From: "Paul E. McKenney" <paulmck@kernel.org>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
rushikesh.s.kadam@intel.com, urezki@gmail.com,
neeraj.iitr10@gmail.com, frederic@kernel.org,
rostedt@goodmis.org, vineeth@bitbyteword.org
Subject: Re: [PATCH v2 1/8] rcu: Introduce call_rcu_lazy() API implementation
Date: Tue, 12 Jul 2022 14:04:06 -0700 [thread overview]
Message-ID: <20220712210406.GF1790663@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <aef0f8a6-cde6-18f3-16ab-7e07a4413f31@joelfernandes.org>
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
next prev parent reply other threads:[~2022-07-12 21:04 UTC|newest]
Thread overview: 60+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-06-22 22:50 [PATCH v2 0/8] Implement call_rcu_lazy() and miscellaneous fixes Joel Fernandes (Google)
2022-06-22 22:50 ` [PATCH v2 1/1] context_tracking: Use arch_atomic_read() in __ct_state for KASAN Joel Fernandes (Google)
2022-06-22 22:58 ` Joel Fernandes
2022-06-22 22:50 ` [PATCH v2 1/8] rcu: Introduce call_rcu_lazy() API implementation Joel Fernandes (Google)
2022-06-22 23:18 ` Joel Fernandes
2022-06-26 4:00 ` Paul E. McKenney
2022-06-23 1:38 ` kernel test robot
2022-06-26 4:00 ` Paul E. McKenney
2022-07-08 18:43 ` Joel Fernandes
2022-07-08 23:10 ` Paul E. McKenney
2022-07-10 2:26 ` Joel Fernandes
2022-07-10 16:03 ` Paul E. McKenney
2022-07-12 20:53 ` Joel Fernandes
2022-07-12 21:04 ` Paul E. McKenney [this message]
2022-07-12 21:10 ` Joel Fernandes
2022-07-12 22:41 ` Paul E. McKenney
2022-06-29 11:53 ` Frederic Weisbecker
2022-06-29 17:05 ` Paul E. McKenney
2022-06-29 20:29 ` Joel Fernandes
2022-06-29 22:01 ` Frederic Weisbecker
2022-06-30 14:08 ` Joel Fernandes
2022-06-22 22:50 ` [PATCH v2 2/8] rcu: shrinker for lazy rcu Joel Fernandes (Google)
2022-06-22 22:50 ` [PATCH v2 3/8] fs: Move call_rcu() to call_rcu_lazy() in some paths Joel Fernandes (Google)
2022-06-22 22:50 ` [PATCH v2 4/8] rcu/nocb: Add option to force all call_rcu() to lazy Joel Fernandes (Google)
2022-06-22 22:50 ` [PATCH v2 5/8] rcu/nocb: Wake up gp thread when flushing Joel Fernandes (Google)
2022-06-26 4:06 ` Paul E. McKenney
2022-06-26 13:45 ` Joel Fernandes
2022-06-26 13:52 ` Paul E. McKenney
2022-06-26 14:37 ` Joel Fernandes
2022-06-22 22:51 ` [PATCH v2 6/8] rcuscale: Add test for using call_rcu_lazy() to emulate kfree_rcu() Joel Fernandes (Google)
2022-06-23 2:09 ` kernel test robot
2022-06-23 3:00 ` kernel test robot
2022-06-23 8:10 ` kernel test robot
2022-06-26 4:13 ` Paul E. McKenney
2022-07-08 4:25 ` Joel Fernandes
2022-07-08 23:06 ` Paul E. McKenney
2022-07-12 20:27 ` Joel Fernandes
2022-07-12 20:58 ` Paul E. McKenney
2022-07-12 21:15 ` Joel Fernandes
2022-07-12 22:41 ` Paul E. McKenney
2022-06-22 22:51 ` [PATCH v2 7/8] rcu/nocb: Rewrite deferred wake up logic to be more clean Joel Fernandes (Google)
2022-06-22 22:51 ` [PATCH v2 8/8] rcu/kfree: Fix kfree_rcu_shrink_count() return value Joel Fernandes (Google)
2022-06-26 4:17 ` Paul E. McKenney
2022-06-27 18:56 ` Uladzislau Rezki
2022-06-27 20:59 ` Paul E. McKenney
2022-06-27 21:18 ` Joel Fernandes
2022-06-27 21:43 ` Paul E. McKenney
2022-06-28 16:56 ` Joel Fernandes
2022-06-28 21:13 ` Joel Fernandes
2022-06-29 16:56 ` Paul E. McKenney
2022-06-29 19:47 ` Joel Fernandes
2022-06-29 21:07 ` Paul E. McKenney
2022-06-30 14:25 ` Joel Fernandes
2022-06-30 15:29 ` Paul E. McKenney
2022-06-29 16:52 ` Paul E. McKenney
2022-06-26 3:12 ` [PATCH v2 0/8] Implement call_rcu_lazy() and miscellaneous fixes Paul E. McKenney
2022-07-08 4:17 ` Joel Fernandes
2022-07-08 22:45 ` Paul E. McKenney
2022-07-10 1:38 ` Joel Fernandes
2022-07-10 15:47 ` Paul E. McKenney
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