On Wed, Nov 2, 2022 at 8:37 AM Uladzislau Rezki <urezki@xxxxxxxxx> wrote:
>
> On Sat, Oct 29, 2022 at 01:28:56PM +0000, Joel Fernandes (Google) wrote:
> > On ChromeOS, I am (almost) always seeing the optimization trigger.
> > Tested boot up and trace_printk'ing how often it triggers.
> >
> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> > ---
> > kernel/rcu/tree.c | 18 +++++++++++++++++-
> > 1 file changed, 17 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 591187b6352e..3e4c50b9fd33 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -2935,6 +2935,7 @@ struct kfree_rcu_cpu_work {
> >
> > /**
> > * struct kfree_rcu_cpu - batch up kfree_rcu() requests for RCU grace period
> > + * @rdp: The rdp of the CPU that this kfree_rcu corresponds to.
> > * @head: List of kfree_rcu() objects not yet waiting for a grace period
> > * @bkvhead: Bulk-List of kvfree_rcu() objects not yet waiting for a grace period
> > * @krw_arr: Array of batches of kfree_rcu() objects waiting for a grace period
> > @@ -2964,6 +2965,8 @@ struct kfree_rcu_cpu {
> > struct kfree_rcu_cpu_work krw_arr[KFREE_N_BATCHES];
> > raw_spinlock_t lock;
> > struct delayed_work monitor_work;
> > + struct rcu_data *rdp;
> > + unsigned long last_gp_seq;
> > bool initialized;
> > int count;
> >
> > @@ -3167,6 +3170,7 @@ schedule_delayed_monitor_work(struct kfree_rcu_cpu *krcp)
> > mod_delayed_work(system_wq, &krcp->monitor_work, delay);
> > return;
> > }
> > + krcp->last_gp_seq = krcp->rdp->gp_seq;
> > queue_delayed_work(system_wq, &krcp->monitor_work, delay);
> > }
> >
> > @@ -3217,7 +3221,17 @@ static void kfree_rcu_monitor(struct work_struct *work)
> > // be that the work is in the pending state when
> > // channels have been detached following by each
> > // other.
> > - queue_rcu_work(system_wq, &krwp->rcu_work);
> > + //
> > + // NOTE about gp_seq wrap: In case of gp_seq overflow,
> > + // it is possible for rdp->gp_seq to be less than
> > + // krcp->last_gp_seq even though a GP might be over. In
> > + // this rare case, we would just have one extra GP.
> > + if (krcp->last_gp_seq &&
> >
> This check can be eliminated i think. A kfree_rcu_cpu is defined as
> static so by default the last_gp_set is set to zero.
Ack.
> > @@ -4802,6 +4816,8 @@ static void __init kfree_rcu_batch_init(void)
> > for_each_possible_cpu(cpu) {
> > struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
> >
> > + krcp->rdp = per_cpu_ptr(&rcu_data, cpu);
> > + krcp->last_gp_seq = 0;
> >
> Yep. This one can be just dropped.
>
> But all the rest looks good :) I will give it a try from test point of
> view. It is interested from the memory footprint point of view.
Ack. Thanks. Even though we should not sample rdp->gp_seq, I think it
is still worth a test.
