Messages in this thread | | | Date | Fri, 9 Jun 2023 13:47:18 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH V11 06/10] arm64/perf: Enable branch stack events via FEAT_BRBE |
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On Fri, Jun 09, 2023 at 10:52:37AM +0530, Anshuman Khandual wrote: > [...] > > On 6/5/23 19:13, Mark Rutland wrote: > >> +void armv8pmu_branch_read(struct pmu_hw_events *cpuc, struct perf_event *event) > >> +{ > >> + struct brbe_hw_attr *brbe_attr = (struct brbe_hw_attr *)cpuc->percpu_pmu->private; > >> + u64 brbfcr, brbcr; > >> + int idx, loop1_idx1, loop1_idx2, loop2_idx1, loop2_idx2, count; > >> + > >> + brbcr = read_sysreg_s(SYS_BRBCR_EL1); > >> + brbfcr = read_sysreg_s(SYS_BRBFCR_EL1); > >> + > >> + /* Ensure pause on PMU interrupt is enabled */ > >> + WARN_ON_ONCE(!(brbcr & BRBCR_EL1_FZP)); > >> + > >> + /* Pause the buffer */ > >> + write_sysreg_s(brbfcr | BRBFCR_EL1_PAUSED, SYS_BRBFCR_EL1); > >> + isb(); > >> + > >> + /* Determine the indices for each loop */ > >> + loop1_idx1 = BRBE_BANK0_IDX_MIN; > >> + if (brbe_attr->brbe_nr <= BRBE_BANK_MAX_ENTRIES) { > >> + loop1_idx2 = brbe_attr->brbe_nr - 1; > >> + loop2_idx1 = BRBE_BANK1_IDX_MIN; > >> + loop2_idx2 = BRBE_BANK0_IDX_MAX; > >> + } else { > >> + loop1_idx2 = BRBE_BANK0_IDX_MAX; > >> + loop2_idx1 = BRBE_BANK1_IDX_MIN; > >> + loop2_idx2 = brbe_attr->brbe_nr - 1; > >> + } > >> + > >> + /* Loop through bank 0 */ > >> + select_brbe_bank(BRBE_BANK_IDX_0); > >> + for (idx = 0, count = loop1_idx1; count <= loop1_idx2; idx++, count++) { > >> + if (!capture_branch_entry(cpuc, event, idx)) > >> + goto skip_bank_1; > >> + } > >> + > >> + /* Loop through bank 1 */ > >> + select_brbe_bank(BRBE_BANK_IDX_1); > >> + for (count = loop2_idx1; count <= loop2_idx2; idx++, count++) { > >> + if (!capture_branch_entry(cpuc, event, idx)) > >> + break; > >> + } > >> + > >> +skip_bank_1: > >> + cpuc->branches->branch_stack.nr = idx; > >> + cpuc->branches->branch_stack.hw_idx = -1ULL; > >> + process_branch_aborts(cpuc); > >> + > >> + /* Unpause the buffer */ > >> + write_sysreg_s(brbfcr & ~BRBFCR_EL1_PAUSED, SYS_BRBFCR_EL1); > >> + isb(); > >> + armv8pmu_branch_reset(); > >> +} > > The loop indicies are rather difficult to follow, and I think those can be made > > quite a lot simpler if split out, e.g. > > > > | int __armv8pmu_branch_read(struct pmu_hw_events *cpuc, struct perf_event *event) > > | { > > | struct brbe_hw_attr *brbe_attr = (struct brbe_hw_attr *)cpuc->percpu_pmu->private; > > | int nr_hw_entries = brbe_attr->brbe_nr; > > | int idx; > > I guess idx needs an init to 0.
Yes, sorry, that should have been:
int idx = 0;
> > | > > | select_brbe_bank(BRBE_BANK_IDX_0); > > | while (idx < nr_hw_entries && idx < BRBE_BANK0_IDX_MAX) { > > | if (!capture_branch_entry(cpuc, event, idx)) > > | return idx; > > | idx++; > > | } > > | > > | select_brbe_bank(BRBE_BANK_IDX_1); > > | while (idx < nr_hw_entries && idx < BRBE_BANK1_IDX_MAX) { > > | if (!capture_branch_entry(cpuc, event, idx)) > > | return idx; > > | idx++; > > | } > > | > > | return idx; > > | } > > These loops are better than the proposed one with indices, will update.
Great!
> > | > > | void armv8pmu_branch_read(struct pmu_hw_events *cpuc, struct perf_event *event) > > | { > > | u64 brbfcr, brbcr; > > | int nr; > > | > > | brbcr = read_sysreg_s(SYS_BRBCR_EL1); > > | brbfcr = read_sysreg_s(SYS_BRBFCR_EL1); > > | > > | /* Ensure pause on PMU interrupt is enabled */ > > | WARN_ON_ONCE(!(brbcr & BRBCR_EL1_FZP)); > > | > > | /* Pause the buffer */ > > | write_sysreg_s(brbfcr | BRBFCR_EL1_PAUSED, SYS_BRBFCR_EL1); > > | isb(); > > | > > | nr = __armv8pmu_branch_read(cpus, event); > > | > > | cpuc->branches->branch_stack.nr = nr; > > | cpuc->branches->branch_stack.hw_idx = -1ULL; > > | process_branch_aborts(cpuc); > > | > > | /* Unpause the buffer */ > > | write_sysreg_s(brbfcr & ~BRBFCR_EL1_PAUSED, SYS_BRBFCR_EL1); > > | isb(); > > | armv8pmu_branch_reset(); > > | } > > > > Looking at <linux/perf_event.h> I see: > > > > | /* > > | * branch stack layout: > > | * nr: number of taken branches stored in entries[] > > | * hw_idx: The low level index of raw branch records > > | * for the most recent branch. > > | * -1ULL means invalid/unknown. > > | * > > | * Note that nr can vary from sample to sample > > | * branches (to, from) are stored from most recent > > | * to least recent, i.e., entries[0] contains the most > > | * recent branch. > > | * The entries[] is an abstraction of raw branch records, > > | * which may not be stored in age order in HW, e.g. Intel LBR. > > | * The hw_idx is to expose the low level index of raw > > | * branch record for the most recent branch aka entries[0]. > > | * The hw_idx index is between -1 (unknown) and max depth, > > | * which can be retrieved in /sys/devices/cpu/caps/branches. > > | * For the architectures whose raw branch records are > > | * already stored in age order, the hw_idx should be 0. > > | */ > > | struct perf_branch_stack { > > | __u64 nr; > > | __u64 hw_idx; > > | struct perf_branch_entry entries[]; > > | }; > > > > ... which seems to indicate we should be setting hw_idx to 0, since IIUC our > > records are in age order. > Branch records are indeed in age order, sure will change hw_idx as 0. Earlier > figured that there was no need for hw_idx and hence marked it as -1UL similar > to other platforms like powerpc.
That's fair enough; looking at power_pmu_bhrb_read() in arch/powerpc/perf/core-book3s.c, I see a comment:
Branches are read most recent first (ie. mfbhrb 0 is the most recent branch).
... which suggests that should be 0 also, or that the documentation is wrong.
Do you know how the perf tool consumes this?
Thanks, Mark.
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