On Tue, Aug 23, 2022 at 11:01 AM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > On Tue, Aug 23, 2022 at 09:50:56AM +0800, Pingfan Liu wrote: > > On Sun, Aug 21, 2022 at 07:45:28PM -0700, Paul E. McKenney wrote: > > > On Mon, Aug 22, 2022 at 10:15:16AM +0800, Pingfan Liu wrote: > > > > In order to support parallel, rcu_state.n_online_cpus should be > > > > atomic_dec() > > > > > > > > Signed-off-by: Pingfan Liu <kernelfans@xxxxxxxxx> > > > > > > I have to ask... What testing have you subjected this patch to? > > > > > > > This patch subjects to [1]. The series aims to enable kexec-reboot in > > parallel on all cpu. As a result, the involved RCU part is expected to > > support parallel. > > I understand (and even sympathize with) the expectation. But results > sometimes diverge from expectations. There have been implicit assumptions > in RCU about only one CPU going offline at a time, and I am not sure > that all of them have been addressed. Concurrent CPU onlining has > been looked at recently here: > > https://docs.google.com/document/d/1jymsaCPQ1PUDcfjIKm0UIbVdrJAaGX-6cXrmcfm0PRU/edit?usp=sharing > > You did us atomic_dec() to make rcu_state.n_online_cpus decrementing be > atomic, which is good. Did you look through the rest of RCU's CPU-offline > code paths and related code paths? > I went through those codes at a shallow level, especially at each cpuhp_step hook in the RCU system. But as you pointed out, there are implicit assumptions about only one CPU going offline at a time, I will chew the google doc which you share. Then I can come to a final result. > > [1]: https://lore.kernel.org/linux-arm-kernel/20220822021520.6996-3-kernelfans@xxxxxxxxx/T/#mf62352138d7b040fdb583ba66f8cd0ed1e145feb > > Perhaps I am more blind than usual today, but I am not seeing anything > in this patch describing the testing. At this point, I am thinking in > terms of making rcutorture test concurrent CPU offlining parallel > Yes, testing results are more convincing in this area. After making clear the implicit assumptions, I will write some code to bridge my code and rcutorture test. Since the series is a little different from parallel cpu offlining. It happens after all devices are torn down, and there is no way to rollback. > Thoughts? > Need a deeper dive into this field. Hope to bring out something soon. Thanks, Pingfan
