linux-kernel - Re: [PATCH v8 0/3] powerpc: Detection and scheduler optimization for POWER9 bigcore

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Date:   Wed, 26 Sep 2018 11:36:13 +0530
From:   Gautham R Shenoy <ego@...ux.vnet.ibm.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     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 v8 0/3] powerpc: Detection and scheduler optimization for
 POWER9 bigcore

Hello Dave,

On Tue, Sep 25, 2018 at 03:16:30PM -0700, Dave Hansen wrote:
> On 09/22/2018 04:03 AM, Gautham R Shenoy wrote:
> > Without this patchset, the SMT domain would be defined as the group of
> > threads that share L2 cache.
> 
> Could you try to make a more clear, concise statement about the current
> state of the art vs. what you want it to be?  Right now, the sched
> domains do something like this in terms of ordering:
> 
> 1. SMT siblings
> 2. Caches
> 3. NUMA

Yes. you are right. The state of art on POWER9 machines having SMT8
cores is as you described above with

1. SMT siblings sharing L2-cache, called the SMT domain
2. Cores on the same die, called the DIE domain
3. NUMA

> 
> It sounds like you don't want SMT siblings to be the things that we use,
> right?  Because some siblings share caches and some do not.  Right?  You
> want something like this:
> 
> 1. SMT siblings (sharing L1)
> 2. SMT siblings (sharing L2)
> 3. Other caches
> 4. NUMA
>

Yes, with the patchset the sched-domain hierarchy on POWER9 machines
having SMT8 will be:

1. SMT siblings sharing L1 cache, called the SMT domain
2. SMT siblings sharing L2 cache, called the CACHE domain (introduced in
   commit 96d91431d691 "powerpc/smp: Add Power9 scheduler topology")
3. Cores on the same die, called the DIE domain.
4. NUMA

--
Thanks and Regards
gautham.