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From: Frederic Weisbecker <frederic@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, paulmck@kernel.org, rostedt@goodmis.org,
	vineeth@bitbyteword.org, boqun.feng@gmail.com
Subject: Re: [PATCH v5 06/18] rcu: Introduce call_rcu_lazy() API implementation
Date: Tue, 6 Sep 2022 21:11:10 +0200	[thread overview]
Message-ID: <20220906191110.GA187950@lothringen> (raw)
In-Reply-To: <4f6061f0-0de7-2916-dc6e-9f5af9b944c0@joelfernandes.org>

On Tue, Sep 06, 2022 at 12:43:52PM -0400, Joel Fernandes wrote:
> On 9/6/2022 12:38 PM, Joel Fernandes wrote:
> Ah, now I know why I got confused. I *used* to flush the bypass list before when
> !lazy CBs showed up. Paul suggested this is overkill. In this old overkill
> method, I was missing a wake up which was likely causing the boot regression.
> Forcing a wake up fixed that. Now in v5 I make it such that I don't do the flush
> on a !lazy rate-limit.
> 
> I am sorry for the confusion. Either way, in my defense this is just an extra
> bit of code that I have to delete. This code is hard. I have mostly relied on a
> test-driven development. But now thanks to this review and I am learning the
> code more and more...

Yeah this code is hard.

Especially as it's possible to flush from both sides and queue the timer
from both sides. And both sides read the bypass/lazy counter locklessly.
But only call_rcu_*() can queue/increase the bypass size whereas only
nocb_gp_wait() can cancel the timer. Phew!

Among the many possible dances between rcu_nocb_try_bypass()
and nocb_gp_wait(), I haven't found a way yet for the timer to be
set to LAZY when it should be BYPASS (or other kind of accident such
as an ignored callback).

In the worst case we may arm an earlier timer than necessary
(RCU_NOCB_WAKE_BYPASS instead of RCU_NOCB_WAKE_LAZY for example).

Famous last words...


> Thanks,
> 
>  - Joel
> 
> 
> 

  reply	other threads:[~2022-09-06 19:11 UTC|newest]

Thread overview: 73+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-09-01 22:17 [PATCH v5 00/18] Implement call_rcu_lazy() and miscellaneous fixes Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 01/18] mm/slub: perform free consistency checks before call_rcu Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 02/18] mm/sl[au]b: rearrange struct slab fields to allow larger rcu_head Joel Fernandes (Google)
2022-09-02  9:26   ` Vlastimil Babka
2022-09-02  9:30     ` Vlastimil Babka
2022-09-02 15:09       ` Joel Fernandes
2022-09-03 13:53         ` Paul E. McKenney
2022-09-01 22:17 ` [PATCH v5 03/18] rcu/tree: Use READ_ONCE() for lockless read of rnp->qsmask Joel Fernandes (Google)
2022-09-06 22:26   ` Boqun Feng
2022-09-06 22:31     ` Joel Fernandes
2022-09-01 22:17 ` [PATCH v5 04/18] rcu: Fix late wakeup when flush of bypass cblist happens Joel Fernandes (Google)
2022-09-02 11:35   ` Frederic Weisbecker
2022-09-02 23:58     ` Joel Fernandes
2022-09-03 15:10       ` Paul E. McKenney
2022-09-04 21:13       ` Frederic Weisbecker
2022-09-03 14:04   ` Paul E. McKenney
2022-09-03 14:05     ` Joel Fernandes
2022-09-06  3:07   ` Joel Fernandes
2022-09-06  9:48     ` Frederic Weisbecker
2022-09-07  2:43       ` Joel Fernandes
2022-09-01 22:17 ` [PATCH v5 05/18] rcu: Move trace_rcu_callback() before bypassing Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 06/18] rcu: Introduce call_rcu_lazy() API implementation Joel Fernandes (Google)
2022-09-02 15:21   ` Frederic Weisbecker
2022-09-02 23:09     ` Joel Fernandes
2022-09-05 12:59       ` Frederic Weisbecker
2022-09-05 20:18         ` Joel Fernandes
2022-09-05 20:32         ` Joel Fernandes
2022-09-06  8:55           ` Paul E. McKenney
2022-09-06 16:16             ` Joel Fernandes
2022-09-06 17:05               ` Paul E. McKenney
2022-09-03 22:00     ` Joel Fernandes
2022-09-04 21:01       ` Frederic Weisbecker
2022-09-05 20:20         ` Joel Fernandes
2022-09-06  3:05     ` Joel Fernandes
2022-09-06 15:17       ` Frederic Weisbecker
2022-09-06 16:15         ` Joel Fernandes
2022-09-06 16:31           ` Joel Fernandes
2022-09-06 16:38             ` Joel Fernandes
2022-09-06 16:43               ` Joel Fernandes
2022-09-06 19:11                 ` Frederic Weisbecker [this message]
2022-09-07  2:56                   ` Joel Fernandes
2022-09-07  9:56                     ` Frederic Weisbecker
2022-09-07 10:03           ` Frederic Weisbecker
2022-09-07 14:01             ` Joel Fernandes
2022-09-07  0:06         ` Joel Fernandes
2022-09-07  9:40           ` Frederic Weisbecker
2022-09-07 13:44             ` Joel Fernandes
2022-09-07 15:38               ` Frederic Weisbecker
2022-09-07 15:39                 ` Joel Fernandes
2022-09-21 23:52             ` Joel Fernandes
2022-09-06 18:16       ` Uladzislau Rezki
2022-09-06 18:21         ` Joel Fernandes
2022-09-07  8:52           ` Uladzislau Rezki
2022-09-07 15:23             ` Joel Fernandes
2022-09-03 14:03   ` Paul E. McKenney
2022-09-03 14:05     ` Joel Fernandes
2022-09-01 22:17 ` [PATCH v5 07/18] rcu: shrinker for lazy rcu Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 08/18] rcu: Add per-CB tracing for queuing, flush and invocation Joel Fernandes (Google)
2022-09-02 16:48   ` kernel test robot
2022-09-03 12:39     ` Paul E. McKenney
2022-09-03 14:07       ` Joel Fernandes
2022-09-02 19:01   ` kernel test robot
2022-09-01 22:17 ` [PATCH v5 09/18] rcuscale: Add laziness and kfree tests Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 10/18] rcutorture: Add test code for call_rcu_lazy() Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 11/18] fs: Move call_rcu() to call_rcu_lazy() in some paths Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 12/18] cred: Move call_rcu() to call_rcu_lazy() Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 13/18] security: " Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 14/18] net/core: " Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 15/18] kernel: Move various core kernel usages " Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 16/18] lib: Move call_rcu() " Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 17/18] i915: " Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 18/18] fork: Move thread_stack_free_rcu() " Joel Fernandes (Google)
2022-09-03 15:22 ` [PATCH v5 00/18] Implement call_rcu_lazy() and miscellaneous fixes Paul E. McKenney

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