linux-kernel - Re: [PATCH] rcu: Make call_rcu() lazy only when CONFIG_RCU

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Message-Id: <6E2C7AC3-9BA6-4C5B-A9D7-59F5F6279177@joelfernandes.org>
Date:   Wed, 19 Oct 2022 14:25:29 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     paulmck@...nel.org
Cc:     Zqiang <qiang1.zhang@...el.com>, frederic@...nel.org,
        rcu@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu: Make call_rcu() lazy only when CONFIG_RCU_LAZY is enabled



> On Oct 19, 2022, at 1:45 PM, Paul E. McKenney <paulmck@...nel.org> wrote:
> 
> On Wed, Oct 19, 2022 at 08:12:30AM -0400, Joel Fernandes wrote:
>>> On Oct 19, 2022, at 8:10 AM, Joel Fernandes <joel@...lfernandes.org> wrote:
>>>>> On Oct 19, 2022, at 6:34 AM, Zqiang <qiang1.zhang@...el.com> wrote:
>>>>> 
>>>>> Currently, regardless of whether the CONFIG_RCU_LAZY is enabled,
>>>>> invoke the call_rcu() is always lazy, it also means that when
>>>>> CONFIG_RCU_LAZY is disabled, invoke the call_rcu_flush() is also
>>>>> lazy. therefore, this commit make call_rcu() lazy only when
>>>>> CONFIG_RCU_LAZY is enabled.
>> 
>> First, good eyes!  Thank you for spotting this!!
>>>>> Signed-off-by: Zqiang <qiang1.zhang@...el.com>
>>>>> ---
>>>>> kernel/rcu/tree.c | 8 +++++++-
>>>>> 1 file changed, 7 insertions(+), 1 deletion(-)
>>>>> 
>>>>> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>>>>> index abc615808b6e..97ef602da3d5 100644
>>>>> --- a/kernel/rcu/tree.c
>>>>> +++ b/kernel/rcu/tree.c
>>>>> @@ -2839,7 +2839,6 @@ void call_rcu_flush(struct rcu_head *head, rcu_callback_t func)
>>>>>  return __call_rcu_common(head, func, false);
>>>>> }
>>>>> EXPORT_SYMBOL_GPL(call_rcu_flush);
>>>>> -#endif
>>>>> 
>>>>> /**
>>>>> * call_rcu() - Queue an RCU callback for invocation after a grace period.
>>>>> @@ -2890,6 +2889,13 @@ void call_rcu(struct rcu_head *head, rcu_callback_t func)
>>>>>  return __call_rcu_common(head, func, true);
>>>>> }
>>>>> EXPORT_SYMBOL_GPL(call_rcu);
>>>>> +#else
>>>>> +void call_rcu(struct rcu_head *head, rcu_callback_t func)
>>>>> +{
>>>>> +    return __call_rcu_common(head, func, false);
>>> 
>>> Thanks. Instead of adding new function, you can also pass IS_ENABLED(CONFIG…) to the existing function of the same name.
> 
> I do like this approach better -- less code, more obvious what is going on.

Sounds good. Zqiang, do you mind updating your patch along these lines? That way you get the proper attribution.

More comments below:
> 
>>> Looks like though I made every one test the patch without having to enable the config option ;-). Hey, I’m a half glass full kind of guy, why do you ask?
>>> 
>>> Paul, I’ll take a closer look once I’m at the desk, but would you prefer to squash a diff into the existing patch, or want a new patch altogether?
>> 
>> On the other hand, what I’d want is to nuke the config option altogether or make it default y, we want to catch issues sooner than later.
> 
> That might be what we do at some point, but one thing at a time.  Let's
> not penalize innocent bystanders, at least not just yet.

It’s a trade off, I thought that’s why we wanted to have the binary search stuff. If no one reports issue on Linux-next, then that code won’t be put to use in the near future at least.

> I do very strongly encourage the ChromeOS and Android folks to test this
> very severely, however.

Agreed. Yes that will happen, though I have to make a note for Android folks other than Vlad, to backports these (and enable the config option), carefully! Especially on pre-5.15 kernels. Luckily I had to do this (not so trivial) exercise myself.

Thanks!

 - Joel

> 
>                            Thanx, Paul
> 
>> Thanks.
>> 
>>> 
>>> Thanks.
>>> 
>>> - Joel
>>> 
>>> 
>>>> +}
>>>> +EXPORT_SYMBOL_GPL(call_rcu);
>>>> +#endif
>>>> 
>>>> /* Maximum number of jiffies to wait before draining a batch. */
>>>> #define KFREE_DRAIN_JIFFIES (5 * HZ)
>>>> -- 
>>>> 2.25.1
>>>>