linux-kernel - Re: [PATCH v9 0/3] powerpc: Detection and scheduler optimization for POWER9 bigcore

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Date:   Wed, 3 Oct 2018 16:49:13 +0530
From:   Gautham R Shenoy <ego@...ux.vnet.ibm.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Michael Neuling <mikey@...ling.org>,
        Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
        Akshay Adiga <akshay.adiga@...ux.vnet.ibm.com>,
        Shilpasri G Bhat <shilpa.bhat@...ux.vnet.ibm.com>,
        "Oliver O'Halloran" <oohall@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Murilo Opsfelder Araujo <muriloo@...ux.ibm.com>,
        Anton Blanchard <anton@...ba.org>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 0/3] powerpc: Detection and scheduler optimization for
 POWER9 bigcore

Hello Dave,

On Mon, Oct 01, 2018 at 07:05:11AM -0700, Dave Hansen wrote:
> On 10/01/2018 06:16 AM, Gautham R. Shenoy wrote:
> > 
> > Patch 3: Creates a pair of sysfs attributes named
> > 	  /sys/devices/system/cpu/cpuN/topology/smallcore_thread_siblings
> > 	  and
> > 	  /sys/devices/system/cpu/cpuN/topology/smallcore_thread_siblings_list
> > 	  exposing the small-core siblings that share the L1 cache
> > 	  to the userspace.
> 
> I really don't think you've justified the existence of a new user/kernel
> interface here.  We already have information about threads share L1
> caches in here:
> 
> 	/sys/devices/system/cpu/cpu0/cache/index0/shared_cpu_list


Hmmm. My bad, I wasn't aware of this sysfs interface. We can use this
to share information about L1 cache. And currently on the upstream
kernel we report incorrect value on POWER9 SMT8 core. I will fix
this. Thanks for calling this out.

> 
> The only question would be if anything would break because it assumes
> that all SMT siblings share all caches.  But, it breaks if your new
> interface is there or not; it's old software that we care about.
>

Up until POWER9 SMT8 cores, this assumption was true that all SMT
siblings share all caches. POWER9 SMT8 cores are the only exception. 

--
Thanks and Regards
gautham.