Re: [PATCH v2 5/8] rcu/nocb: Wake up gp thread when flushing

[Joel Fernandes]

On Sat, Jun 25, 2022 at 09:06:22PM -0700, Paul E. McKenney wrote:
> On Wed, Jun 22, 2022 at 10:50:59PM +0000, Joel Fernandes (Google) wrote:
> > We notice that rcu_barrier() can take a really long time. It appears
> > that this can happen when all CBs are lazy and the timer does not fire
> > yet. So after flushing, nothing wakes up GP thread. This patch forces
> > GP thread to wake when bypass flushing happens, this fixes the
> > rcu_barrier() delays with lazy CBs.
> 
> I am wondering if there is a bug in non-rcu_barrier() lazy callback
> processing hiding here as well?

I don't think so because in both nocb_try_bypass and nocb_gp_wait, we are not
going to an indefinite sleep after the flush. However, with rcu_barrier() ,
there is nothing to keep the RCU GP thread awake. That's my theory at least.
In practice, I have not been able to reproduce this issue with
non-rcu_barrier().

With rcu_barrier() I happen to hit it thanks to the rcuscale changes I did.
That's an interesting story! As I apply call_rcu_lazy() to the file table
code, turns out that on boot, the initram unpacking code continously triggers
call_rcu_lazy(). This happens apparently in a different thread than the one
that rcuscale is running in. In rcuscale, I did rcu_barrier() at init time
and this stalled for a long time to my surprise, and this patch fixed it.

thanks,

 - Joel


> 
> 							Thanx, Paul
> 
> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> > ---
> >  kernel/rcu/tree_nocb.h | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> > index 2f5da12811a5..b481f1ea57c0 100644
> > --- a/kernel/rcu/tree_nocb.h
> > +++ b/kernel/rcu/tree_nocb.h
> > @@ -325,6 +325,8 @@ static bool rcu_nocb_do_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
> >  	rcu_segcblist_insert_pend_cbs(&rdp->cblist, &rcl);
> >  	WRITE_ONCE(rdp->nocb_bypass_first, j);
> >  	rcu_nocb_bypass_unlock(rdp);
> > +
> > +	wake_nocb_gp(rdp, true);
> >  	return true;
> >  }
> >  
> > -- 
> > 2.37.0.rc0.104.g0611611a94-goog
> >