On Wed, Aug 24, 2022 at 3:21 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > On Wed, Aug 24, 2022 at 01:26:01PM -0400, Joel Fernandes wrote: > > > > > > 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 :) > > Looks like it should be regardless of Pingfan's patches, given that > the grace-period kthread might report a quiescent state concurrently. Thanks for confirming, I'll queue it into my next revision of the series. - Joel
