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    SubjectRe: [RFC v2] perf: Rewrite core context handling
    On Mon, Jun 13, 2022 at 04:35:11PM +0200, Peter Zijlstra wrote:

    > @@ -3652,17 +3697,28 @@ static noinline int visit_groups_merge(s
    > .size = ARRAY_SIZE(itrs),
    > };
    > /* Events not within a CPU context may be on any CPU. */
    > - __heap_add(&event_heap, perf_event_groups_first(groups, -1, NULL));
    > + __heap_add(&event_heap, perf_event_groups_first(groups, -1, pmu, NULL));
    > }
    > evt = event_heap.data;
    >
    > - __heap_add(&event_heap, perf_event_groups_first(groups, cpu, NULL));
    > + __heap_add(&event_heap, perf_event_groups_first(groups, cpu, pmu, NULL));
    >
    > #ifdef CONFIG_CGROUP_PERF
    > for (; css; css = css->parent)
    > - __heap_add(&event_heap, perf_event_groups_first(groups, cpu, css->cgroup));
    > + __heap_add(&event_heap, perf_event_groups_first(groups, cpu, pmu, css->cgroup));
    > #endif
    >
    > + if (event_heap.nr) {
    > + /*
    > + * XXX: For now, visit_groups_merge() gets called with pmu
    > + * pointer never NULL. But these functions needs to be called
    > + * once for each pmu if I implement pmu=NULL optimization.
    > + */
    > + __link_epc((*evt)->pmu_ctx);
    > + perf_assert_pmu_disabled((*evt)->pmu_ctx->pmu);
    > + }
    > +
    > +
    > min_heapify_all(&event_heap, &perf_min_heap);
    >
    > while (event_heap.nr) {

    > @@ -3741,39 +3799,67 @@ static int merge_sched_in(struct perf_ev
    > return 0;
    > }
    >
    > -static void
    > -ctx_pinned_sched_in(struct perf_event_context *ctx,
    > - struct perf_cpu_context *cpuctx)
    > +static void ctx_pinned_sched_in(struct perf_event_context *ctx, struct pmu *pmu)
    > {
    > + struct perf_event_pmu_context *pmu_ctx;
    > int can_add_hw = 1;
    >
    > - if (ctx != &cpuctx->ctx)
    > - cpuctx = NULL;
    > -
    > - visit_groups_merge(cpuctx, &ctx->pinned_groups,
    > - smp_processor_id(),
    > - merge_sched_in, &can_add_hw);
    > + if (pmu) {
    > + visit_groups_merge(ctx, &ctx->pinned_groups,
    > + smp_processor_id(), pmu,
    > + merge_sched_in, &can_add_hw);
    > + } else {
    > + /*
    > + * XXX: This can be optimized for per-task context by calling
    > + * visit_groups_merge() only once with:
    > + * 1) pmu=NULL
    > + * 2) Ignoring pmu in perf_event_groups_cmp() when it's NULL
    > + * 3) Making can_add_hw a per-pmu variable
    > + *
    > + * Though, it can not be opimized for per-cpu context because
    > + * per-cpu rb-tree consist of pmu-subtrees and pmu-subtrees
    > + * consist of cgroup-subtrees. i.e. a cgroup events of same
    > + * cgroup but different pmus are seperated out into respective
    > + * pmu-subtrees.
    > + */
    > + list_for_each_entry(pmu_ctx, &ctx->pmu_ctx_list, pmu_ctx_entry) {
    > + can_add_hw = 1;
    > + visit_groups_merge(ctx, &ctx->pinned_groups,
    > + smp_processor_id(), pmu_ctx->pmu,
    > + merge_sched_in, &can_add_hw);
    > + }
    > + }
    > }

    I'm not sure I follow.. task context can have multiple PMUs just the
    same as CPU context can, that's more or less the entire point of the
    patch.

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