Re: [PATCH RFC] rcu/kfree: Do not request RCU when not needed

[Uladzislau Rezki <urezki@gmail.com>]

On Wed, Nov 02, 2022 at 09:35:44AM -0700, Paul E. McKenney wrote:
> On Wed, Nov 02, 2022 at 12:13:17PM -0400, Joel Fernandes wrote:
> > On Wed, Nov 2, 2022 at 8:37 AM Uladzislau Rezki <urezki@gmail.com> 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@joelfernandes.org>
> > > > ---
> > > >  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.
> 
> Just for completeness, the main purpose of rdp->gp_seq is to reject
> quiescent states that were seen during already-completed grace periods.
> 
So it means that instead of gp_seq reading we should take a snaphshot
of the current state:

snp = get_state_synchronize_rcu();

and later on do a:

cond_synchronize_rcu(snp);

to wait for a GP. Or if the poll_state_synchronize_rcu(oldstate)) != 0
queue_rcu_work().

Sorry for a description using the RCU API functions name :) 

--
Uladzislau Rezki