On Mon, Sep 05, 2022 at 11:53:52AM +0800, Pingfan Liu wrote: > On Thu, Sep 1, 2022 at 12:15 AM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > > > On Wed, Aug 24, 2022 at 09:20:50AM -0700, 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.) > > > > > Sorry to reply late, since I am interrupted by some other things. > I have took a different way and posted a series ([PATCH 1/3] rcu: > remove redundant cpu affinity setting during teardown) for that on > https://lore.kernel.org/rcu/20220905033852.18988-1-kernelfans@xxxxxxxxx/T/#t And I took patch #3, thank you! #1 allows the kthread to run on the outgoing CPU, which is to be avoided, and #2 depends on #1. > Besides, for the integration of the concurrency cpu hot-removing into > the rcu torture test, I begin to do it. Very good! I am looking forward to seeing what you come up with. > > > There are probably more where that one came from. ;-) > > > > And here is one more from this week's session. > > Thanks for the update. > > > The calls to tick_dep_set() and tick_dep_clear() use atomic operations, > > but they operate on a global variable. This means that the first call > > to rcutree_offline_cpu() would enable the tick and the first call to > > rcutree_dead_cpu() would disable the tick. This might be OK, but it > > is at the very least bad practice. There needs to be a counter > > mediating these calls. > > I will see what I can do here. > > > For more detail, please see the Google document: > > > > https://docs.google.com/document/d/1jymsaCPQ1PUDcfjIKm0UIbVdrJAaGX-6cXrmcfm0PRU/edit?usp=sharing > > > > Have read it and hope that both online and offline concurrency can > come to true in near future. Indeed, I suspect that a lot of people would like to see faster kexec! Thanx, Paul
