Messages in this thread | | | Date | Thu, 19 Oct 2023 12:52:31 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH V4 4/7] perf/x86/intel: Support LBR event logging |
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On Wed, Oct 04, 2023 at 11:40:41AM -0700, kan.liang@linux.intel.com wrote:
> +#define ARCH_LBR_EVENT_LOG_WIDTH 2 > +#define ARCH_LBR_EVENT_LOG_MASK 0x3
event log ?
> +static __always_inline void intel_pmu_update_lbr_event(u64 *lbr_events, int idx, int pos) > +{ > + u64 logs = *lbr_events >> (LBR_INFO_EVENTS_OFFSET + > + idx * ARCH_LBR_EVENT_LOG_WIDTH); > + > + logs &= ARCH_LBR_EVENT_LOG_MASK; > + *lbr_events |= logs << (pos * ARCH_LBR_EVENT_LOG_WIDTH); > +} > + > +/* > + * The enabled order may be different from the counter order. > + * Update the lbr_events with the enabled order. > + */ > +static void intel_pmu_lbr_event_reorder(struct cpu_hw_events *cpuc, > + struct perf_event *event) > +{ > + int i, j, pos = 0, enabled[X86_PMC_IDX_MAX]; > + struct perf_event *leader, *sibling; > + > + leader = event->group_leader; > + if (branch_sample_counters(leader)) > + enabled[pos++] = leader->hw.idx; > + > + for_each_sibling_event(sibling, leader) { > + if (!branch_sample_counters(sibling)) > + continue; > + enabled[pos++] = sibling->hw.idx; > + }
Ok, so far so good: enabled[x] = y, is a mapping of hardware index (y) to group order (x).
Although I would perhaps name that order[] instead of enabled[].
> + > + if (!pos) > + return;
How would we ever get here if this is the case?
> + > + for (i = 0; i < cpuc->lbr_stack.nr; i++) { > + for (j = 0; j < pos; j++) > + intel_pmu_update_lbr_event(&cpuc->lbr_events[i], enabled[j], j);
But this confuses me... per that function it:
- extracts counter value for enabled[j] and, - or's it into the same variable at j
But what if j is already taken by something else?
That is, suppose enabled[] = {3,2,1,0}, and lbr_events = 11 10 01 00
Then: for (j) intel_pmu_update_lbt_event(&lbr_event, enabled[j], j);
0: 3->0, 11 10 01 00 -> 11 10 01 11 1: 2->1, 11 10 01 11 -> 11 10 11 11 2: 1->2, 11 10 11 11 -> 11 11 11 11
> + > + /* Clear the original counter order */ > + cpuc->lbr_events[i] &= ~LBR_INFO_EVENTS; > + } > +}
Would not something like:
src = lbr_events[i]; dst = 0; for (j = 0; j < pos; j++) { cnt = (src >> enabled[j]*2) & 3; dst |= cnt << j*2 } lbr_events[i] = dst;
be *FAR* clearer, and actually work?
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