Messages in this thread | | | Date | Mon, 22 Aug 2022 20:01:25 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [RFC 06/10] rcu/hotplug: Make rcutree_dead_cpu() parallel |
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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@gmail.com> > > > > 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@gmail.com/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@kernel.org> > > > Cc: Frederic Weisbecker <frederic@kernel.org> > > > Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com> > > > Cc: Josh Triplett <josh@joshtriplett.org> > > > Cc: Steven Rostedt <rostedt@goodmis.org> > > > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> > > > Cc: Lai Jiangshan <jiangshanlai@gmail.com> > > > Cc: Joel Fernandes <joel@joelfernandes.org> > > > Cc: Thomas Gleixner <tglx@linutronix.de> > > > Cc: Steven Price <steven.price@arm.com> > > > Cc: "Peter Zijlstra > > > Cc: Mark Rutland <mark.rutland@arm.com> > > > Cc: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com> > > > Cc: "Jason A. Donenfeld" <Jason@zx2c4.com> > > > To: linux-kernel@vger.kernel.org > > > To: rcu@vger.kernel.org > > > --- > > > 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 > > >
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