On 10/2/2022 10:06 AM, Pingfan Liu wrote:
> On Fri, Sep 30, 2022 at 9:04 PM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
>>
>> On Thu, Sep 29, 2022 at 4:21 AM Pingfan Liu <kernelfans@xxxxxxxxx> wrote:
>>>
>>> On Thu, Sep 29, 2022 at 4:19 PM Pingfan Liu <kernelfans@xxxxxxxxx> wrote:
>>>>
>>> [...]
>>>> "
>>>>
>>>> I have no idea whether this is related to the reverted commit.
>>>>
>>>
>>> I have started another test against clean v6.0-rc7 to see whether this
>>> is an issue with the mainline.
>>
>> I am not sure what exactly you are reverting (you could clarify that),
>
> commit 96926686deab ("rcu: Make CPU-hotplug removal operations enable tick").
> But due to conflict, "git revert" can not work directly. So I applied
> it by hand.
>
>> but if you are just removing the entire TICK_DEP_BIT_RCU, I do
>> remember (and mentioned on IRC to others recently) that without this
>> NOHZ_FULL has a hard time ending grace-periods because the forcing of
>> tick is needed for this configuration if we are spinning in the kernel
>> with the tick turned off. That seems to align with your TREE04
>> (NOHZ_FULL) configuration.
>>
>
> Yes, that is the scenario.
>
>> Also, the commit Frederic suggested to revert seems to be a cosmetic
>> optimization in the interrupt-entry path. That should not change
>> functionality I believe. So I did not fully follow why reverting that
>> is relevant (maybe Frederic can clarify?).
>>
>
> Leave this question to Frederic.
I take this comment back, Sorry. Indeed the commits Frederic mentioned will make
a functional change to CPU hotplug path.
Sorry for the noise.
Excited to see exact reason why TICK_DEP_BIT_RCU matters in the hotplug paths. I
might jump into the investigation with you guys, but I have to make time for
Lazy-RCU v7 next :)
Thanks.
