Re: [PATCH v1 1/6] perf report: Sort child tasks by tid

From: Ian Rogers
Date: Tue Feb 27 2024 - 02:12:42 EST

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On Mon, Feb 26, 2024 at 10:39 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>
> On Tue, Feb 13, 2024 at 10:37 PM Ian Rogers <irogers@xxxxxxxxxx> wrote:
> >
> > Commit 91e467bc568f ("perf machine: Use hashtable for machine
> > threads") made the iteration of thread tids unordered. The perf report
> > --tasks output now shows child threads in an order determined by the
> > hashing. For example, in this snippet tid 3 appears after tid 256 even
> > though they have the same ppid 2:
> >
> > ```
> > $ perf report --tasks
> > % pid tid ppid comm
> > 0 0 -1 |swapper
> > 2 2 0 | kthreadd
> > 256 256 2 | kworker/12:1H-k
> > 693761 693761 2 | kworker/10:1-mm
> > 1301762 1301762 2 | kworker/1:1-mm_
> > 1302530 1302530 2 | kworker/u32:0-k
> > 3 3 2 | rcu_gp
> > ...
> > ```
> >
> > The output is easier to read if threads appear numerically
> > increasing. To allow for this, read all threads into a list then sort
> > with a comparator that orders by the child task's of the first common
> > parent. The list creation and deletion are created as utilities on
> > machine. The indentation is possible by counting the number of
> > parents a child has.
> >
> > With this change the output for the same data file is now like:
> > ```
> > $ perf report --tasks
> > % pid tid ppid comm
> > 0 0 -1 |swapper
> > 1 1 0 | systemd
> > 823 823 1 | systemd-journal
> > 853 853 1 | systemd-udevd
> > 3230 3230 1 | systemd-timesyn
> > 3236 3236 1 | auditd
> > 3239 3239 3236 | audisp-syslog
> > 3321 3321 1 | accounts-daemon
> > ...
> > ```
> >
> > Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
> > ---
> > tools/perf/builtin-report.c | 203 ++++++++++++++++++++----------------
> > tools/perf/util/machine.c | 30 ++++++
> > tools/perf/util/machine.h | 10 ++
> > 3 files changed, 155 insertions(+), 88 deletions(-)
> >
> > diff --git a/tools/perf/builtin-report.c b/tools/perf/builtin-report.c
> > index 8e16fa261e6f..b48f1d5309e3 100644
> > --- a/tools/perf/builtin-report.c
> > +++ b/tools/perf/builtin-report.c
> > @@ -59,6 +59,7 @@
> > #include <linux/ctype.h>
> > #include <signal.h>
> > #include <linux/bitmap.h>
> > +#include <linux/list_sort.h>
> > #include <linux/string.h>
> > #include <linux/stringify.h>
> > #include <linux/time64.h>
> > @@ -828,35 +829,6 @@ static void tasks_setup(struct report *rep)
> > rep->tool.no_warn = true;
> > }
> >
> > -struct task {
> > - struct thread *thread;
> > - struct list_head list;
> > - struct list_head children;
> > -};
> > -
> > -static struct task *tasks_list(struct task *task, struct machine *machine)
> > -{
> > - struct thread *parent_thread, *thread = task->thread;
> > - struct task *parent_task;
> > -
> > - /* Already listed. */
> > - if (!list_empty(&task->list))
> > - return NULL;
> > -
> > - /* Last one in the chain. */
> > - if (thread__ppid(thread) == -1)
> > - return task;
> > -
> > - parent_thread = machine__find_thread(machine, -1, thread__ppid(thread));
> > - if (!parent_thread)
> > - return ERR_PTR(-ENOENT);
> > -
> > - parent_task = thread__priv(parent_thread);
> > - thread__put(parent_thread);
> > - list_add_tail(&task->list, &parent_task->children);
> > - return tasks_list(parent_task, machine);
> > -}
> > -
> > struct maps__fprintf_task_args {
> > int indent;
> > FILE *fp;
> > @@ -900,89 +872,144 @@ static size_t maps__fprintf_task(struct maps *maps, int indent, FILE *fp)
> > return args.printed;
> > }
> >
> > -static void task__print_level(struct task *task, FILE *fp, int level)
> > +static int thread_level(struct machine *machine, const struct thread *thread)
> > {
> > - struct thread *thread = task->thread;
> > - struct task *child;
> > - int comm_indent = fprintf(fp, " %8d %8d %8d |%*s",
> > - thread__pid(thread), thread__tid(thread),
> > - thread__ppid(thread), level, "");
> > + struct thread *parent_thread;
> > + int res;
> >
> > - fprintf(fp, "%s\n", thread__comm_str(thread));
> > + if (thread__tid(thread) <= 0)
> > + return 0;
> >
> > - maps__fprintf_task(thread__maps(thread), comm_indent, fp);
> > + if (thread__ppid(thread) <= 0)
> > + return 1;
> >
> > - if (!list_empty(&task->children)) {
> > - list_for_each_entry(child, &task->children, list)
> > - task__print_level(child, fp, level + 1);
> > + parent_thread = machine__find_thread(machine, -1, thread__ppid(thread));
> > + if (!parent_thread) {
> > + pr_err("Missing parent thread of %d\n", thread__tid(thread));
> > + return 0;
> > }
> > + res = 1 + thread_level(machine, parent_thread);
> > + thread__put(parent_thread);
> > + return res;
> > }
> >
> > -static int tasks_print(struct report *rep, FILE *fp)
> > +static void task__print_level(struct machine *machine, struct thread *thread, FILE *fp)
> > {
> > - struct perf_session *session = rep->session;
> > - struct machine *machine = &session->machines.host;
> > - struct task *tasks, *task;
> > - unsigned int nr = 0, itask = 0, i;
> > - struct rb_node *nd;
> > - LIST_HEAD(list);
> > + int level = thread_level(machine, thread);
> > + int comm_indent = fprintf(fp, " %8d %8d %8d |%*s",
> > + thread__pid(thread), thread__tid(thread),
> > + thread__ppid(thread), level, "");
> >
> > - /*
> > - * No locking needed while accessing machine->threads,
> > - * because --tasks is single threaded command.
> > - */
> > + fprintf(fp, "%s\n", thread__comm_str(thread));
> >
> > - /* Count all the threads. */
> > - for (i = 0; i < THREADS__TABLE_SIZE; i++)
> > - nr += machine->threads[i].nr;
> > + maps__fprintf_task(thread__maps(thread), comm_indent, fp);
> > +}
> >
> > - tasks = malloc(sizeof(*tasks) * nr);
> > - if (!tasks)
> > - return -ENOMEM;
> > +static int task_list_cmp(void *priv, const struct list_head *la, const struct list_head *lb)
>
> I'm a little afraid that this comparison logic becomes complex.
> But I think it's better than having a tree of thread relationship.
> Just a comment that explains why we need this would be nice.

I can add something in v2.

>
> > +{
> > + struct machine *machine = priv;
> > + struct thread_list *task_a = list_entry(la, struct thread_list, list);
> > + struct thread_list *task_b = list_entry(lb, struct thread_list, list);
> > + struct thread *a = task_a->thread;
> > + struct thread *b = task_b->thread;
> > + int level_a, level_b, res;
> > +
> > + /* Compare a and b to root. */
> > + if (thread__tid(a) == thread__tid(b))
> > + return 0;
> >
> > - for (i = 0; i < THREADS__TABLE_SIZE; i++) {
> > - struct threads *threads = &machine->threads[i];
> > + if (thread__tid(a) == 0)
> > + return -1;
> >
> > - for (nd = rb_first_cached(&threads->entries); nd;
> > - nd = rb_next(nd)) {
> > - task = tasks + itask++;
> > + if (thread__tid(b) == 0)
> > + return 1;
> >
> > - task->thread = rb_entry(nd, struct thread_rb_node, rb_node)->thread;
> > - INIT_LIST_HEAD(&task->children);
> > - INIT_LIST_HEAD(&task->list);
> > - thread__set_priv(task->thread, task);
> > - }
> > + /* If parents match sort by tid. */
> > + if (thread__ppid(a) == thread__ppid(b)) {
> > + return thread__tid(a) < thread__tid(b)
> > + ? -1
> > + : (thread__tid(a) > thread__tid(b) ? 1 : 0);
>
> Can it be simply like this? We know tid(a) != tid(b).
>
> return thread__tid(a) < thread__tid(b) ? -1 : 1;

Yes, but the parent check is still required.

> > }
> >
> > /*
> > - * Iterate every task down to the unprocessed parent
> > - * and link all in task children list. Task with no
> > - * parent is added into 'list'.
> > + * Find a and b such that if they are a child of each other a and b's
> > + * tid's match, otherwise a and b have a common parent and distinct
> > + * tid's to sort by. First make the depths of the threads match.
> > */
> > - for (itask = 0; itask < nr; itask++) {
> > - task = tasks + itask;
> > -
> > - if (!list_empty(&task->list))
> > - continue;
> > -
> > - task = tasks_list(task, machine);
> > - if (IS_ERR(task)) {
> > - pr_err("Error: failed to process tasks\n");
> > - free(tasks);
> > - return PTR_ERR(task);
> > + level_a = thread_level(machine, a);
> > + level_b = thread_level(machine, b);
> > + a = thread__get(a);
> > + b = thread__get(b);
> > + for (int i = level_a; i > level_b; i--) {
> > + struct thread *parent = machine__find_thread(machine, -1, thread__ppid(a));
> > +
> > + thread__put(a);
> > + if (!parent) {
> > + pr_err("Missing parent thread of %d\n", thread__tid(a));
> > + thread__put(b);
> > + return -1;
> > }
> > + a = parent;
> > + }
> > + for (int i = level_b; i > level_a; i--) {
> > + struct thread *parent = machine__find_thread(machine, -1, thread__ppid(b));
> >
> > - if (task)
> > - list_add_tail(&task->list, &list);
> > + thread__put(b);
> > + if (!parent) {
> > + pr_err("Missing parent thread of %d\n", thread__tid(b));
> > + thread__put(a);
> > + return 1;
> > + }
> > + b = parent;
> > + }
> > + /* Search up to a common parent. */
> > + while (thread__ppid(a) != thread__ppid(b)) {
> > + struct thread *parent;
> > +
> > + parent = machine__find_thread(machine, -1, thread__ppid(a));
> > + thread__put(a);
> > + if (!parent)
> > + pr_err("Missing parent thread of %d\n", thread__tid(a));
> > + a = parent;
> > + parent = machine__find_thread(machine, -1, thread__ppid(b));
> > + thread__put(b);
> > + if (!parent)
> > + pr_err("Missing parent thread of %d\n", thread__tid(b));
> > + b = parent;
> > + if (!a || !b)
> > + return !a && !b ? 0 : (!a ? -1 : 1);
>
> Wouldn't it leak a refcount if either a or b is NULL (not both)?

It would, but this would be an error condition anyway. I can add puts.

>
> > + }
> > + if (thread__tid(a) == thread__tid(b)) {
> > + /* a is a child of b or vice-versa, deeper levels appear later. */
> > + res = level_a < level_b ? -1 : (level_a > level_b ? 1 : 0);
> > + } else {
> > + /* Sort by tid now the parent is the same. */
> > + res = thread__tid(a) < thread__tid(b) ? -1 : 1;
> > }
> > + thread__put(a);
> > + thread__put(b);
> > + return res;
> > +}
> > +
> > +static int tasks_print(struct report *rep, FILE *fp)
> > +{
> > + struct machine *machine = &rep->session->machines.host;
> > + LIST_HEAD(tasks);
> > + int ret;
> >
> > - fprintf(fp, "# %8s %8s %8s %s\n", "pid", "tid", "ppid", "comm");
> > + ret = machine__thread_list(machine, &tasks);
> > + if (!ret) {
> > + struct thread_list *task;
>
> Do we really need this thread_list? Why not use an
> array of threads directly?

The code isn't particularly performance critical. I used a list as it
best approximated how the rbtree was being used. The code is reused in
subsequent patches, there's no initial pass to size an array and I
think the reallocarray/qsort logic is generally more problematic than
the list ones. If we were worried about performance then I think
arrays could make sense for optimization, but I think this is good
enough for now.

Thanks,
Ian

> Thanks,
> Namhyung
>
> >
> > - list_for_each_entry(task, &list, list)
> > - task__print_level(task, fp, 0);
> > + list_sort(machine, &tasks, task_list_cmp);
> >
> > - free(tasks);
> > - return 0;
> > + fprintf(fp, "# %8s %8s %8s %s\n", "pid", "tid", "ppid", "comm");
> > +
> > + list_for_each_entry(task, &tasks, list)
> > + task__print_level(machine, task->thread, fp);
> > + }
> > + thread_list__delete(&tasks);
> > + return ret;
> > }
> >
> > static int __cmd_report(struct report *rep)
> > diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
> > index 3da92f18814a..7872ce92c9fc 100644
> > --- a/tools/perf/util/machine.c
> > +++ b/tools/perf/util/machine.c
> > @@ -3261,6 +3261,36 @@ int machines__for_each_thread(struct machines *machines,
> > return rc;
> > }
> >
> > +
> > +static int thread_list_cb(struct thread *thread, void *data)
> > +{
> > + struct list_head *list = data;
> > + struct thread_list *entry = malloc(sizeof(*entry));
> > +
> > + if (!entry)
> > + return -ENOMEM;
> > +
> > + entry->thread = thread__get(thread);
> > + list_add_tail(&entry->list, list);
> > + return 0;
> > +}
> > +
> > +int machine__thread_list(struct machine *machine, struct list_head *list)
> > +{
> > + return machine__for_each_thread(machine, thread_list_cb, list);
> > +}
> > +
> > +void thread_list__delete(struct list_head *list)
> > +{
> > + struct thread_list *pos, *next;
> > +
> > + list_for_each_entry_safe(pos, next, list, list) {
> > + thread__zput(pos->thread);
> > + list_del(&pos->list);
> > + free(pos);
> > + }
> > +}
> > +
> > pid_t machine__get_current_tid(struct machine *machine, int cpu)
> > {
> > if (cpu < 0 || (size_t)cpu >= machine->current_tid_sz)
> > diff --git a/tools/perf/util/machine.h b/tools/perf/util/machine.h
> > index 1279acda6a8a..b738ce84817b 100644
> > --- a/tools/perf/util/machine.h
> > +++ b/tools/perf/util/machine.h
> > @@ -280,6 +280,16 @@ int machines__for_each_thread(struct machines *machines,
> > int (*fn)(struct thread *thread, void *p),
> > void *priv);
> >
> > +struct thread_list {
> > + struct list_head list;
> > + struct thread *thread;
> > +};
> > +
> > +/* Make a list of struct thread_list based on threads in the machine. */
> > +int machine__thread_list(struct machine *machine, struct list_head *list);
> > +/* Free up the nodes within the thread_list list. */
> > +void thread_list__delete(struct list_head *list);
> > +
> > pid_t machine__get_current_tid(struct machine *machine, int cpu);
> > int machine__set_current_tid(struct machine *machine, int cpu, pid_t pid,
> > pid_t tid);
> > --
> > 2.43.0.687.g38aa6559b0-goog
> >

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