Re: [PATCH v2 1/8] rcu: Introduce call_rcu_lazy() API implementation

[Frederic Weisbecker]

On Wed, Jun 29, 2022 at 08:29:48PM +0000, Joel Fernandes wrote:
> On Wed, Jun 29, 2022 at 01:53:49PM +0200, Frederic Weisbecker wrote:
> > On Wed, Jun 22, 2022 at 10:50:55PM +0000, Joel Fernandes (Google) wrote:
> > > @@ -414,30 +427,37 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
> > >  	}
> > >  	WRITE_ONCE(rdp->nocb_nobypass_count, c);
> > >  
> > > -	// If there hasn't yet been all that many ->cblist enqueues
> > > -	// this jiffy, tell the caller to enqueue onto ->cblist.  But flush
> > > -	// ->nocb_bypass first.
> > > -	if (rdp->nocb_nobypass_count < nocb_nobypass_lim_per_jiffy) {
> > > +	// If caller passed a non-lazy CB and there hasn't yet been all that
> > > +	// many ->cblist enqueues this jiffy, tell the caller to enqueue it
> > > +	// onto ->cblist.  But flush ->nocb_bypass first. Also do so, if total
> > > +	// number of CBs (lazy + non-lazy) grows too much.
> > > +	//
> > > +	// Note that if the bypass list has lazy CBs, and the main list is
> > > +	// empty, and rhp happens to be non-lazy, then we end up flushing all
> > > +	// the lazy CBs to the main list as well. That's the right thing to do,
> > > +	// since we are kick-starting RCU GP processing anyway for the non-lazy
> > > +	// one, we can just reuse that GP for the already queued-up lazy ones.
> > > +	if ((rdp->nocb_nobypass_count < nocb_nobypass_lim_per_jiffy && !lazy) ||
> > > +	    (lazy && n_lazy_cbs >= qhimark)) {
> > >  		rcu_nocb_lock(rdp);
> > >  		*was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
> > >  		if (*was_alldone)
> > >  			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
> > > -					    TPS("FirstQ"));
> > > -		WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, j));
> > > +					    lazy ? TPS("FirstLazyQ") : TPS("FirstQ"));
> > > +		WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, j, false));
> > 
> > That's outside the scope of this patchset but this makes me realize we
> > unconditionally try to flush the bypass from call_rcu() fastpath, and
> > therefore we unconditionally lock the bypass lock from call_rcu() fastpath.
> > 
> > It shouldn't be contended at this stage since we are holding the nocb_lock
> > already, and only the local CPU can hold the nocb_bypass_lock without holding
> > the nocb_lock. But still...
> > 
> > It looks safe to locklessly early check if (rcu_cblist_n_cbs(&rdp->nocb_bypass))
> > before doing anything. Only the local CPU can enqueue to the bypass list.
> > 
> > Adding that to my TODO list...
> > 
> 
> I am afraid I did not understand your comment. The bypass list lock is held
> once we have decided to use the bypass list to queue something on to it.
> 
> The bypass flushing is also conditional on either the bypass cblist growing
> too big or a jiffie elapsing since the first bypass queue.
> 
> So in both cases, acquiring the lock is conditional. What do you mean it is
> unconditionally acquiring the bypass lock? Where?

Just to make sure we are talking about the same thing, I'm referring to this
path:

	// If there hasn't yet been all that many ->cblist enqueues
	// this jiffy, tell the caller to enqueue onto ->cblist.  But flush
	// ->nocb_bypass first.
	if (rdp->nocb_nobypass_count < nocb_nobypass_lim_per_jiffy) {
		rcu_nocb_lock(rdp);
		*was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
		if (*was_alldone)
			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
					    TPS("FirstQ"));
		WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, j));
		WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass));
		return false; // Caller must enqueue the callback.
	}

This is called whenever we decide not to queue to the bypass list because
there is no flooding detected (rdp->nocb_nobypass_count hasn't reached
nocb_nobypass_lim_per_jiffy for the current jiffy). I call this the fast path
because this is what I would except in a normal load, as opposed to callbacks
flooding.

And in this fastpath, the above rcu_nocb_flush_bypass() is unconditional.

> 
> Thanks!
> 
>  - Joel
>