On 8/24/2022 12:20 PM, Paul E. McKenney wrote: > On Wed, Aug 24, 2022 at 09:53:11PM +0800, Pingfan Liu wrote: >> 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. > > And that is fine, at least as a first step. > >> 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. > > Boqun Feng, Neeraj Upadhyay, Uladzislau Rezki, and I took a quick look, > and rcu_boost_kthread_setaffinity() seems to need some help. As it > stands, it appears that concurrent invocations of this function from the > CPU-offline path will cause all but the last outgoing CPU's bit to be > (incorrectly) set in the cpumask_var_t passed to set_cpus_allowed_ptr(). > > This should not be difficult to fix, for example, by maintaining a > separate per-leaf-rcu_node-structure bitmask of the concurrently outgoing > CPUs for that rcu_node structure. (Similar in structure to the > ->qsmask field.) > > There are probably more where that one came from. ;-) Should rcutree_dying_cpu() access to rnp->qsmask have a READ_ONCE() ? I was thinking grace period initialization or qs reporting paths racing with that. Its just tracing, still :) Thanks, - Joel
