late on boot matter for anybody in practice?

So shouldn't we disable lazy callbacks by default when CONFIG_RCU_LAZY=y and then
turn it on with "sysctl kernel.rcu.lazy=1" only whenever userspace feels ready
about it? We can still keep the current call to rcu_end_inkernel_boot().

And if suddenly disabling lazy by default is an ABI breakage we can still add
CONFIG_RCU_LAZY_DEFAULT_DISABLED.