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

From: John Garry
Date: Fri Jul 29 2022 - 11:03:48 EST

On 29/07/2022 08:43, Ian Rogers wrote:
jevents.py creates a number of large arrays from the json events. The
arrays contain pointers to strings that need relocating. The
relocations have file size, run time and memory costs. These changes
refactor the pmu_events API so that the storage of the pmu_event
struct isn't exposed. The format is then changed to an offset within a
combined big string, with adjacent pmu_event struct variables being
next to each other in the string separated by \0 - meaning only the
first variable of the struct needs its offset recording.

Some related fixes are contained with the patches. The architecture
jevents.py creates tables for can now be set by the JEVENTS_ARCH make
variable, with a new 'all' that generates the events and metrics for
all architectures.

Hi Ian,

I am going through this series currently.

But I just wanted to mention my idea again on how to compress the tables. Maybe you thought that there was no value in my idea or didn't get it, but I'll mention it again just in case...

Background:
There is much duplication in events between cores. And currently we have something like this:

pmu-events/pmu-events.c:
struct pmu_event core0[] {
{
.name = event0,
.event = "event=0x0",
},
{
.name = event1,
.event = "event=0x1",
},
{
.name = event2,
.event = "event=0x2",
.desc = "event2 common desc",
},
...
};

struct pmu_event core1[] {
{
.name = event0,
.event = "event=0x0",
},
{
.name = event1,
.event = "event=0x1",
},
{
.name = event2,
.event = "event=0x2",
.desc = "event2 desc for core1",
},
...
};


struct pmu_evenets_map map[] = {
{
.cpuid = "0000",
.table = core0,
},
{
.cpuid = "0001",
.table = core1,
},
...
};

If you check broadwell and broadwellde frontent.json you will notice that they are identical, which is an extreme example of duplication.

Proposal for change:
Make each event in the per-core pmu event table point to common event. Each common event is unique, and each event per-core will point to a common event. So if 2x cores have same event but small difference, then there would be still 2x common events.

pmu-events/pmu-events.c:
struct pmu_event common_events[] {
{
.name = event0,
.event = "event=0x0",
},
{
.name = event1,
.event = "event=0x1",
},
{
.name = event2,
.event = "event=0x2",
.desc = "event2 common desc",
},
{
.name = event2,
.event = "event=0x2",
.desc = "event2 desc for core1",
},
...
};

struct pmu_event_ptr {
struct pmu_event *pmu_event;
}

struct pmu_event_ptr core0[] {
{
.pmu_event = &common_events[0],
},
{
.pmu_event = &common_events[1],
},
{
.pmu_event = &common_events[2],
},
...
};

struct pmu_event_ptr core0[] {
{
.pmu_event = &common_events[0],
},
{
.pmu_event = &common_events[1],
},
{
.pmu_event = &common_events[3],
},
...
};

struct pmu_evenets_map map[] = {
{
.cpuid = "0000",
.table = core0,
},
{
.cpuid = "0001",
.table = core1,
},
...
};

For x86, first step in JSON parsing would be to go through the JSON files and compile a list of unique events. Then second step is to process each per-core JSON to create the pmu events table, using the common events. Using a per common event hash would make the lookup quicker.

I'm not sure what you think. From figures below you seem to be saving ~20% at best - I would guess (with a capital G) that my method could save a lot more.

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


An example of the improvement to the file size on x86 is:
no jevents - the same 19,788,464bytes
x86 jevents - ~16.7% file size saving 23,744,288bytes vs 28,502,632bytes
all jevents - ~19.5% file size saving 24,469,056bytes vs 30,379,920bytes
default build options plus NO_LIBBFD=1.

I originally suggested fixing this problem in:
https://lore.kernel.org/linux-perf-users/CAP-5=fVB8G4bdb9T=FncRTh9oBVKCS=+=eowAO+YSgAhab+Dtg@xxxxxxxxxxxxxx/

v3. Fix an ARM build issue with a missed weak symbol. Perform some
pytype clean up.
v2. Split the substring folding optimization to its own patch and
comment tweaks as suggested by Namhyung Kim
<namhyung@xxxxxxxxxx>. Recompute the file size savings with the
latest json events and metrics.

Ian Rogers (17):
perf jevents: Clean up pytype warnings
perf jevents: Simplify generation of C-string
perf jevents: Add JEVENTS_ARCH make option
perf jevent: Add an 'all' architecture argument
perf jevents: Remove the type/version variables
perf jevents: Provide path to json file on error
perf jevents: Sort json files entries
perf pmu-events: Hide pmu_sys_event_tables
perf pmu-events: Avoid passing pmu_events_map
perf pmu-events: Hide pmu_events_map
perf test: Use full metric resolution
perf pmu-events: Move test events/metrics to json
perf pmu-events: Don't assume pmu_event is an array
perf pmu-events: Hide the pmu_events
perf metrics: Copy entire pmu_event in find metric
perf jevents: Compress the pmu_events_table
perf jevents: Fold strings optimization

tools/perf/arch/arm64/util/pmu.c | 4 +-
tools/perf/pmu-events/Build | 6 +-
.../arch/test/test_soc/cpu/metrics.json | 64 +++
tools/perf/pmu-events/empty-pmu-events.c | 204 +++++++-
tools/perf/pmu-events/jevents.py | 495 ++++++++++++++----
tools/perf/pmu-events/pmu-events.h | 40 +-
tools/perf/tests/expand-cgroup.c | 25 +-
tools/perf/tests/parse-metric.c | 77 +--
tools/perf/tests/pmu-events.c | 466 +++++++----------
tools/perf/util/metricgroup.c | 275 ++++++----
tools/perf/util/metricgroup.h | 5 +-
tools/perf/util/pmu.c | 139 ++---
tools/perf/util/pmu.h | 8 +-
tools/perf/util/s390-sample-raw.c | 50 +-
14 files changed, 1140 insertions(+), 718 deletions(-)
create mode 100644 tools/perf/pmu-events/arch/test/test_soc/cpu/metrics.json