linux-kernel - Re: [PATCH] arch/arm64: Fix topology initialization for core scheduling

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Date:   Wed, 30 Mar 2022 11:52:49 -0400
From:   Phil Auld <pauld@...hat.com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     linux-kernel@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] arch/arm64: Fix topology initialization for core
 scheduling

On Wed, Mar 30, 2022 at 05:48:34PM +0200 Dietmar Eggemann wrote:
> On 29/03/2022 21:50, Phil Auld wrote:
> > On Tue, Mar 29, 2022 at 08:55:08PM +0200 Dietmar Eggemann wrote:
> >> On 29/03/2022 17:20, Phil Auld wrote:
> >>> On Tue, Mar 29, 2022 at 04:02:22PM +0200 Dietmar Eggemann wrote:
> >>>> On 22/03/2022 17:03, Phil Auld wrote:
> 
> [...]
> 
> >>> This instance is an HPE Apollo 70 set to smt-4.  I believe it's ThunderX2
> >>> chips.
> >>>
> >>> ARM (CN9980-2200LG4077-Y21-G) 
> >> I'm using the same processor just with ACPI/PPTT.
> >>
> > 
> > Maybe I'm misinformed about these systems having no PPTT...  
> > 
> > I'm reclaiming the system. Is there a way I can tell from userspace?
> 
> # cat /sys/firmware/acpi/tables/PPTT > pptt.dat
> # iasl -d pptt.dat
> # vim pptt.dsl
>

Thanks, I'll git that a try.  I suspect these are the same as yours though
and I was just mistaken :)


> [...]
> 
> >> so no SMT sched domain. The MPIDR-based topology fallback code in
> >> store_cpu_topology() forces `cpuid_topo->thread_id  = -1`.
> > 
> > Right. So since I'm getting SMT it must not have package_id == -1.
> > In which case you should be able to reproduce it because it must
> > be that the call the update_siblings_masks() is required.  That
> > appears to only be called from store_cpu_topology() which is
> > after the scheduler has already setup the core pointers.
> > 
> > The fix could be the same but I should reword the commit message
> > since it should effect all SMT arm systems I'd think.
> > 
> > Or maybe the ACPI topology code should call update_sibling_masks(). 
> >>
> >> IMHO this is why on my machine I don't see this issue while running:
> >>
> >> root@...-apollo7007:~# stress-ng --prctl 256 -t 60
> >> stress-ng: info:  [2388042] dispatching hogs: 256 prctl
> >>
> >> Is there something I miss in my setup to provoke this issue?
> >>
> > 
> > Make sure you have a stress-ng that is new enough and built against
> > headers that have the CORE_SCHED prctls defined.
> 
> Ah, I was using a pretty old version 0.11.07. Now I switched to 0.13.12
> which includes:
> 
>   9038e442b92d - stress-prctl: add Linux 5.14 PR_SCHED_CORE prctl
> 
> To get SCHED_CORE activated in stress-prctl.c, as a quick hack, I had to
> add the definitions of PR_SCHED_CORE, PR_SCHED_CORE_GET, etc. to this file.
> 
> Now the issue you described triggers on this machine immediately.
>

Great!  I'll repost the patch with a more accurate commit message then.

And if you come up with something different that works for me too. Let
me know and I'll test it here.


Cheers,
Phil


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