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

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Date:   Wed, 24 Nov 2021 08:28:43 -0800
From:   Peter Oskolkov <posk@...k.io>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-api@...r.kernel.org, Paul Turner <pjt@...gle.com>,
        Ben Segall <bsegall@...gle.com>,
        Peter Oskolkov <posk@...gle.com>,
        Andrei Vagin <avagin@...gle.com>, Jann Horn <jannh@...gle.com>,
        Thierry Delisle <tdelisle@...terloo.ca>
Subject: 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 <peterz@...radead.org> wrote:
>
> On Mon, Nov 22, 2021 at 01:13:21PM -0800, Peter Oskolkov wrote:
> > 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/:
> >
> > - 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.

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