linux-kernel - Re: [RFC PATCH v3 0/2] scheduler: expose the topology of clusters and add cluster scheduler

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Message-ID: <20210108151241.GA47324@e123083-lin>
Date:   Fri, 8 Jan 2021 16:12:41 +0100
From:   Morten Rasmussen <morten.rasmussen@....com>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
Cc:     Barry Song <song.bao.hua@...ilicon.com>,
        valentin.schneider@....com, catalin.marinas@....com,
        will@...nel.org, rjw@...ysocki.net, vincent.guittot@...aro.org,
        lenb@...nel.org, gregkh@...uxfoundation.org,
        jonathan.cameron@...wei.com, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, mark.rutland@....com, sudeep.holla@....com,
        aubrey.li@...ux.intel.com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
        linuxarm@...neuler.org, xuwei5@...wei.com,
        prime.zeng@...ilicon.com, tiantao6@...ilicon.com
Subject: Re: [RFC PATCH v3 0/2] scheduler: expose the topology of clusters
 and add cluster scheduler

On Thu, Jan 07, 2021 at 03:16:47PM -0800, Tim Chen wrote:
> On 1/6/21 12:30 AM, Barry Song wrote:
> > ARM64 server chip Kunpeng 920 has 6 clusters in each NUMA node, and each
> > cluster has 4 cpus. All clusters share L3 cache data while each cluster
> > has local L3 tag. On the other hand, each cluster will share some
> > internal system bus. This means cache is much more affine inside one cluster
> > than across clusters.
> 
> There is a similar need for clustering in x86.  Some x86 cores could share L2 caches that
> is similar to the cluster in Kupeng 920 (e.g. on Jacobsville there are 6 clusters
> of 4 Atom cores, each cluster sharing a separate L2, and 24 cores sharing L3).  
> Having a sched domain at the L2 cluster helps spread load among 
> L2 domains.  This will reduce L2 cache contention and help with
> performance for low to moderate load scenarios.

IIUC, you are arguing for the exact opposite behaviour, i.e. balancing
between L2 caches while Barry is after consolidating tasks within the
boundaries of a L3 tag cache. One helps cache utilization, the other
communication latency between tasks. Am I missing something? 

IMHO, we need some numbers on the table to say which way to go. Looking
at just benchmarks of one type doesn't show that this is a good idea in
general.

Morten