Re: [RFC PATCH 0/8] cgroup/cpuset: Support RCU_NOCB on isolated partitions

Waiman Long
2024-02-10T04:20:08Z

On 2/6/24 07:56, Frederic Weisbecker wrote:
> Le Wed, Jan 17, 2024 at 12:15:07PM -0500, Waiman Long a écrit :
>> On 1/17/24 12:07, Tejun Heo wrote:
>>> Hello,
>>>
>>> On Wed, Jan 17, 2024 at 11:35:03AM -0500, Waiman Long wrote:
>>>> The first 2 patches are adopted from Federic with minor twists to fix
>>>> merge conflicts and compilation issue. The rests are for implementing
>>>> the new cpuset.cpus.isolation_full interface which is essentially a flag
>>>> to globally enable or disable full CPU isolation on isolated partitions.
>>> I think the interface is a bit premature. The cpuset partition feature is
>>> already pretty restrictive and makes it really clear that it's to isolate
>>> the CPUs. I think it'd be better to just enable all the isolation features
>>> by default. If there are valid use cases which can't be served without
>>> disabling some isolation features, we can worry about adding the interface
>>> at that point.
>> My current thought is to make isolated partitions act like isolcpus=domain,
>> additional CPU isolation capabilities are optional and can be turned on
>> using isolation_full. However, I am fine with making all these turned on by
>> default if it is the consensus.
> Right it was the consensus last time I tried. Along with the fact that mutating
> this isolation_full set has to be done on offline CPUs to simplify the whole
> picture.
>
> So lemme try to summarize what needs to be done:
>
> 1) An all-isolation feature file (that is, all the HK_TYPE_* things) on/off for
> now. And if it ever proves needed, provide a way later for more finegrained
> tuning.
That is more or less the current plan. As detailed below, HK_TYPE_DOMAIN
& HK_TYPE_WQ isolation are included in the isolated partitions by
default. I am also thinking about including other relatively cheap
isolation flags by default. The expensive ones will have to be enabled
via isolation_full.
>
> 2) This file must only apply to offline CPUs because it avoids migrations and
> stuff.
Well, the process of first moving the CPUs offline first is rather
expensive. I won't mind doing some partial offlining based on the
existing set of teardown and bringup callbacks, but I would try to avoid
fully offlining the CPUs first.
>
> 3) I need to make RCU NOCB tunable only on offline CPUs, which isn't that much
> changes.
>
> 4) HK_TYPE_TIMER:
> * Wrt. timers in general, not much needs to be done, the CPUs are
> offline. But:
> * arch/x86/kvm/x86.c does something weird
> * drivers/char/random.c might need some care
> * watchdog needs to be (de-)activated
>
> 5) HK_TYPE_DOMAIN:
> * This one I fear is not mutable, this is isolcpus...

HK_TYPE_DOMAIN is already available via the current cpuset isolated
partition functionality. What I am currently doing is to extend that to
other HK_TYPE* flags.

>
> 6) HK_TYPE_MANAGED_IRQ:
> * I prefer not to think about it :-)
>
> 7) HK_TYPE_TICK:
> * Maybe some tiny ticks internals to revisit, I'll check that.
> * There is a remote tick to take into consideration, but again the
> CPUs are offline so it shouldn't be too complicated.
>
> 8) HK_TYPE_WQ:
> * Fortunately we already have all the mutable interface in place.
> But we must make it live nicely with the sysfs workqueue affinity
> files.

HK_TYPE_WQ is basically done and it is going to work properly with the
workqueue affinity sysfs files. From the workqueue of view, HK_TYPE_WQ
is currently treated the same as HK_TYPE_DOMAIN.

>
> 9) HK_FLAG_SCHED:
> * Oops, this one is ignored by nohz_full/isolcpus, isn't it?
> Should be removed?
I don't think HK_FLAG_SCHED is being used at all. So I believe we should
remove it to avoid confusion.
>
> 10) HK_TYPE_RCU:
> * That's point 3) and also some kthreads to affine, which leads us
> to the following in HK_TYPE_KTHREAD:
>
> 11) HK_FLAG_KTHREAD:
> * I'm guessing it's fine as long as isolation_full is also an
> isolated partition. Then unbound kthreads shouldn't run there.

Yes, isolation_full applies only to isolated partitions. It extends the
amount of CPU isolation by enabling all the other CPU available
isolation flags.

Cheers,
Longman