linux-kernel - Re: [PATCH v6 1/4] rcu: Make call

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Date:   Mon, 26 Sep 2022 00:09:36 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        rushikesh.s.kadam@...el.com, urezki@...il.com,
        neeraj.iitr10@...il.com, paulmck@...nel.org, rostedt@...dmis.org
Subject: Re: [PATCH v6 1/4] rcu: Make call_rcu() lazy to save power

On Sat, Sep 24, 2022 at 07:28:16PM -0400, Joel Fernandes wrote:
> > And then FLUSH_BP_WAKE is probably not needed anymore. 
> 
> It is needed as the API is in tree_nocb.h and we
> have to have that handle the details of laziness
> there rather than tree.c. We could add new apis
> to get rid of flag but it’s cleaner (and Paul seemed
> to be ok with it).

If the wake up is handled outside the flush function, as in the
diff I just posted, there is no more user left of FLUSH_BP_WAKE, IIRC...

> >> @@ -512,9 +598,16 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
> >>    }
> >>    // Need to actually to a wakeup.
> >>    len = rcu_segcblist_n_cbs(&rdp->cblist);
> >> +    bypass_len = rcu_cblist_n_cbs(&rdp->nocb_bypass);
> >> +    lazy_len = READ_ONCE(rdp->lazy_len);
> >>    if (was_alldone) {
> >>        rdp->qlen_last_fqs_check = len;
> >> -        if (!irqs_disabled_flags(flags)) {
> >> +        // Only lazy CBs in bypass list
> >> +        if (lazy_len && bypass_len == lazy_len) {
> >> +            rcu_nocb_unlock_irqrestore(rdp, flags);
> >> +            wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_LAZY,
> >> +                       TPS("WakeLazy"));
> > 
> > I'm trying to think of a case where rcu_nocb_try_bypass() returns false
> > (queue to regular list) but then call_rcu() -> __call_rcu_nocb_wake() ends up
> > seeing a lazy bypass queue even though we are queueing a non-lazy callback
> > (should have flushed in this case).
> > 
> > Looks like it shouldn't happen, even with concurrent (de-offloading) but just
> > in case, can we add:
> 
> Yes I also feel this couldn’t happen because irq is
> off and nocb lock is held throughout the calls to
> the above 2 functions. Unless I missed the race
> you’re describing?

At least I can't find any either...

> 
> > 
> >      WARN_ON_ONCE(lazy_len != len)
> 
> But this condition can be true even in normal
> circumstances? len also contains DONE CBs
> which are ready to be invoked. Or did I miss
> something?

Duh, good point, nevermind then :-)

Thanks.

> 
> Thanks,
> 
>   - Joel
> 
> > 
> >> +        } else if (!irqs_disabled_flags(flags)) {
> >>            /* ... if queue was empty ... */
> >>            rcu_nocb_unlock_irqrestore(rdp, flags);
> >>            wake_nocb_gp(rdp, false);
> > 
> > Thanks.