I want to understand which events can have the precise modifier on my CPU (Sandy Bridge).
Intel Software Developer’s Manual (Table 18-32. PEBS Performance
Events for Intel Microarchitecture Code Name Sandy Bridge) contains
only the following events: INST_RETIRED, UOPS_RETIRED,
BR_INST_RETIRED, BR_MISP_RETIRED, MEM_UOPS_RETIRED,
MEM_LOAD_UOPS_RETIRED, MEM_LOAD_UOPS_LLC_HIT_RETIRED. And SandyBridge_core_V15.json lists the same events with PEBS > 0.
However there are some examples of using perf, which add :p to the cycles event. And I can successfully run perf record -e cycles:p on my machine.
Also perf record -e cycles:p -vv -- sleep 1 prints precise_ip 1. So does it mean that CPU_CLK_UNHALTED event actually uses PEBS?
Is it possible to get the full list of events, which support :p?
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Answer
There is hack to support cycles:p on SandyBridge which has no PEBS for CPU_CLK_UNHALTED.*. The hack is implemented in the kernel part of perf in intel_pebs_aliases_snb(). When user requests -e cycles which is PERF_COUNT_HW_CPU_CYCLES (translates to CPU_CLK_UNHALTED.CORE) with nonzero precise modifier, this function will change hardware event to UOPS_RETIRED.ALL with PEBS:
29 [PERF_COUNT_HW_CPU_CYCLES] = 0x003c,
2739 static void intel_pebs_aliases_snb(struct perf_event *event)
2740 {
2741 if ((event->hw.config & X86_RAW_EVENT_MASK) == 0x003c) {
2742 /*
2743 * Use an alternative encoding for CPU_CLK_UNHALTED.THREAD_P
2744 * (0x003c) so that we can use it with PEBS.
2745 *
2746 * The regular CPU_CLK_UNHALTED.THREAD_P event (0x003c) isn't
2747 * PEBS capable. However we can use UOPS_RETIRED.ALL
2748 * (0x01c2), which is a PEBS capable event, to get the same
2749 * count.
2750 *
2751 * UOPS_RETIRED.ALL counts the number of cycles that retires
2752 * CNTMASK micro-ops. By setting CNTMASK to a value (16)
2753 * larger than the maximum number of micro-ops that can be
2754 * retired per cycle (4) and then inverting the condition, we
2755 * count all cycles that retire 16 or less micro-ops, which
2756 * is every cycle.
2757 *
2758 * Thereby we gain a PEBS capable cycle counter.
2759 */
2760 u64 alt_config = X86_CONFIG(.event=0xc2, .umask=0x01, .inv=1, .cmask=16);
2761
2762 alt_config |= (event->hw.config & ~X86_RAW_EVENT_MASK);
2763 event->hw.config = alt_config;
2764 }
2765 }
The intel_pebs_aliases_snb hack is registered in 3557 __init int intel_pmu_init(void) for case INTEL_FAM6_SANDYBRIDGE: / case INTEL_FAM6_SANDYBRIDGE_X: as
3772 x86_pmu.event_constraints = intel_snb_event_constraints; 3773 x86_pmu.pebs_constraints = intel_snb_pebs_event_constraints; 3774 x86_pmu.pebs_aliases = intel_pebs_aliases_snb;
pebs_aliases is called from intel_pmu_hw_config() when precise_ip is set to non-zero:
2814 static int intel_pmu_hw_config(struct perf_event *event)
2815 {
2821 if (event->attr.precise_ip) {
2828 if (x86_pmu.pebs_aliases)
2829 x86_pmu.pebs_aliases(event);
2830 }
The hack was implemented in 2012, lkml threads “[PATCH] perf, x86: Make cycles:p working on SNB”, “[tip:perf/core] perf/x86: Implement cycles:p for SNB/IVB”, cccb9ba9e4ee0d750265f53de9258df69655c40b, http://git.kernel.org/cgit/linux/kernel/git/tip/tip.git/commit/?id=cccb9ba9e4ee0d750265f53de9258df69655c40b:
perf/x86: Implement cycles:p for SNB/IVB
Now that there’s finally a chip with working PEBS (IvyBridge), we can enable the hardware and implement cycles:p for SNB/IVB.
And I think, there is no full list of such “precise” converting hack besides the linux source code in arch/x86/events/intel/core.c, grep for static void intel_pebs_aliases (usually cycles:p / CPU_CLK_UNHALTED 0x003c is implemented) and check intel_pmu_init for actual model and exact x86_pmu.pebs_aliases variant selected:
- intel_pebs_aliases_core2,
INST_RETIRED.ANY_P (0x00c0) CNTMASK=16instead ofcycles:p - intel_pebs_aliases_snb,
UOPS_RETIRED.ALL (0x01c2) CNTMASK=16instead ofcycles:p - intel_pebs_aliases_precdist for highest values of
precise_ip,INST_RETIRED.PREC_DIST (0x01c0)instead ofcycles:pppon SKL, IVB, HSW, BDW