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Message-ID: <87fvk6ji10.fsf@sejong.aot.lge.com>
Date: Mon, 19 May 2014 17:21:15 +0900
From: Namhyung Kim <namhyung@...il.com>
To: Andi Kleen <andi@...stfloor.org>
Cc: jolsa@...hat.com, linux-kernel@...r.kernel.org, acme@...radead.org,
Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 1/9] perf, tools: Support handling complete branch stacks as histograms v6
Hi Andi,
On Fri, 16 May 2014 10:05:30 -0700, Andi Kleen wrote:
> From: Andi Kleen <ak@...ux.intel.com>
>
> Currently branch stacks can be only shown as edge histograms for
> individual branches. I never found this display particularly useful.
>
> This implements an alternative mode that creates histograms over complete
> branch traces, instead of individual branches, similar to how normal
> callgraphs are handled. This is done by putting it in
> front of the normal callgraph and then using the normal callgraph
> histogram infrastructure to unify them.
>
> This way in complex functions we can understand the control flow
> that lead to a particular sample, and may even see some control
> flow in the caller for short functions.
[SNIP]
> +static int add_callchain_ip(struct machine *machine,
> + struct thread *thread,
> + struct symbol **parent,
> + struct addr_location *root_al,
> + int cpumode,
> + u64 ip)
> +{
> + struct addr_location al;
> +
> + al.filtered = 0;
> + al.sym = NULL;
> + thread__find_addr_location(thread, machine, cpumode, MAP__FUNCTION,
> + ip, &al);
> + if (al.sym != NULL) {
> + if (sort__has_parent && !*parent &&
> + symbol__match_regex(al.sym, &parent_regex))
> + *parent = al.sym;
> + else if (have_ignore_callees && root_al &&
> + symbol__match_regex(al.sym, &ignore_callees_regex)) {
> + /* Treat this symbol as the root,
> + forgetting its callees. */
> + *root_al = al;
> + callchain_cursor_reset(&callchain_cursor);
> + }
> + if (!symbol_conf.use_callchain)
> + return -EINVAL;
This is gone with 540476de74c9 ("perf tools: Remove
symbol_conf.use_callchain check").
> + }
> +
> + return callchain_cursor_append(&callchain_cursor, ip, al.map, al.sym);
> +}
> +
> +#define CHASHSZ 127
> +#define CHASHBITS 7
> +#define NO_ENTRY 0xff
> +
> +#define PERF_MAX_BRANCH_DEPTH 127
> +
> +/* Remove loops. */
> +static int remove_loops(struct branch_entry *l, int nr)
> +{
> + int i, j, off;
> + unsigned char chash[CHASHSZ];
> + memset(chash, -1, sizeof(chash));
s/-1/NO_ENTRY/ ?
> +
> + BUG_ON(nr >= 256);
> + for (i = 0; i < nr; i++) {
> + int h = hash_64(l[i].from, CHASHBITS) % CHASHSZ;
> +
> + /* no collision handling for now */
> + if (chash[h] == NO_ENTRY) {
> + chash[h] = i;
> + } else if (l[chash[h]].from == l[i].from) {
> + bool is_loop = true;
> + /* check if it is a real loop */
> + off = 0;
> + for (j = chash[h]; j < i && i + off < nr; j++, off++)
> + if (l[j].from != l[i + off].from) {
> + is_loop = false;
> + break;
> + }
> + if (is_loop) {
> + memmove(l + i, l + i + off,
> + (nr - (i + off))
> + * sizeof(struct branch_entry));
> + nr -= off;
> + }
> + }
> + }
> + return nr;
> +}
> +
> static int machine__resolve_callchain_sample(struct machine *machine,
> struct thread *thread,
> struct ip_callchain *chain,
> + struct branch_stack *branch,
> struct symbol **parent,
> struct addr_location *root_al,
> int max_stack)
> @@ -1290,17 +1363,73 @@ static int machine__resolve_callchain_sample(struct machine *machine,
> int chain_nr = min(max_stack, (int)chain->nr);
> int i;
> int err;
> + int first_call = 0;
>
> callchain_cursor_reset(&callchain_cursor);
>
> + /*
> + * Add branches to call stack for easier browsing. This gives
> + * more context for a sample than just the callers.
> + *
> + * This uses individual histograms of paths compared to the
> + * aggregated histograms the normal LBR mode uses.
> + *
> + * Limitations for now:
> + * - No extra filters
> + * - No annotations (should annotate somehow)
> + */
> +
> + if (branch->nr > PERF_MAX_BRANCH_DEPTH) {
> + pr_warning("corrupted branch chain. skipping...\n");
> + return 0;
> + }
> +
> + if (callchain_param.branch_callstack) {
> + int nr = min(max_stack, (int)branch->nr);
> + struct branch_entry be[nr];
> +
> + for (i = 0; i < nr; i++) {
> + if (callchain_param.order == ORDER_CALLEE) {
> + be[i] = branch->entries[i];
> + /*
> + * Check for overlap into the callchain.
> + * The return address is one off compared to
> + * the branch entry. To adjust for this
> + * assume the calling instruction is not longer
> + * than 8 bytes.
> + */
> + if (be[i].from < chain->ips[first_call] &&
> + be[i].from >= chain->ips[first_call] - 8)
> + first_call++;
It seems that you need to check chain->ips[first_call] is greater than
PERF_CONTEXT_MAX and use such value as the cpumode...
> + } else
> + be[i] = branch->entries[branch->nr - i - 1];
> + }
> +
> + nr = remove_loops(be, nr);
> +
> + for (i = 0; i < nr; i++) {
> + err = add_callchain_ip(machine, thread, parent,
> + root_al,
> + -1, be[i].to);
> + if (!err)
> + err = add_callchain_ip(machine, thread,
> + parent, root_al,
> + -1, be[i].from);
... for here.
> + if (err == -EINVAL)
> + break;
> + if (err)
> + return err;
> + }
> + chain_nr -= nr;
It seems it could make some callchain nodes being ignored. What if a
case like small callchains with matches to only 2 nodes in the LBR?
nr = 16, chain_nr = 10 and first_call = 2
Thanks,
Namhyung
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