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

[Namhyung Kim <namhyung@kernel.org>]

On Mon, Feb 26, 2024 at 11:12 PM Ian Rogers <irogers@google.com> wrote:
>
> On Mon, Feb 26, 2024 at 10:39 PM Namhyung Kim <namhyung@kernel.org> wrote:
> >
> > On Tue, Feb 13, 2024 at 10:37 PM Ian Rogers <irogers@google.com> 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@google.com>

I know you sent out v2 already, but let me continue the discussion
here.


> > > ---
> > >  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.

Sure.  I only meant the return statement.

>
> > >         }
> > >
> > >         /*
> > > -        * 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.

Well, it's not about performance.  It made me think why we need
this thread_list but I couldn't find the reason.  If you can move
machine__threads_nr() here then you won't need realloc().

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
> > >