linux-kernel - Re: [PATCH v4 7/7] selftests/resctrl: Adjust effective L3 cache size when SNC enabled

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Message-ID: <ZOzwGQhGucBXZGO+@agluck-desk3>
Date:   Mon, 28 Aug 2023 12:06:01 -0700
From:   Tony Luck <tony.luck@...el.com>
To:     Reinette Chatre <reinette.chatre@...el.com>
Cc:     Fenghua Yu <fenghua.yu@...el.com>,
        Peter Newman <peternewman@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Shuah Khan <skhan@...uxfoundation.org>, x86@...nel.org,
        Shaopeng Tan <tan.shaopeng@...itsu.com>,
        James Morse <james.morse@....com>,
        Jamie Iles <quic_jiles@...cinc.com>,
        Babu Moger <babu.moger@....com>,
        Randy Dunlap <rdunlap@...radead.org>,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        patches@...ts.linux.dev
Subject: Re: [PATCH v4 7/7] selftests/resctrl: Adjust effective L3 cache size
 when SNC enabled

On Mon, Aug 28, 2023 at 10:06:32AM -0700, Reinette Chatre wrote:
> Hi Tony,
> 
> On 8/25/2023 10:56 AM, Tony Luck wrote:
> > On Fri, Aug 11, 2023 at 10:33:43AM -0700, Reinette Chatre wrote:
> >> Hi Tony,
> >>
> >> On 7/22/2023 12:07 PM, Tony Luck wrote:
> 
> ...
> 
> >>> @@ -190,6 +245,8 @@ int get_cache_size(int cpu_no, char *cache_type, unsigned long *cache_size)
> >>>  			break;
> >>>  	}
> >>>  
> >>> +	if (cache_num == 3)
> >>> +		*cache_size /= snc_ways();
> >>>  	return 0;
> >>>  }
> >>>  
> >>
> >> I am surprised that this small change is sufficient. The resctrl
> >> selftests are definitely not NUMA aware and the CAT and CMT tests
> >> are not taking that into account when picking CPUs to run on. From
> >> what I understand LLC occupancy counters need to be added in this
> >> scenario but I do not see that done either.
> > 
> > This is a first step (the tests are definitely going to fail if
> > they have incorrect information about the cache size).
> > 
> > For a fully reliable set of tests some major surgery will be required
> > to bind to CPUs and memory to control allocation and access.
> > 
> 
> What is the plan for making the tests more reliable? What is the
> use of this patch if it is just the first step?

Reinette,

I have no immediate plan to re-architect the the resctrl self-tests.
If you feel this step towards a solution is useless unless it is part
of a complete solution, then I can drop it from this series.

-Tony