Re: [PATCH] rcu: Make call_rcu() lazy only when CONFIG_RCU_LAZY is enabled

From: Joel Fernandes
Date: Wed Oct 19 2022 - 14:25:42 EST




> On Oct 19, 2022, at 1:45 PM, Paul E. McKenney <paulmck@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxxx> wrote:
>>>>> On Oct 19, 2022, at 6:34 AM, Zqiang <qiang1.zhang@xxxxxxxxx> 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@xxxxxxxxx>
>>>>> ---
>>>>> 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
>>>>