linux-kernel - Re: [PATCH v4 00/14] Implement call_rcu

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Date:   Thu, 1 Sep 2022 12:07:56 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Frederic Weisbecker <frederic@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        Rushikesh S Kadam <rushikesh.s.kadam@...el.com>,
        "Uladzislau Rezki (Sony)" <urezki@...il.com>,
        Neeraj upadhyay <neeraj.iitr10@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        rcu <rcu@...r.kernel.org>,
        Vineeth Pillai <vineeth@...byteword.org>
Subject: Re: [PATCH v4 00/14] Implement call_rcu_lazy() and miscellaneous
 fixes



On 9/1/2022 10:58 AM, Frederic Weisbecker wrote:
> On Thu, Sep 01, 2022 at 07:39:07AM -0700, Paul E. McKenney wrote:
>> On Wed, Aug 31, 2022 at 05:26:58PM +0200, Frederic Weisbecker wrote:
>>> On Tue, Aug 30, 2022 at 09:46:34AM -0700, Paul E. McKenney wrote:
>>>>> Although who knows, may be some periodic file operation while idle are specific
>>>>> to Android. I'll try to trace lazy callbacks while idle and the number of grace
>>>>> periods associated.
>>>>
>>>> Sounds like a good start.
>>>>
>>>> And yes, we don't need to show that the whole !NOCB world needs this,
>>>> just some significant portion of it.  But we do need some decent evidence.
>>>> After all, it is all too easy to do a whole lot of work and find that
>>>> the expected benefits fail to materialize.
>>>
>>> So here is some quick test. I made a patch that replaces Joel's 1st patch
>>> with an implementation of call_rcu_lazy() that queues lazy callbacks
>>> through the regular call_rcu() way but it counts them in a lazy_count.
>>>
>>> Upon idle entry it reports whether the tick is retained solely by lazy
>>> callbacks or not.
>>>
>>> I get periodic and frequent results on my idle test box, something must be
>>> opening/closing some file periodically perhaps.
>>>
>>> Anyway the thing can be tested with this branch:
>>>
>>> git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git
>>> 	rcu/lazy-trace
>>>
>>> Excerpt:
>>>
>>>           <idle>-0       [007] d..1.   414.226966: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
>>>           <idle>-0       [007] d..1.   414.228271: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
>>>           <idle>-0       [007] d..1.   414.232269: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
>>>           <idle>-0       [007] d..1.   414.236269: rcu_needs_cpu: BAD: 1 lazy callbacks retaining dynticks-idle
>>
>> Just to make sure that I understand, at this point, there is only the
>> one lazy callback (and no non-lazy callbacks) on this CPU, and that
>> CPU is therefore keeping the tick on only for the benefit of that one
>> lazy callback.  And for the above four traces, this is likely the same
>> lazy callback.
>>
>> Did I get it right, or is there something else going on?
> 
> Exactly that!

Interesting!

 - Joel