linux-kernel - Re: [PATCH v2 1/8] rcu: Introduce call_rcu

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Message-ID: <20220712210406.GF1790663@paulmck-ThinkPad-P17-Gen-1>
Date:   Tue, 12 Jul 2022 14:04:06 -0700
From:   "Paul E. McKenney" <paulmck@...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, frederic@...nel.org, rostedt@...dmis.org,
        vineeth@...byteword.org
Subject: Re: [PATCH v2 1/8] rcu: Introduce call_rcu_lazy() API implementation

On Tue, Jul 12, 2022 at 04:53:48PM -0400, Joel Fernandes wrote:
> 
> On 7/10/2022 12:03 PM, Paul E. McKenney wrote:
> [..]
> >>>> +	// 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));
> >>>
> >>> The "false" here instead of "lazy" is because the caller is to do the
> >>> enqueuing, correct?
> >>
> >> There is no difference between using false or lazy here, because the bypass
> >> flush is not also enqueuing the lazy callback, right?
> >>
> >> We can also pass lazy instead of false if that's less confusing.
> >>
> >> Or maybe I missed the issue you're raising?
> > 
> > I am mostly checking up on your intended meaning of "lazy" in various
> > contexts.  It could mean only that the caller requested laziness, or in
> > some cases it could mean that the callback actually will be lazy.
> > 
> > I can rationalize the "false" above as a "don't care" in this case
> > because (as you say) there is not callback.  In which case this code
> > is OK as is, as long as the header comment for rcu_nocb_flush_bypass()
> > clearly states that this parameter has meaning only when there really
> > is a callback being queued.
> 
> I decided to change this and the below to "lazy" variable instead of
> true/false, as the code is cleaner and less confusing IMO. It makes
> sense to me and in my testing it works fine. Hope that's Ok with you.

Sounds plausible.

> About changing the lazy length count to a flag, one drawback of doing
> that is, say if there are some non-lazy CBs in the bypass list, then the
> lazy shrinker will end up reporting an inaccurate count. Also
> considering that it might be harder to add the count back later say if
> we need it for tracing, I would say lets leave it as is. I will keep the
> counter for v3 and we can discuss. Does that sound good to you?

You lost me on this one.  If there are any non-lazy callbacks, the whole
bypass list is already being treated as non-lazy, right?  If so, then
the lazy shrinker should report the full count if all callbacks are lazy,
and should report none otherwise.  Or am I missing something here?

> I think some more testing, checkpatch running etc and I should be good
> to send v3 :)

Sounds good!

							Thanx, Paul

> Thanks!
> 
>  - Joel
> 
> 
> > 
> >>>>  		WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass));
> >>>>  		return false; // Caller must enqueue the callback.
> >>>>  	}
> >>>>  
> >>>>  	// If ->nocb_bypass has been used too long or is too full,
> >>>>  	// flush ->nocb_bypass to ->cblist.
> >>>> -	if ((ncbs && j != READ_ONCE(rdp->nocb_bypass_first)) ||
> >>>> -	    ncbs >= qhimark) {
> >>>> +	if ((ncbs && j != READ_ONCE(rdp->nocb_bypass_first)) || ncbs >= qhimark) {
> >>>>  		rcu_nocb_lock(rdp);
> >>>> -		if (!rcu_nocb_flush_bypass(rdp, rhp, j)) {
> >>>> +		if (!rcu_nocb_flush_bypass(rdp, rhp, j, true)) {
> >>>
> >>> But shouldn't this "true" be "lazy"?  I don't see how we are guaranteed
> >>> that the callback is in fact lazy at this point in the code.  Also,
> >>> there is not yet a guarantee that the caller will do the enqueuing.
> >>> So what am I missing?
> >>
> >> Sorry I screwed this part up. I think I meant 'false' here, if the list grew
> >> too big- then I think I would prefer if the new lazy CB instead is treated as
> >> non-lazy. But if that's too confusing, I will just pass 'lazy' instead. What
> >> do you think?
> > 
> > Good point, if we are choosing to override the laziness requested by the
> > caller, then it should say "true".  It would be good to have a comment
> > saying that is what we are doing, correct?
> > 
> >> Will reply more to the rest of the comments soon, thanks!
> > 
> > Sounds good!  (Hey, wouldn't want you to be bored!)
> > 
> > 							Thanx, Paul