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McKenney" Cc: linux-kernel@vger.kernel.org, rcu@vger.kernel.org, Frederic Weisbecker , Neeraj Upadhyay , Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Joel Fernandes , Thomas Gleixner , Steven Price , Mark Rutland , Kuppuswamy Sathyanarayanan , "Jason A. Donenfeld" , boqun.feng@gmail.com Subject: Re: [RFC 06/10] rcu/hotplug: Make rcutree_dead_cpu() parallel Message-ID: References: <20220822021520.6996-1-kernelfans@gmail.com> <20220822021520.6996-7-kernelfans@gmail.com> <20220822024528.GC6159@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220822024528.GC6159@paulmck-ThinkPad-P17-Gen-1> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > > 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. [1]: https://lore.kernel.org/linux-arm-kernel/20220822021520.6996-3-kernelfans@gmail.com/T/#mf62352138d7b040fdb583ba66f8cd0ed1e145feb Thanks, Pingfan > Thanx, Paul > > > Cc: "Paul E. McKenney" > > Cc: Frederic Weisbecker > > Cc: Neeraj Upadhyay > > Cc: Josh Triplett > > Cc: Steven Rostedt > > Cc: Mathieu Desnoyers > > Cc: Lai Jiangshan > > Cc: Joel Fernandes > > Cc: Thomas Gleixner > > Cc: Steven Price > > Cc: "Peter Zijlstra > > Cc: Mark Rutland > > Cc: Kuppuswamy Sathyanarayanan > > Cc: "Jason A. Donenfeld" > > 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 > >