Re: [PATCH v3 00/17] Compress the pmu_event tables

From: John Garry <john.garry@huawei.com>
To: Ian Rogers <irogers@google.com>
Cc: Will Deacon <will@kernel.org>, James Clark <james.clark@arm.com>,
	"Mike Leach" <mike.leach@linaro.org>,
	Leo Yan <leo.yan@linaro.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	"Arnaldo Carvalho de Melo" <acme@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	"Alexander Shishkin" <alexander.shishkin@linux.intel.com>,
	Jiri Olsa <jolsa@kernel.org>, Namhyung Kim <namhyung@kernel.org>,
	Andi Kleen <ak@linux.intel.com>,
	"Zhengjun Xing" <zhengjun.xing@linux.intel.com>,
	Ravi Bangoria <ravi.bangoria@amd.com>,
	Kan Liang <kan.liang@linux.intel.com>,
	Adrian Hunter <adrian.hunter@intel.com>,
	<linux-kernel@vger.kernel.org>,
	<linux-arm-kernel@lists.infradead.org>,
	<linux-perf-users@vger.kernel.org>,
	Stephane Eranian <eranian@google.com>
Subject: Re: [PATCH v3 00/17] Compress the pmu_event tables
Date: Tue, 2 Aug 2022 10:08:47 +0100	[thread overview]
Message-ID: <3d0c1ec0-42ec-8c51-743b-5d93cabb53fb@huawei.com> (raw)
In-Reply-To: <CAP-5=fV0CMpBGkNOBMRjbdZfdw1mQjrDhLVG38jLtpsdWXtQ_Q@mail.gmail.com>

On 29/07/2022 18:27, Ian Rogers wrote:
>> This implementation would require core pmu.c to be changed, but there is
>> ways that this could be done without needing to change core pmu.c
>>
>> Thanks,
>> John
> Thanks John!
> 
> You are right about broadwell, it is an extreme case of sharing. IIRC
> BDX is the server core/uncore events, BDW is the consumer core/uncore
> and BDW-DE is consumer core with server uncore - so the sharing is
> inherent in this. Metrics become interesting as they may mix core and
> uncore, but I'll ignore that here.
> 
> In the old code every event needs 15 char*s, with 64-bit that is 15*8
> bytes per entry with 741 core and 23 uncore entries for BDW, and 372
> core and 1284 uncore for BDX. I expect the strings themselves will be
> shared by the C compiler, and so I just focus on the pointer sizes.
> With the new code every event is just 1 32-bit int. So for BDW we go
> from 15*8*(741+23)=91,680 to 4*(741+23)=3056, BDX is
> 15*8*(372+1284)=198720 to 4*(372+1284)=6624. This means we've gone
> from 290,400bytes to 9,680bytes for BDW and BDX. BDW-DE goes from
> 243,000bytes to 8,100bytes -

> we can ignore the costs of the strings as
> they should be fully shared, especially for BDW-DE.

Are you sure about this? I did not think that the compiler would have 
the intelligence to try to share strings. That is the basis of the size 
optimisation which I was proposing (that the compiler would not share 
strings).

> 
> If we added some kind of table sharing, so BDW-DE was core from BDW
> and uncore from BDX and the tables shared, then in the old code you
> could save nearly 0.25MB but with the new code the saving is only
> around 8KB. I think we can go after that 8KB but it is less urgent
> after this change which gets 96% of the benefit. We have 72
> architectures for jevents at the moment and so I'd ball park (assuming
> they all saved as much as BDW-DE) the max saving as about 0.5MB, which
> is 12% of what is saved here.
> 
> Longer term I'd like to make the pmu-events.c logic look closer to the
> sysfs API. Perhaps we can unify the uncore events for BDX and BDW-DE
> with some notion of a common PMU, or PMUs with common events and
> tables, and automate deduction of this. It also isn't clear to me the
> advantage of storing the metrics inside the events, separate tables
> feel cleaner. Anyway, there's lots of follow up.

Thanks,
John

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