[PATCH v6 00/29] context_tracking,x86: Defer some IPIs until a user->kernel transition

[Valentin Schneider]

On 28/10/25 17:25, Frederic Weisbecker wrote:
> +Cc Phil Auld
>
> Le Fri, Oct 10, 2025 at 05:38:10PM +0200, Valentin Schneider a écrit :
>> Patches
>> =======
>>
>> o Patches 1-2 are standalone objtool cleanups.
>
> Would be nice to get these merged.
>
>> o Patches 3-4 add an RCU testing feature.
>
> I'm taking this one.
>

Thanks!

>>
>> o Patches 5-6 add infrastructure for annotating static keys and static calls
>>   that may be used in noinstr code (courtesy of Josh).
>> o Patches 7-20 use said annotations on relevant keys / calls.
>> o Patch 21 enforces proper usage of said annotations (courtesy of Josh).
>>
>> o Patch 22 deals with detecting NOINSTR text in modules
>
> Not sure how to route those. If we wait for each individual subsystem,
> this may take a while.
>

At least the __ro_after_init ones could go as their own thing since they're
standalone, but yeah they're the ones touching all sorts of subsystems :/

>> o Patches 23-24 deal with kernel text sync IPIs
>
> I would be fine taking those (the concerns I had are just details)
> but they depend on all the annotations. Alternatively I can take the whole
> thing but then we'll need some acks.
>
>> o Patches 25-29 deal with kernel range TLB flush IPIs
>
> I'll leave these more time for now ;o)
> And if they ever go somewhere, it should be through x86 tree.
>
> Also, here is another candidate usecase for this deferral thing.
> I remember Phil Auld complaining that stop_machine() on CPU offlining was
> a big problem for nohz_full. Especially while we are working on
> a cpuset interface to toggle nohz_full but this will require the CPUs
> to be offline.
>

Yeah that does ring a bell...

> So my point is that when a CPU goes offline, stop_machine() puts all
> CPUs into a loop with IRQs disabled. CPUs in userspace could possibly
> escape that since they don't touch the kernel anyway. But as soon as
> they enter the kernel, they should either acquire the final state of
> stop_machine if completed or join the global loop if in the middle.
>

I need to have a think about that one; one pain point I see is the context
tracking work has to be NMI safe since e.g. an NMI can take us out of
userspace. Another is that NOHZ-full CPUs need to be special cased in the
stop machine queueing / completion.

/me goes fetch a new notebook

> Thanks.
>
> --
> Frederic Weisbecker
> SUSE Labs