[PATCH 5.15 096/124] powerpc/perf: Fix power10 event alternatives

Greg Kroah-Hartman
2022-04-26T09:12:42Z

From: Athira Rajeev <atrajeev@linux.vnet.ibm.com>

[ Upstream commit c6cc9a852f123301d5271f1484df8e961b2b64f1 ]

When scheduling a group of events, there are constraint checks done to
make sure all events can go in a group. Example, one of the criteria is
that events in a group cannot use the same PMC. But platform specific
PMU supports alternative event for some of the event codes. During
perf_event_open(), if any event group doesn't match constraint check
criteria, further lookup is done to find alternative event.

By current design, the array of alternatives events in PMU code is
expected to be sorted by column 0. This is because in
find_alternative() the return criteria is based on event code
comparison. ie. "event < ev_alt[i][0])". This optimisation is there
since find_alternative() can be called multiple times. In power10 PMU
code, the alternative event array is not sorted properly and hence there
is breakage in finding alternative event.

To work with existing logic, fix the alternative event array to be
sorted by column 0 for power10-pmu.c

Results:

In case where an alternative event is not chosen when we could, events
will be multiplexed. ie, time sliced where it could actually run
concurrently.

Example, in power10 PM_INST_CMPL_ALT(0x00002) has alternative event,
PM_INST_CMPL(0x500fa). Without the fix, if a group of events with PMC1
to PMC4 is used along with PM_INST_CMPL_ALT, it will be time sliced
since all programmable PMC's are consumed already. But with the fix,
when it picks alternative event on PMC5, all events will run
concurrently.

Before:

# perf stat -e r00002,r100fc,r200fa,r300fc,r400fc

Performance counter stats for 'system wide':

328668935 r00002 (79.94%)
56501024 r100fc (79.95%)
49564238 r200fa (79.95%)
376 r300fc (80.19%)
660 r400fc (79.97%)

4.039150522 seconds time elapsed

With the fix, since alternative event is chosen to run on PMC6, events
will be run concurrently.

After:

# perf stat -e r00002,r100fc,r200fa,r300fc,r400fc

Performance counter stats for 'system wide':

23596607 r00002
4907738 r100fc
2283608 r200fa
135 r300fc
248 r400fc

1.664671390 seconds time elapsed

Fixes: a64e697cef23 ("powerpc/perf: power10 Performance Monitoring support")
Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Reviewed-by: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220419114828.89843-2-atrajeev@linux.vnet.ibm.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/powerpc/perf/power10-pmu.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/perf/power10-pmu.c b/arch/powerpc/perf/power10-pmu.c
index 9dd75f385837..07ca62d084d9 100644
--- a/arch/powerpc/perf/power10-pmu.c
+++ b/arch/powerpc/perf/power10-pmu.c
@@ -91,8 +91,8 @@ extern u64 PERF_REG_EXTENDED_MASK;

/* Table of alternatives, sorted by column 0 */
static const unsigned int power10_event_alternatives[][MAX_ALT] = {
- { PM_CYC_ALT, PM_CYC },
{ PM_INST_CMPL_ALT, PM_INST_CMPL },
+ { PM_CYC_ALT, PM_CYC },
};

static int power10_get_alternatives(u64 event, unsigned int flags, u64 alt[])
--
2.35.1