perf hists: Resort hist entries using group members for output

[deliverable/linux.git] / tools / perf / util / hist.c

#include "annotate.h"
#include "util.h"
#include "build-id.h"
#include "hist.h"
#include "session.h"
#include "sort.h"
#include "evsel.h"
#include <math.h>

static bool hists__filter_entry_by_dso(struct hists *hists,
                                       struct hist_entry *he);
static bool hists__filter_entry_by_thread(struct hists *hists,
                                          struct hist_entry *he);
static bool hists__filter_entry_by_symbol(struct hists *hists,
                                          struct hist_entry *he);

enum hist_filter {
        HIST_FILTER__DSO,
        HIST_FILTER__THREAD,
        HIST_FILTER__PARENT,
        HIST_FILTER__SYMBOL,
};

struct callchain_param  callchain_param = {
        .mode   = CHAIN_GRAPH_REL,
        .min_percent = 0.5,
        .order  = ORDER_CALLEE
};

u16 hists__col_len(struct hists *hists, enum hist_column col)
{
        return hists->col_len[col];
}

void hists__set_col_len(struct hists *hists, enum hist_column col, u16 len)
{
        hists->col_len[col] = len;
}

bool hists__new_col_len(struct hists *hists, enum hist_column col, u16 len)
{
        if (len > hists__col_len(hists, col)) {
                hists__set_col_len(hists, col, len);
                return true;
        }
        return false;
}

void hists__reset_col_len(struct hists *hists)
{
        enum hist_column col;

        for (col = 0; col < HISTC_NR_COLS; ++col)
                hists__set_col_len(hists, col, 0);
}

static void hists__set_unres_dso_col_len(struct hists *hists, int dso)
{
        const unsigned int unresolved_col_width = BITS_PER_LONG / 4;

        if (hists__col_len(hists, dso) < unresolved_col_width &&
            !symbol_conf.col_width_list_str && !symbol_conf.field_sep &&
            !symbol_conf.dso_list)
                hists__set_col_len(hists, dso, unresolved_col_width);
}

void hists__calc_col_len(struct hists *hists, struct hist_entry *h)
{
        const unsigned int unresolved_col_width = BITS_PER_LONG / 4;
        u16 len;

        if (h->ms.sym)
                hists__new_col_len(hists, HISTC_SYMBOL, h->ms.sym->namelen + 4);
        else
                hists__set_unres_dso_col_len(hists, HISTC_DSO);

        len = thread__comm_len(h->thread);
        if (hists__new_col_len(hists, HISTC_COMM, len))
                hists__set_col_len(hists, HISTC_THREAD, len + 6);

        if (h->ms.map) {
                len = dso__name_len(h->ms.map->dso);
                hists__new_col_len(hists, HISTC_DSO, len);
        }

        if (h->parent)
                hists__new_col_len(hists, HISTC_PARENT, h->parent->namelen);

        if (h->branch_info) {
                int symlen;
                /*
                 * +4 accounts for '[x] ' priv level info
                 * +2 account of 0x prefix on raw addresses
                 */
                if (h->branch_info->from.sym) {
                        symlen = (int)h->branch_info->from.sym->namelen + 4;
                        hists__new_col_len(hists, HISTC_SYMBOL_FROM, symlen);

                        symlen = dso__name_len(h->branch_info->from.map->dso);
                        hists__new_col_len(hists, HISTC_DSO_FROM, symlen);
                } else {
                        symlen = unresolved_col_width + 4 + 2;
                        hists__new_col_len(hists, HISTC_SYMBOL_FROM, symlen);
                        hists__set_unres_dso_col_len(hists, HISTC_DSO_FROM);
                }

                if (h->branch_info->to.sym) {
                        symlen = (int)h->branch_info->to.sym->namelen + 4;
                        hists__new_col_len(hists, HISTC_SYMBOL_TO, symlen);

                        symlen = dso__name_len(h->branch_info->to.map->dso);
                        hists__new_col_len(hists, HISTC_DSO_TO, symlen);
                } else {
                        symlen = unresolved_col_width + 4 + 2;
                        hists__new_col_len(hists, HISTC_SYMBOL_TO, symlen);
                        hists__set_unres_dso_col_len(hists, HISTC_DSO_TO);
                }
        }
}

void hists__output_recalc_col_len(struct hists *hists, int max_rows)
{
        struct rb_node *next = rb_first(&hists->entries);
        struct hist_entry *n;
        int row = 0;

        hists__reset_col_len(hists);

        while (next && row++ < max_rows) {
                n = rb_entry(next, struct hist_entry, rb_node);
                if (!n->filtered)
                        hists__calc_col_len(hists, n);
                next = rb_next(&n->rb_node);
        }
}

static void hist_entry__add_cpumode_period(struct hist_entry *he,
                                           unsigned int cpumode, u64 period)
{
        switch (cpumode) {
        case PERF_RECORD_MISC_KERNEL:
                he->stat.period_sys += period;
                break;
        case PERF_RECORD_MISC_USER:
                he->stat.period_us += period;
                break;
        case PERF_RECORD_MISC_GUEST_KERNEL:
                he->stat.period_guest_sys += period;
                break;
        case PERF_RECORD_MISC_GUEST_USER:
                he->stat.period_guest_us += period;
                break;
        default:
                break;
        }
}

static void he_stat__add_period(struct he_stat *he_stat, u64 period)
{
        he_stat->period         += period;
        he_stat->nr_events      += 1;
}

static void he_stat__add_stat(struct he_stat *dest, struct he_stat *src)
{
        dest->period            += src->period;
        dest->period_sys        += src->period_sys;
        dest->period_us         += src->period_us;
        dest->period_guest_sys  += src->period_guest_sys;
        dest->period_guest_us   += src->period_guest_us;
        dest->nr_events         += src->nr_events;
}

static void hist_entry__decay(struct hist_entry *he)
{
        he->stat.period = (he->stat.period * 7) / 8;
        he->stat.nr_events = (he->stat.nr_events * 7) / 8;
}

static bool hists__decay_entry(struct hists *hists, struct hist_entry *he)
{
        u64 prev_period = he->stat.period;

        if (prev_period == 0)
                return true;

        hist_entry__decay(he);

        if (!he->filtered)
                hists->stats.total_period -= prev_period - he->stat.period;

        return he->stat.period == 0;
}

static void __hists__decay_entries(struct hists *hists, bool zap_user,
                                   bool zap_kernel, bool threaded)
{
        struct rb_node *next = rb_first(&hists->entries);
        struct hist_entry *n;

        while (next) {
                n = rb_entry(next, struct hist_entry, rb_node);
                next = rb_next(&n->rb_node);
                /*
                 * We may be annotating this, for instance, so keep it here in
                 * case some it gets new samples, we'll eventually free it when
                 * the user stops browsing and it agains gets fully decayed.
                 */
                if (((zap_user && n->level == '.') ||
                     (zap_kernel && n->level != '.') ||
                     hists__decay_entry(hists, n)) &&
                    !n->used) {
                        rb_erase(&n->rb_node, &hists->entries);

                        if (sort__need_collapse || threaded)
                                rb_erase(&n->rb_node_in, &hists->entries_collapsed);

                        hist_entry__free(n);
                        --hists->nr_entries;
                }
        }
}

void hists__decay_entries(struct hists *hists, bool zap_user, bool zap_kernel)
{
        return __hists__decay_entries(hists, zap_user, zap_kernel, false);
}

void hists__decay_entries_threaded(struct hists *hists,
                                   bool zap_user, bool zap_kernel)
{
        return __hists__decay_entries(hists, zap_user, zap_kernel, true);
}

/*
 * histogram, sorted on item, collects periods
 */

static struct hist_entry *hist_entry__new(struct hist_entry *template)
{
        size_t callchain_size = symbol_conf.use_callchain ? sizeof(struct callchain_root) : 0;
        struct hist_entry *he = malloc(sizeof(*he) + callchain_size);

        if (he != NULL) {
                *he = *template;

                if (he->ms.map)
                        he->ms.map->referenced = true;

                if (he->branch_info) {
                        if (he->branch_info->from.map)
                                he->branch_info->from.map->referenced = true;
                        if (he->branch_info->to.map)
                                he->branch_info->to.map->referenced = true;
                }

                if (symbol_conf.use_callchain)
                        callchain_init(he->callchain);

                INIT_LIST_HEAD(&he->pairs.node);
        }

        return he;
}

void hists__inc_nr_entries(struct hists *hists, struct hist_entry *h)
{
        if (!h->filtered) {
                hists__calc_col_len(hists, h);
                ++hists->nr_entries;
                hists->stats.total_period += h->stat.period;
        }
}

static u8 symbol__parent_filter(const struct symbol *parent)
{
        if (symbol_conf.exclude_other && parent == NULL)
                return 1 << HIST_FILTER__PARENT;
        return 0;
}

static struct hist_entry *add_hist_entry(struct hists *hists,
                                      struct hist_entry *entry,
                                      struct addr_location *al,
                                      u64 period)
{
        struct rb_node **p;
        struct rb_node *parent = NULL;
        struct hist_entry *he;
        int cmp;

        pthread_mutex_lock(&hists->lock);

        p = &hists->entries_in->rb_node;

        while (*p != NULL) {
                parent = *p;
                he = rb_entry(parent, struct hist_entry, rb_node_in);

                /*
                 * Make sure that it receives arguments in a same order as
                 * hist_entry__collapse() so that we can use an appropriate
                 * function when searching an entry regardless which sort
                 * keys were used.
                 */
                cmp = hist_entry__cmp(he, entry);

                if (!cmp) {
                        he_stat__add_period(&he->stat, period);

                        /* If the map of an existing hist_entry has
                         * become out-of-date due to an exec() or
                         * similar, update it.  Otherwise we will
                         * mis-adjust symbol addresses when computing
                         * the history counter to increment.
                         */
                        if (he->ms.map != entry->ms.map) {
                                he->ms.map = entry->ms.map;
                                if (he->ms.map)
                                        he->ms.map->referenced = true;
                        }
                        goto out;
                }

                if (cmp < 0)
                        p = &(*p)->rb_left;
                else
                        p = &(*p)->rb_right;
        }

        he = hist_entry__new(entry);
        if (!he)
                goto out_unlock;

        rb_link_node(&he->rb_node_in, parent, p);
        rb_insert_color(&he->rb_node_in, hists->entries_in);
out:
        hist_entry__add_cpumode_period(he, al->cpumode, period);
out_unlock:
        pthread_mutex_unlock(&hists->lock);
        return he;
}

struct hist_entry *__hists__add_branch_entry(struct hists *self,
                                             struct addr_location *al,
                                             struct symbol *sym_parent,
                                             struct branch_info *bi,
                                             u64 period)
{
        struct hist_entry entry = {
                .thread = al->thread,
                .ms = {
                        .map    = bi->to.map,
                        .sym    = bi->to.sym,
                },
                .cpu    = al->cpu,
                .ip     = bi->to.addr,
                .level  = al->level,
                .stat = {
                        .period = period,
                        .nr_events = 1,
                },
                .parent = sym_parent,
                .filtered = symbol__parent_filter(sym_parent),
                .branch_info = bi,
                .hists  = self,
        };

        return add_hist_entry(self, &entry, al, period);
}

struct hist_entry *__hists__add_entry(struct hists *self,
                                      struct addr_location *al,
                                      struct symbol *sym_parent, u64 period)
{
        struct hist_entry entry = {
                .thread = al->thread,
                .ms = {
                        .map    = al->map,
                        .sym    = al->sym,
                },
                .cpu    = al->cpu,
                .ip     = al->addr,
                .level  = al->level,
                .stat = {
                        .period = period,
                        .nr_events = 1,
                },
                .parent = sym_parent,
                .filtered = symbol__parent_filter(sym_parent),
                .hists  = self,
        };

        return add_hist_entry(self, &entry, al, period);
}

int64_t
hist_entry__cmp(struct hist_entry *left, struct hist_entry *right)
{
        struct sort_entry *se;
        int64_t cmp = 0;

        list_for_each_entry(se, &hist_entry__sort_list, list) {
                cmp = se->se_cmp(left, right);
                if (cmp)
                        break;
        }

        return cmp;
}

int64_t
hist_entry__collapse(struct hist_entry *left, struct hist_entry *right)
{
        struct sort_entry *se;
        int64_t cmp = 0;

        list_for_each_entry(se, &hist_entry__sort_list, list) {
                int64_t (*f)(struct hist_entry *, struct hist_entry *);

                f = se->se_collapse ?: se->se_cmp;

                cmp = f(left, right);
                if (cmp)
                        break;
        }

        return cmp;
}

void hist_entry__free(struct hist_entry *he)
{
        free(he->branch_info);
        free(he);
}

/*
 * collapse the histogram
 */

static bool hists__collapse_insert_entry(struct hists *hists __maybe_unused,
                                         struct rb_root *root,
                                         struct hist_entry *he)
{
        struct rb_node **p = &root->rb_node;
        struct rb_node *parent = NULL;
        struct hist_entry *iter;
        int64_t cmp;

        while (*p != NULL) {
                parent = *p;
                iter = rb_entry(parent, struct hist_entry, rb_node_in);

                cmp = hist_entry__collapse(iter, he);

                if (!cmp) {
                        he_stat__add_stat(&iter->stat, &he->stat);

                        if (symbol_conf.use_callchain) {
                                callchain_cursor_reset(&callchain_cursor);
                                callchain_merge(&callchain_cursor,
                                                iter->callchain,
                                                he->callchain);
                        }
                        hist_entry__free(he);
                        return false;
                }

                if (cmp < 0)
                        p = &(*p)->rb_left;
                else
                        p = &(*p)->rb_right;
        }

        rb_link_node(&he->rb_node_in, parent, p);
        rb_insert_color(&he->rb_node_in, root);
        return true;
}

static struct rb_root *hists__get_rotate_entries_in(struct hists *hists)
{
        struct rb_root *root;

        pthread_mutex_lock(&hists->lock);

        root = hists->entries_in;
        if (++hists->entries_in > &hists->entries_in_array[1])
                hists->entries_in = &hists->entries_in_array[0];

        pthread_mutex_unlock(&hists->lock);

        return root;
}

static void hists__apply_filters(struct hists *hists, struct hist_entry *he)
{
        hists__filter_entry_by_dso(hists, he);
        hists__filter_entry_by_thread(hists, he);
        hists__filter_entry_by_symbol(hists, he);
}

static void __hists__collapse_resort(struct hists *hists, bool threaded)
{
        struct rb_root *root;
        struct rb_node *next;
        struct hist_entry *n;

        if (!sort__need_collapse && !threaded)
                return;

        root = hists__get_rotate_entries_in(hists);
        next = rb_first(root);

        while (next) {
                n = rb_entry(next, struct hist_entry, rb_node_in);
                next = rb_next(&n->rb_node_in);

                rb_erase(&n->rb_node_in, root);
                if (hists__collapse_insert_entry(hists, &hists->entries_collapsed, n)) {
                        /*
                         * If it wasn't combined with one of the entries already
                         * collapsed, we need to apply the filters that may have
                         * been set by, say, the hist_browser.
                         */
                        hists__apply_filters(hists, n);
                }
        }
}

void hists__collapse_resort(struct hists *hists)
{
        return __hists__collapse_resort(hists, false);
}

void hists__collapse_resort_threaded(struct hists *hists)
{
        return __hists__collapse_resort(hists, true);
}

/*
 * reverse the map, sort on period.
 */

static int period_cmp(u64 period_a, u64 period_b)
{
        if (period_a > period_b)
                return 1;
        if (period_a < period_b)
                return -1;
        return 0;
}

static int hist_entry__sort_on_period(struct hist_entry *a,
                                      struct hist_entry *b)
{
        int ret;
        int i, nr_members;
        struct perf_evsel *evsel;
        struct hist_entry *pair;
        u64 *periods_a, *periods_b;

        ret = period_cmp(a->stat.period, b->stat.period);
        if (ret || !symbol_conf.event_group)
                return ret;

        evsel = hists_to_evsel(a->hists);
        nr_members = evsel->nr_members;
        if (nr_members <= 1)
                return ret;

        periods_a = zalloc(sizeof(periods_a) * nr_members);
        periods_b = zalloc(sizeof(periods_b) * nr_members);

        if (!periods_a || !periods_b)
                goto out;

        list_for_each_entry(pair, &a->pairs.head, pairs.node) {
                evsel = hists_to_evsel(pair->hists);
                periods_a[perf_evsel__group_idx(evsel)] = pair->stat.period;
        }

        list_for_each_entry(pair, &b->pairs.head, pairs.node) {
                evsel = hists_to_evsel(pair->hists);
                periods_b[perf_evsel__group_idx(evsel)] = pair->stat.period;
        }

        for (i = 1; i < nr_members; i++) {
                ret = period_cmp(periods_a[i], periods_b[i]);
                if (ret)
                        break;
        }

out:
        free(periods_a);
        free(periods_b);

        return ret;
}

static void __hists__insert_output_entry(struct rb_root *entries,
                                         struct hist_entry *he,
                                         u64 min_callchain_hits)
{
        struct rb_node **p = &entries->rb_node;
        struct rb_node *parent = NULL;
        struct hist_entry *iter;

        if (symbol_conf.use_callchain)
                callchain_param.sort(&he->sorted_chain, he->callchain,
                                      min_callchain_hits, &callchain_param);

        while (*p != NULL) {
                parent = *p;
                iter = rb_entry(parent, struct hist_entry, rb_node);

                if (hist_entry__sort_on_period(he, iter) > 0)
                        p = &(*p)->rb_left;
                else
                        p = &(*p)->rb_right;
        }

        rb_link_node(&he->rb_node, parent, p);
        rb_insert_color(&he->rb_node, entries);
}

static void __hists__output_resort(struct hists *hists, bool threaded)
{
        struct rb_root *root;
        struct rb_node *next;
        struct hist_entry *n;
        u64 min_callchain_hits;

        min_callchain_hits = hists->stats.total_period * (callchain_param.min_percent / 100);

        if (sort__need_collapse || threaded)
                root = &hists->entries_collapsed;
        else
                root = hists->entries_in;

        next = rb_first(root);
        hists->entries = RB_ROOT;

        hists->nr_entries = 0;
        hists->stats.total_period = 0;
        hists__reset_col_len(hists);

        while (next) {
                n = rb_entry(next, struct hist_entry, rb_node_in);
                next = rb_next(&n->rb_node_in);

                __hists__insert_output_entry(&hists->entries, n, min_callchain_hits);
                hists__inc_nr_entries(hists, n);
        }
}

void hists__output_resort(struct hists *hists)
{
        return __hists__output_resort(hists, false);
}

void hists__output_resort_threaded(struct hists *hists)
{
        return __hists__output_resort(hists, true);
}

static void hists__remove_entry_filter(struct hists *hists, struct hist_entry *h,
                                       enum hist_filter filter)
{
        h->filtered &= ~(1 << filter);
        if (h->filtered)
                return;

        ++hists->nr_entries;
        if (h->ms.unfolded)
                hists->nr_entries += h->nr_rows;
        h->row_offset = 0;
        hists->stats.total_period += h->stat.period;
        hists->stats.nr_events[PERF_RECORD_SAMPLE] += h->stat.nr_events;

        hists__calc_col_len(hists, h);
}


static bool hists__filter_entry_by_dso(struct hists *hists,
                                       struct hist_entry *he)
{
        if (hists->dso_filter != NULL &&
            (he->ms.map == NULL || he->ms.map->dso != hists->dso_filter)) {
                he->filtered |= (1 << HIST_FILTER__DSO);
                return true;
        }

        return false;
}

void hists__filter_by_dso(struct hists *hists)
{
        struct rb_node *nd;

        hists->nr_entries = hists->stats.total_period = 0;
        hists->stats.nr_events[PERF_RECORD_SAMPLE] = 0;
        hists__reset_col_len(hists);

        for (nd = rb_first(&hists->entries); nd; nd = rb_next(nd)) {
                struct hist_entry *h = rb_entry(nd, struct hist_entry, rb_node);

                if (symbol_conf.exclude_other && !h->parent)
                        continue;

                if (hists__filter_entry_by_dso(hists, h))
                        continue;

                hists__remove_entry_filter(hists, h, HIST_FILTER__DSO);
        }
}

static bool hists__filter_entry_by_thread(struct hists *hists,
                                          struct hist_entry *he)
{
        if (hists->thread_filter != NULL &&
            he->thread != hists->thread_filter) {
                he->filtered |= (1 << HIST_FILTER__THREAD);
                return true;
        }

        return false;
}

void hists__filter_by_thread(struct hists *hists)
{
        struct rb_node *nd;

        hists->nr_entries = hists->stats.total_period = 0;
        hists->stats.nr_events[PERF_RECORD_SAMPLE] = 0;
        hists__reset_col_len(hists);

        for (nd = rb_first(&hists->entries); nd; nd = rb_next(nd)) {
                struct hist_entry *h = rb_entry(nd, struct hist_entry, rb_node);

                if (hists__filter_entry_by_thread(hists, h))
                        continue;

                hists__remove_entry_filter(hists, h, HIST_FILTER__THREAD);
        }
}

static bool hists__filter_entry_by_symbol(struct hists *hists,
                                          struct hist_entry *he)
{
        if (hists->symbol_filter_str != NULL &&
            (!he->ms.sym || strstr(he->ms.sym->name,
                                   hists->symbol_filter_str) == NULL)) {
                he->filtered |= (1 << HIST_FILTER__SYMBOL);
                return true;
        }

        return false;
}

void hists__filter_by_symbol(struct hists *hists)
{
        struct rb_node *nd;

        hists->nr_entries = hists->stats.total_period = 0;
        hists->stats.nr_events[PERF_RECORD_SAMPLE] = 0;
        hists__reset_col_len(hists);

        for (nd = rb_first(&hists->entries); nd; nd = rb_next(nd)) {
                struct hist_entry *h = rb_entry(nd, struct hist_entry, rb_node);

                if (hists__filter_entry_by_symbol(hists, h))
                        continue;

                hists__remove_entry_filter(hists, h, HIST_FILTER__SYMBOL);
        }
}

int hist_entry__inc_addr_samples(struct hist_entry *he, int evidx, u64 ip)
{
        return symbol__inc_addr_samples(he->ms.sym, he->ms.map, evidx, ip);
}

int hist_entry__annotate(struct hist_entry *he, size_t privsize)
{
        return symbol__annotate(he->ms.sym, he->ms.map, privsize);
}

void events_stats__inc(struct events_stats *stats, u32 type)
{
        ++stats->nr_events[0];
        ++stats->nr_events[type];
}

void hists__inc_nr_events(struct hists *hists, u32 type)
{
        events_stats__inc(&hists->stats, type);
}

static struct hist_entry *hists__add_dummy_entry(struct hists *hists,
                                                 struct hist_entry *pair)
{
        struct rb_root *root;
        struct rb_node **p;
        struct rb_node *parent = NULL;
        struct hist_entry *he;
        int cmp;

        if (sort__need_collapse)
                root = &hists->entries_collapsed;
        else
                root = hists->entries_in;

        p = &root->rb_node;

        while (*p != NULL) {
                parent = *p;
                he = rb_entry(parent, struct hist_entry, rb_node_in);

                cmp = hist_entry__collapse(he, pair);

                if (!cmp)
                        goto out;

                if (cmp < 0)
                        p = &(*p)->rb_left;
                else
                        p = &(*p)->rb_right;
        }

        he = hist_entry__new(pair);
        if (he) {
                memset(&he->stat, 0, sizeof(he->stat));
                he->hists = hists;
                rb_link_node(&he->rb_node_in, parent, p);
                rb_insert_color(&he->rb_node_in, root);
                hists__inc_nr_entries(hists, he);
        }
out:
        return he;
}

static struct hist_entry *hists__find_entry(struct hists *hists,
                                            struct hist_entry *he)
{
        struct rb_node *n;

        if (sort__need_collapse)
                n = hists->entries_collapsed.rb_node;
        else
                n = hists->entries_in->rb_node;

        while (n) {
                struct hist_entry *iter = rb_entry(n, struct hist_entry, rb_node_in);
                int64_t cmp = hist_entry__collapse(iter, he);

                if (cmp < 0)
                        n = n->rb_left;
                else if (cmp > 0)
                        n = n->rb_right;
                else
                        return iter;
        }

        return NULL;
}

/*
 * Look for pairs to link to the leader buckets (hist_entries):
 */
void hists__match(struct hists *leader, struct hists *other)
{
        struct rb_root *root;
        struct rb_node *nd;
        struct hist_entry *pos, *pair;

        if (sort__need_collapse)
                root = &leader->entries_collapsed;
        else
                root = leader->entries_in;

        for (nd = rb_first(root); nd; nd = rb_next(nd)) {
                pos  = rb_entry(nd, struct hist_entry, rb_node_in);
                pair = hists__find_entry(other, pos);

                if (pair)
                        hist_entry__add_pair(pair, pos);
        }
}

/*
 * Look for entries in the other hists that are not present in the leader, if
 * we find them, just add a dummy entry on the leader hists, with period=0,
 * nr_events=0, to serve as the list header.
 */
int hists__link(struct hists *leader, struct hists *other)
{
        struct rb_root *root;
        struct rb_node *nd;
        struct hist_entry *pos, *pair;

        if (sort__need_collapse)
                root = &other->entries_collapsed;
        else
                root = other->entries_in;

        for (nd = rb_first(root); nd; nd = rb_next(nd)) {
                pos = rb_entry(nd, struct hist_entry, rb_node_in);

                if (!hist_entry__has_pairs(pos)) {
                        pair = hists__add_dummy_entry(leader, pos);
                        if (pair == NULL)
                                return -1;
                        hist_entry__add_pair(pos, pair);
                }
        }

        return 0;
}