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From:   Joel Fernandes <joel@joelfernandes.org>
Mime-Version: 1.0 (1.0)
Subject: Re: [PATCH] rcu: Make call_rcu() lazy only when CONFIG_RCU_LAZY is enabled
Date:   Wed, 19 Oct 2022 14:25:29 -0400
Message-Id: <6E2C7AC3-9BA6-4C5B-A9D7-59F5F6279177@joelfernandes.org>
References: <20221019174458.GD5600@paulmck-ThinkPad-P17-Gen-1>
Cc:     Zqiang <qiang1.zhang@intel.com>, frederic@kernel.org,
        rcu@vger.kernel.org, linux-kernel@vger.kernel.org
In-Reply-To: <20221019174458.GD5600@paulmck-ThinkPad-P17-Gen-1>
To:     paulmck@kernel.org
Precedence: bulk


> On Oct 19, 2022, at 1:45 PM, Paul E. McKenney <paulmck@kernel.org> wrote:
>=20
> =EF=BB=BFOn Wed, Oct 19, 2022 at 08:12:30AM -0400, Joel Fernandes wrote:
>>> On Oct 19, 2022, at 8:10 AM, Joel Fernandes <joel@joelfernandes.org> wro=
te:
>>>>> On Oct 19, 2022, at 6:34 AM, Zqiang <qiang1.zhang@intel.com> wrote:
>>>>>=20
>>>>> =EF=BB=BFCurrently, regardless of whether the CONFIG_RCU_LAZY is enabl=
ed,
>>>>> 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.
>>=20
>> First, good eyes!  Thank you for spotting this!!
>>>>> Signed-off-by: Zqiang <qiang1.zhang@intel.com>
>>>>> ---
>>>>> kernel/rcu/tree.c | 8 +++++++-
>>>>> 1 file changed, 7 insertions(+), 1 deletion(-)
>>>>>=20
>>>>> 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_c=
allback_t func)
>>>>>  return __call_rcu_common(head, func, false);
>>>>> }
>>>>> EXPORT_SYMBOL_GPL(call_rcu_flush);
>>>>> -#endif
>>>>>=20
>>>>> /**
>>>>> * call_rcu() - Queue an RCU callback for invocation after a grace peri=
od.
>>>>> @@ -2890,6 +2889,13 @@ void call_rcu(struct rcu_head *head, rcu_callba=
ck_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);
>>>=20
>>> Thanks. Instead of adding new function, you can also pass IS_ENABLED(CON=
FIG=E2=80=A6) to the existing function of the same name.
>=20
> 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:
>=20
>>> Looks like though I made every one test the patch without having to enab=
le the config option ;-). Hey, I=E2=80=99m a half glass full kind of guy, wh=
y do you ask?
>>>=20
>>> Paul, I=E2=80=99ll take a closer look once I=E2=80=99m at the desk, but w=
ould you prefer to squash a diff into the existing patch, or want a new patc=
h altogether?
>>=20
>> On the other hand, what I=E2=80=99d want is to nuke the config option alt=
ogether or make it default y, we want to catch issues sooner than later.
>=20
> 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=E2=80=99s a trade off, I thought that=E2=80=99s why we wanted to have the=
 binary search stuff. If no one reports issue on Linux-next, then that code w=
on=E2=80=99t 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), careful=
ly! Especially on pre-5.15 kernels. Luckily I had to do this (not so trivial=
) exercise myself.

Thanks!

 - Joel

>=20
>                            Thanx, Paul
>=20
>> Thanks.
>>=20
>>>=20
>>> Thanks.
>>>=20
>>> - Joel
>>>=20
>>>=20
>>>> +}
>>>> +EXPORT_SYMBOL_GPL(call_rcu);
>>>> +#endif
>>>>=20
>>>> /* Maximum number of jiffies to wait before draining a batch. */
>>>> #define KFREE_DRAIN_JIFFIES (5 * HZ)
>>>> --=20
>>>> 2.25.1
>>>>=20