Re: [PATCH v2] rcu/kfree: Do not request RCU when not needed

[Uladzislau Rezki]

> Hi,
> 
> On Thu, Nov 10, 2022 at 8:05 AM Uladzislau Rezki <urezki@xxxxxxxxx> wrote:
> 
> > > On ChromeOS, using this with the increased timeout, we see that we
> > almost always
> > > never need to initiate a new grace period. Testing also shows this frees
> > large
> > > amounts of unreclaimed memory, under intense kfree_rcu() pressure.
> > >
> > > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> > > ---
> > > v1->v2: Same logic but use polled grace periods instead of sampling
> > gp_seq.
> > >
> > >  kernel/rcu/tree.c | 8 +++++++-
> > >  1 file changed, 7 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index 591187b6352e..ed41243f7a49 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
> > > + * @gp_snap: The GP snapshot recorded at the last scheduling of monitor
> > work.
> > >   * @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 +2965,7 @@ struct kfree_rcu_cpu {
> > >       struct kfree_rcu_cpu_work krw_arr[KFREE_N_BATCHES];
> > >       raw_spinlock_t lock;
> > >       struct delayed_work monitor_work;
> > > +     unsigned long gp_snap;
> > >       bool initialized;
> > >       int count;
> > >
> > > @@ -3167,6 +3169,7 @@ schedule_delayed_monitor_work(struct kfree_rcu_cpu
> > *krcp)
> > >                       mod_delayed_work(system_wq, &krcp->monitor_work,
> > delay);
> > >               return;
> > >       }
> > > +     krcp->gp_snap = get_state_synchronize_rcu();
> > >       queue_delayed_work(system_wq, &krcp->monitor_work, delay);
> > >  }
> > >
> > How do you guarantee a full grace period for objects which proceed
> > to be placed into an input stream that is not yet detached?
> 
> 
> Just replying from phone as I’m OOO today.
> 
> Hmm, so you’re saying that objects can be queued after the delayed work has
> been queued, but processed when the delayed work is run? I’m looking at
> this code after few years so I may have missed something.
> 
> That’s a good point and I think I missed that. I think your version did too
> but I’ll have to double check.
> 
> The fix then is to sample the clock for the latest object queued, not for
> when the delayed work is queued.
> 
The patch i sent gurantee it. Just in case see v2: