linux-kernel - Re: [PATCH] x86/resctrl: avoid compiler optimization in __resctrl_sched

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Date:   Tue, 7 Mar 2023 22:13:27 -0800
From:   Stephane Eranian <eranian@...gle.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Jakub Jelinek <jakub@...hat.com>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, tony.luck@...el.com,
        reinette.chatre@...el.com, fenghua.yu@...el.com,
        peternewman@...gle.com, james.morse@....com, babu.moger@....com,
        ananth.narayan@....com, vschneid@...hat.com,
        Nathan Chancellor <nathan@...nel.org>,
        clang-built-linux <llvm@...ts.linux.dev>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, linux-toolchains@...r.kernel.org
Subject: Re: [PATCH] x86/resctrl: avoid compiler optimization in __resctrl_sched_in

On Tue, Mar 7, 2023 at 1:06 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Tue, Mar 7, 2023 at 12:54 PM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > I think the problem is that the <asm/resctrl.h> code is disgusting and
> > horrible in multiple ways:
> >
> >  (a) it shouldn't define and declare a static function in a header file
> >
> >  (b) the resctrl_sched_in() inline function is misdesigned to begin with
>
> Ok, so here's a *ttoally* untested and mindless patch to maybe fix
> what I dislike about that resctl code.
>
> Does it fix the code generation issue? I have no idea. But this is
> what I would suggest is the right answer, without actually knowing the
> code any better, and just going on a mindless rampage.
>
> It seems to compile for me, fwiw.
>

On Tue, Mar 7, 2023 at 3:01 PM Nick Desaulniers <ndesaulniers@...gle.com> wrote:
>
> On Tue, Mar 7, 2023 at 2:56 PM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > On Tue, Mar 7, 2023 at 2:03 PM Nick Desaulniers <ndesaulniers@...gle.com> wrote:
> > >
> > > Sounds like Stephane is going to re-run the internal tests he used to
> > > discover the issue with your diff applied, if you don't mind holding
> > > out for another Tested-by tag. EOM
> >
> > Ack. I am in no hurry.
> >
> > In fact, I'd prefer to just get the patch sent back to me with a
> > commit message too, if somebody has the energy. I don't need the
> > credit for a trivial thing like that.
>
> Sure, then maybe Stephane you can supply a v2 with updated commit message and a
>
> Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
>
I verified Linus' patch on my test case on AMD Zen3 and it works as
 expected, i.e., the limit is enforced. I had tried a similar approach myself
as well and it worked.

I think passing the task pointer is the proper approach because we are
in a sched_in routine
and I would expect the task scheduled in to be passed as argument
instead of having the function
retrieve it from the current pointer.

Thanks.

Tested-by: Stephane Eranian <eranian@...gle.com>

>              Linus