Re: [PATCH v0.9.1 0/6] sched,mm,x86/uaccess: implement User Managed Concurrency Groups

On Wed, Nov 24, 2021 at 6:06 AM Peter Zijlstra [off-list ref] wrote:

On Mon, Nov 22, 2021 at 01:13:21PM -0800, Peter Oskolkov wrote:

quoted

User Managed Concurrency Groups (UMCG) is an M:N threading
subsystem/toolkit that lets user space application developers implement
in-process user space schedulers.

This v0.9.1 patchset is the same as v0.9, where u32/u64 in
uapi/linux/umcg.h are replaced with __u32/__u64, as test robot/lkp
does not recognize u32/u64 for some reason.

v0.9 is v0.8 rebased on top of the current tip/sched/core,
with a fix in umcg_update_state of an issue reported by Tao Zhou.

Key changes from patchset v0.7:
https://lore.kernel.org/all/20211012232522.714898-1-posk@google.com/ (local):

- added libumcg tools/lib/umcg;
- worker "wakeup" is reworked so that it is now purely a userspace op,
  instead of waking the thread in order for it to block on return
  to the userspace immediately;
- a couple of minor fixes and refactorings.

These big things remain to be addressed (in no particular order):
- support tracing/debugging
- make context switches faster (see umcg_do_context_switch in umcg.c)
- support other architectures
- cleanup and post selftests in tools/testing/selftests/umcg/
- allow cross-mm wakeups (securely)

*groan*... so these patches do *NOT* support the very thing this all
started with, namely block + wakeup notifications. I'm really not sure
how that happened, as that was the sole purpose of the exercise.

I'm not sure why you say this - in-process block/wakeup is very much
supported - please see the third patch. Cross-process (cross-mm)
wakeups are not supported at the moment, as the security story has to
be fleshed out.

Aside of that, the whole uaccess stuff is horrific :-( I'll reply to
that email separately, but the alternative is also included in the
random hackery below.

Thanks - I'll try to make uaccess more to your liking, unless you say
the whole thing is a no-go.

I'm still trying to make sense of it all, but I'm really not seeing how
any of this satisfies the initial goals, also it is once again 100% new
code :/

I believe the initial goals of in-process block/wakeup detection,
on-cpu context switching, etc. are all achieved here. Re: new code:
the code in the third patch evolved into what it is today based on
feedback/discussions in this list.

[...]