linux-kernel - Re: [PATCH 23/24] selftests/resctrl: Add L2 CAT test

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Message-ID: <d3dc1393-f51d-1fea-2787-4063abdc7c33@linux.intel.com>
Date:   Tue, 7 Nov 2023 11:33:40 +0200 (EET)
From:   Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To:     Reinette Chatre <reinette.chatre@...el.com>
cc:     linux-kselftest@...r.kernel.org, Shuah Khan <shuah@...nel.org>,
        Shaopeng Tan <tan.shaopeng@...fujitsu.com>,
        Maciej Wieczór-Retman 
        <maciej.wieczor-retman@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 23/24] selftests/resctrl: Add L2 CAT test

On Mon, 6 Nov 2023, Reinette Chatre wrote:
> On 11/6/2023 9:03 AM, Reinette Chatre wrote:
> > On 11/6/2023 1:53 AM, Ilpo Järvinen wrote:
> >> On Fri, 3 Nov 2023, Reinette Chatre wrote:
> >>> On 11/3/2023 3:39 AM, Ilpo Järvinen wrote:
> >>>> On Thu, 2 Nov 2023, Reinette Chatre wrote:
> >>>>> On 10/24/2023 2:26 AM, Ilpo Järvinen wrote:
> >>>>
> >>>>>> Add L2 CAT selftest. As measuring L2 misses is not easily available
> >>>>>> with perf, use L3 accesses as a proxy for L2 CAT working or not.
> >>>>>
> >>>>> I understand the exact measurement is not available but I do notice some
> >>>>> L2 related symbolic counters when I run "perf list". l2_rqsts.all_demand_miss
> >>>>> looks promising.
> >>>>
> >>>> Okay, I was under impression that L2 misses are not available. Both based 
> >>>> on what you mentioned to me half an year ago and because of what flags I 
> >>>> found from the header. But I'll take another look into it.
> >>>
> >>> You are correct that when I did L2 testing a long time ago I used
> >>> the model specific L2 miss counts. I was hoping that things have improved
> >>> so that model specific counters are not needed, as you have tried here.
> >>> I found the l2_rqsts symbol while looking for alternatives but I am not
> >>> familiar enough with perf to know how these symbolic names are mapped.
> >>> I was hoping that they could be a simple drop-in replacement to
> >>> experiment with.
> >>
> >> According to perf_event_open() manpage, mapping those symbolic names 
> >> requires libpfm so this would add a library dependency?
> > 
> > I do not see perf list using this library to determine the event and
> > umask but I am in unfamiliar territory. I'll have to spend some more
> > time here to determine options.
> 
> tools/perf/pmu-events/README cleared it up for me. The architecture specific
> tables are included in the perf binary. Potentially pmu-events.h could be
> included or the test could just stick with the architectural events.
> A quick look at the various cache.json files created the impression that
> the events of interest may actually have the same event code and umask across
> platforms.
> I am not familiar with libpfm. This can surely be considered if it supports
> this testing. Several selftests have library dependencies.

man perf_event_open() says this:

"If type is PERF_TYPE_RAW, then a custom "raw" config  value  is  needed.
Most  CPUs  support  events  that  are  not covered by the "generalized"
events.  These are implementation defined; see your CPU manual (for  ex-
ample  the  Intel Volume 3B documentation or the AMD BIOS and Kernel De-
veloper Guide).  The libpfm4 library can be used to translate  from  the
name in the architectural manuals to the raw hex value perf_event_open()
expects in this field."

...I've not come across libpfm myself either but to me it looks libpfm 
bridges between those architecture specific tables and perf_event_open(). 
That is, it could provide the binary value necessary in constructing the 
perf_event_attr struct.

I think this is probably the function which maps string -> 
perf_event_attr:

https://man7.org/linux/man-pages/man3/pfm_get_os_event_encoding.3.html


-- 
 i.