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

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Message-Id: <D7F0EE3F-0A06-4868-87E3-B88B9BD6480C@joelfernandes.org>
Date:   Thu, 20 Oct 2022 04:42:05 -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 7:34 PM, Paul E. McKenney <paulmck@...nel.org> wrote:
> 
> On Wed, Oct 19, 2022 at 02:25:29PM -0400, Joel Fernandes wrote:
>> 
>> 
>>>> 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!!

Indeed.

>>>>>>> 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.

Acked that patch.

>> 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.
> 
> Well, not to put too fine a point on it, but we currently really are
> exposing -next to lazy call_rcu().  ;-)

This is true. I think I assumed nobody will enable a default off config option but I probably meant a smaller percentage will.

>>> 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.
> 
> And this is another situation in which the binary search stuff may prove
> extremely useful.

Agreed. Thanks. Very least I owe per-rdp splitting of the hashtable, to that code.  Steven and me talked today that probably the hashtable can go into the rcu_segcblist itself, and protect it by the nocb lock.

Thanks,

 - Joel


>                            Thanx, Paul
> 
>> 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
>>>>>>