On Mon, Sep 05, 2022 at 11:38:50AM +0800, Pingfan Liu wrote:
> At present, during the cpu teardown, rcu_boost_kthread_setaffinity() is
> called twice. Firstly by rcutree_offline_cpu(), then by
> rcutree_dead_cpu() as the CPUHP_RCUTREE_PREP cpuhp_step callback.
>
> >From the scheduler's perspective, a bit in cpu_online_mask means that the cpu
> is visible to the scheduler. Furthermore, a bit in cpu_active_mask
> means that the cpu is suitable as a migration destination.
>
> Now turning back to the case in rcu offlining. sched_cpu_deactivate()
> has disabled the dying cpu as the migration destination before
> rcutree_offline_cpu(). Furthermore, if the boost kthread is on the dying
> cpu, it will be migrated to another suitable online cpu by the scheduler.
> So the affinity setting by rcutree_offline_cpu() is redundant and can be
> eliminated.
>
> Besides, this patch does an trival code rearrangement by unfolding
> rcutree_affinity_setting() into rcutree_online_cpu(), considering that
> the latter one is the only user of the former.
>
> Signed-off-by: Pingfan Liu <kernelfans@xxxxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> Cc: David Woodhouse <dwmw@xxxxxxxxxxxx>
> Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Cc: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx>
> Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
> Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> Cc: "Jason A. Donenfeld" <Jason@xxxxxxxxx>
> To: rcu@xxxxxxxxxxxxxxx
> ---
> kernel/rcu/tree.c | 14 +-------------
> 1 file changed, 1 insertion(+), 13 deletions(-)
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 79aea7df4345..b90f6487fd45 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3978,16 +3978,6 @@ int rcutree_prepare_cpu(unsigned int cpu)
> return 0;
> }
>
> -/*
> - * Update RCU priority boot kthread affinity for CPU-hotplug changes.
> - */
> -static void rcutree_affinity_setting(unsigned int cpu, int outgoing)
> -{
> - struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
> -
> - rcu_boost_kthread_setaffinity(rdp->mynode, outgoing);
> -}
Good point, tiven how simple a wrapper this is and how little it is used,
getting rid of it does sound like a reasonable idea.
> /*
> * Near the end of the CPU-online process. Pretty much all services
> * enabled, and the CPU is now very much alive.
> @@ -4006,7 +3996,7 @@ int rcutree_online_cpu(unsigned int cpu)
> if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
> return 0; /* Too early in boot for scheduler work. */
> sync_sched_exp_online_cleanup(cpu);
> - rcutree_affinity_setting(cpu, -1);
> + rcu_boost_kthread_setaffinity(rdp->mynode, -1);
>
> // Stop-machine done, so allow nohz_full to disable tick.
> tick_dep_clear(TICK_DEP_BIT_RCU);
> @@ -4029,8 +4019,6 @@ int rcutree_offline_cpu(unsigned int cpu)
> rnp->ffmask &= ~rdp->grpmask;
> raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
>
> - rcutree_affinity_setting(cpu, cpu);
We do need to keep this one because the CPU is going away.
One the other hand, it might well be that we could get rid of the call
to rcutree_affinity_setting() in rcutree_dead_cpu().
Or am I missing something subtle here?
Thanx, Paul
> -
> // nohz_full CPUs need the tick for stop-machine to work quickly
> tick_dep_set(TICK_DEP_BIT_RCU);
> return 0;
> --
> 2.31.1
>
