On Sun, Jun 26, 2022 at 01:45:32PM +0000, Joel Fernandes wrote: > 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. Cool! Then should this wake_nocb_gp() instead go into the rcu_barrier() code path? As shown below, wouldn't we be doing some spurious wakeups? Thanx, Paul > 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 > > >
