On Fri, Sep 02, 2022 at 07:58:42PM -0400, Joel Fernandes wrote:
>
>
> On 9/2/2022 7:35 AM, Frederic Weisbecker wrote:
> > On Thu, Sep 01, 2022 at 10:17:06PM +0000, Joel Fernandes (Google) wrote:
> >> When the bypass cblist gets too big or its timeout has occurred, it is
> >> flushed into the main cblist. However, the bypass timer is still running
> >> and the behavior is that it would eventually expire and wake the GP
> >> thread.
> >>
> >> Since we are going to use the bypass cblist for lazy CBs, do the wakeup
> >> soon as the flush happens. Otherwise, the lazy-timer will go off much
> >> later and the now-non-lazy cblist CBs can get stranded for the duration
> >> of the timer.
> >>
> >> This is a good thing to do anyway (regardless of this series), since it
> >> makes the behavior consistent with behavior of other code paths where queueing
> >> something into the ->cblist makes the GP kthread in a non-sleeping state
> >> quickly.
> >>
> >> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> >> ---
> >> kernel/rcu/tree_nocb.h | 8 +++++++-
> >> 1 file changed, 7 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> >> index 0a5f0ef41484..31068dd31315 100644
> >> --- a/kernel/rcu/tree_nocb.h
> >> +++ b/kernel/rcu/tree_nocb.h
> >> @@ -447,7 +447,13 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
> >> rcu_advance_cbs_nowake(rdp->mynode, rdp);
> >> rdp->nocb_gp_adv_time = j;
> >> }
> >> - rcu_nocb_unlock_irqrestore(rdp, flags);
> >> +
> >> + // The flush succeeded and we moved CBs into the ->cblist.
> >> + // However, the bypass timer might still be running. Wakeup the
> >> + // GP thread by calling a helper with was_all_done set so that
> >> + // wake up happens (needed if main CB list was empty before).
> >> + __call_rcu_nocb_wake(rdp, true, flags)
> >> +
> >
> > Ok so there are two different changes here:
> >
> > 1) wake up nocb_gp as we just flushed the bypass list. Indeed if the regular
> > callback list was empty before flushing, we rather want to immediately wake
> > up nocb_gp instead of waiting for the bypass timer to process them.
> >
> > 2) wake up nocb_gp unconditionally (ie: even if the regular queue was not empty
> > before bypass flushing) so that nocb_gp_wait() is forced through another loop
> > starting with cancelling the bypass timer (I suggest you put such explanation
> > in the comment btw because that process may not be obvious for mortals).
> >
> > The change 1) looks like a good idea to me.
> >
> > The change 2) has unclear motivation. It forces nocb_gp_wait() through another
> > costly loop even though the timer might have been cancelled into some near
> > future, eventually avoiding that extra costly loop. Also it abuses the
> > was_alldone stuff and we may get rcu_nocb_wake with incoherent meanings
> > (WakeEmpty/WakeEmptyIsDeferred) when it's actually not empty.
>
> Yes #2 can be optimized as follows I think on top of this patch, good point:
> =============
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index ee5924ba2f3b..24aabd723abd 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -514,12 +514,13 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp,
> struct rcu_head *rhp,
> ncbs >= qhimark) {
> rcu_nocb_lock(rdp);
>
> + *was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
> +
> rcu_cblist_set_flush(&rdp->nocb_bypass,
> lazy ? BIT(CB_DEBUG_BYPASS_LAZY_FLUSHED) :
> BIT(CB_DEBUG_BYPASS_FLUSHED),
> (j - READ_ONCE(cb_debug_jiffies_first)));
>
> if (!rcu_nocb_flush_bypass(rdp, rhp, j, lazy, false)) {
> - *was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
> if (*was_alldone)
> trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
> TPS("FirstQ"));
> @@ -537,7 +538,7 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct
> rcu_head *rhp,
> // However, the bypass timer might still be running. Wakeup the
> // GP thread by calling a helper with was_all_done set so that
> // wake up happens (needed if main CB list was empty before).
> - __call_rcu_nocb_wake(rdp, true, flags)
> + __call_rcu_nocb_wake(rdp, *was_all_done, flags)
>
> return true; // Callback already enqueued.
>
That looks right!
}
> =============
>
> > So you may need to clarify the purpose. And I would suggest to make two patches
> > here.
> I guess this change only #2 is no longer a concern? And splitting is not needed
> then as it is only #1.
Sounds good!
Thanks!
