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? > [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. Thoughts? Thanx, Paul > Thanks, > > Pingfan > > > > Thanx, Paul > > > > > Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx> > > > Cc: Frederic Weisbecker <frederic@xxxxxxxxxx> > > > Cc: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx> > > > Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx> > > > Cc: Steven Rostedt <rostedt@xxxxxxxxxxx> > > > Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> > > > Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx> > > > Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> > > > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > > > Cc: Steven Price <steven.price@xxxxxxx> > > > Cc: "Peter Zijlstra > > > Cc: Mark Rutland <mark.rutland@xxxxxxx> > > > Cc: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx> > > > Cc: "Jason A. Donenfeld" <Jason@xxxxxxxxx> > > > To: linux-kernel@xxxxxxxxxxxxxxx > > > To: rcu@xxxxxxxxxxxxxxx > > > --- > > > kernel/cpu.c | 1 + > > > kernel/rcu/tree.c | 3 ++- > > > 2 files changed, 3 insertions(+), 1 deletion(-) > > > > > > diff --git a/kernel/cpu.c b/kernel/cpu.c > > > index 1261c3f3be51..90debbe28e85 100644 > > > --- a/kernel/cpu.c > > > +++ b/kernel/cpu.c > > > @@ -1872,6 +1872,7 @@ static struct cpuhp_step cpuhp_hp_states[] = { > > > .name = "RCU/tree:prepare", > > > .startup.single = rcutree_prepare_cpu, > > > .teardown.single = rcutree_dead_cpu, > > > + .support_kexec_parallel = true, > > > }, > > > /* > > > * On the tear-down path, timers_dead_cpu() must be invoked > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > > index 79aea7df4345..07d31e16c65e 100644 > > > --- a/kernel/rcu/tree.c > > > +++ b/kernel/rcu/tree.c > > > @@ -2168,7 +2168,8 @@ int rcutree_dead_cpu(unsigned int cpu) > > > if (!IS_ENABLED(CONFIG_HOTPLUG_CPU)) > > > return 0; > > > > > > - WRITE_ONCE(rcu_state.n_online_cpus, rcu_state.n_online_cpus - 1); > > > + /* Hot remove path allows parallel, while Hot add races against remove on lock */ > > > + atomic_dec((atomic_t *)&rcu_state.n_online_cpus); > > > /* Adjust any no-longer-needed kthreads. */ > > > rcu_boost_kthread_setaffinity(rnp, -1); > > > // Stop-machine done, so allow nohz_full to disable tick. > > > -- > > > 2.31.1 > > >
