On Thu, Aug 25, 2022 at 07:24:05PM +0000, Joel Fernandes wrote:
> On Thu, Aug 25, 2022 at 10:26:54AM -0700, Paul E. McKenney wrote:
> > On Thu, Aug 25, 2022 at 05:23:11PM +0800, Zhen Lei wrote:
> > > When CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y or CONFIG_NO_HZ_FULL=y, additional
> > > CPUs need to be added to 'rcu_nocb_mask'. But 'rcu_nocb_mask' may be not
> > > available now, due to 'rcu_nocbs' is not specified. Check and initialize
> > > 'rcu_nocb_mask' before using it. This code simplification strictly follows
> > > this logic, compared with old implementations, unnecessary crossovers are
> > > avoided and easy to understand.
> > >
> > > Signed-off-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
> >
> > Much nicer, thank you!
> >
> > As usual, I could not resist the urge to wordsmith. Could you please
> > check to make sure that I did not mess anything up?
> >
> > Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> > commit 4ac3b3d1a19943b1522c0b1d0895aefbb80ec294
> > Author: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
> > Date: Thu Aug 25 17:23:11 2022 +0800
> >
> > rcu: Simplify rcu_init_nohz() cpumask handling
> >
> > In kernels built with either CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y or
> > CONFIG_NO_HZ_FULL=y, additional CPUs must be added to rcu_nocb_mask.
> > Except that kernels booted without the rcu_nocb_mask= will not have
> > allocated rcu_nocb_mask. And the current rcu_init_nohz() function uses
> > its need_rcu_nocb_mask and offload_all local variables to track the
> > rcu_nocb and nohz_full state.
> >
> > But there is a much simpler approach, namely creating a cpumask pointer
> > to track the default and then using cpumask_available() to check the
> > rcu_nocb_mask state. This commit takes this approach, thereby simplifying
> > and shortening the rcu_init_nohz() function.
> >
> > Signed-off-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
>
> Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
Thank you, and I will apply this on the next rebase.
Thanx, Paul
> thanks,
>
> - Joel
>
>
> >
> > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> > index 0a5f0ef414845..c8167be2288fa 100644
> > --- a/kernel/rcu/tree_nocb.h
> > +++ b/kernel/rcu/tree_nocb.h
> > @@ -1210,45 +1210,31 @@ EXPORT_SYMBOL_GPL(rcu_nocb_cpu_offload);
> > void __init rcu_init_nohz(void)
> > {
> > int cpu;
> > - bool need_rcu_nocb_mask = false;
> > - bool offload_all = false;
> > struct rcu_data *rdp;
> > + const struct cpumask *cpumask = NULL;
> >
> > #if defined(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL)
> > - if (!rcu_state.nocb_is_setup) {
> > - need_rcu_nocb_mask = true;
> > - offload_all = true;
> > - }
> > -#endif /* #if defined(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL) */
> > -
> > -#if defined(CONFIG_NO_HZ_FULL)
> > - if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask)) {
> > - need_rcu_nocb_mask = true;
> > - offload_all = false; /* NO_HZ_FULL has its own mask. */
> > - }
> > -#endif /* #if defined(CONFIG_NO_HZ_FULL) */
> > + cpumask = cpu_possible_mask;
> > +#elif defined(CONFIG_NO_HZ_FULL)
> > + if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
> > + cpumask = tick_nohz_full_mask;
> > +#endif
> >
> > - if (need_rcu_nocb_mask) {
> > + if (cpumask) {
> > if (!cpumask_available(rcu_nocb_mask)) {
> > if (!zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL)) {
> > pr_info("rcu_nocb_mask allocation failed, callback offloading disabled.\n");
> > return;
> > }
> > }
> > +
> > + cpumask_or(rcu_nocb_mask, rcu_nocb_mask, cpumask);
> > rcu_state.nocb_is_setup = true;
> > }
> >
> > if (!rcu_state.nocb_is_setup)
> > return;
> >
> > -#if defined(CONFIG_NO_HZ_FULL)
> > - if (tick_nohz_full_running)
> > - cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask);
> > -#endif /* #if defined(CONFIG_NO_HZ_FULL) */
> > -
> > - if (offload_all)
> > - cpumask_setall(rcu_nocb_mask);
> > -
> > if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
> > pr_info("\tNote: kernel parameter 'rcu_nocbs=', 'nohz_full', or 'isolcpus=' contains nonexistent CPUs.\n");
> > cpumask_and(rcu_nocb_mask, cpu_possible_mask,
