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 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() function, the return criteria is based on event code comparison. ie "event < ev_alt[i][0])". This optimisation is there since find_alternative() can get called multiple times. In power9 PMU code, the alternative event array is not sorted list 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 power9-pmu.c Results: With alternative events, multiplexing can be avoided. That is, for example, in power9 PM_LD_MISS_L1 (0x3e054) has alternative event, PM_LD_MISS_L1_ALT (0x400f0). This is an identical event which can be programmed in a different PMC. << Before patch >> # perf stat -e r3e054,r300fc ^C Performance counter stats for 'system wide': 1057860 r3e054 (50.21%) 379 r300fc (49.79%) 0.944329741 seconds time elapsed Since both the events are using PMC3 in this case, they are multiplexed here. <<With patch>> # perf stat -e r3e054,r300fc ^C Performance counter stats for 'system wide': 1006948 r3e054 182 r300fc <<>> Fixes: 91e0bd1e6251 ("powerpc/perf: Add PM_LD_MISS_L1 and PM_BR_2PATH to power9 event list") Signed-off-by: Athira Rajeev <atraj...@linux.vnet.ibm.com> Reviewed-by: Madhavan Srinivasan <ma...@linux.vnet.ibm.com> --- Changelog: v1 -> v2: Added Fixes tag and reworded commit message. Added Reviewed-by from Maddy. v2 -> v3: Added info about what is the breakage with current code. arch/powerpc/perf/power9-pmu.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/arch/powerpc/perf/power9-pmu.c b/arch/powerpc/perf/power9-pmu.c index c9eb5232e68b..c393e837648e 100644 --- a/arch/powerpc/perf/power9-pmu.c +++ b/arch/powerpc/perf/power9-pmu.c @@ -133,11 +133,11 @@ int p9_dd22_bl_ev[] = { /* Table of alternatives, sorted by column 0 */ static const unsigned int power9_event_alternatives[][MAX_ALT] = { - { PM_INST_DISP, PM_INST_DISP_ALT }, - { PM_RUN_CYC_ALT, PM_RUN_CYC }, - { PM_RUN_INST_CMPL_ALT, PM_RUN_INST_CMPL }, - { PM_LD_MISS_L1, PM_LD_MISS_L1_ALT }, { PM_BR_2PATH, PM_BR_2PATH_ALT }, + { PM_INST_DISP, PM_INST_DISP_ALT }, + { PM_RUN_CYC_ALT, PM_RUN_CYC }, + { PM_LD_MISS_L1, PM_LD_MISS_L1_ALT }, + { PM_RUN_INST_CMPL_ALT, PM_RUN_INST_CMPL }, }; static int power9_get_alternatives(u64 event, unsigned int flags, u64 alt[]) -- 2.35.1