linux-kernel - Re: [PATCH v3 4/7] perf machine: Move machine's threads into its own abstraction

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Date: Thu, 29 Feb 2024 21:02:45 -0800
From: Ian Rogers <irogers@...gle.com>
To: Namhyung Kim <namhyung@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, Mark Rutland <mark.rutland@....com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, 
	Adrian Hunter <adrian.hunter@...el.com>, Oliver Upton <oliver.upton@...ux.dev>, 
	Yang Jihong <yangjihong1@...wei.com>, linux-kernel@...r.kernel.org, 
	linux-perf-users@...r.kernel.org
Subject: Re: [PATCH v3 4/7] perf machine: Move machine's threads into its own abstraction

On Thu, Feb 29, 2024 at 5:36 PM Namhyung Kim <namhyung@...nel.org> wrote:
>
> On Wed, Feb 28, 2024 at 10:33 PM Ian Rogers <irogers@...gle.com> wrote:
> >
> > Move thread_rb_node into the machine.c file. This hides the
> > implementation of threads from the rest of the code allowing for it to
> > be refactored.
> >
> > Locking discipline is tightened up in this change. As the lock is now
> > encapsulated in threads, the findnew function requires holding it (as
> > it already did in machine). Rather than do conditionals with locks
> > based on whether the thread should be created (which could potentially
> > be error prone with a read lock match with a write unlock), have a
> > separate threads__find that won't create the thread and only holds the
> > read lock. This effectively duplicates the findnew logic, with the
> > existing findnew logic only operating under a write lock assuming
> > creation is necessary as a previous find failed. The creation may
> > still fail with the write lock due to another thread. The duplication
> > is removed in a later next patch that delegates the implementation to
> > hashtable.
> >
> > Signed-off-by: Ian Rogers <irogers@...gle.com>
>
> Thanks for doing this!  A nit below..
>
> > ---
> [SNIP]
> > @@ -3228,27 +3258,31 @@ int thread__resolve_callchain(struct thread *thread,
> >         return ret;
> >  }
> >
> > -int machine__for_each_thread(struct machine *machine,
> > -                            int (*fn)(struct thread *thread, void *p),
> > -                            void *priv)
> > +int threads__for_each_thread(struct threads *threads,
> > +                            int (*fn)(struct thread *thread, void *data),
> > +                            void *data)
> >  {
> > -       struct threads *threads;
> > -       struct rb_node *nd;
> > -       int rc = 0;
> > -       int i;
> > +       for (int i = 0; i < THREADS__TABLE_SIZE; i++) {
> > +               struct threads_table_entry *table = &threads->table[i];
> > +               struct rb_node *nd;
> >
> > -       for (i = 0; i < THREADS__TABLE_SIZE; i++) {
> > -               threads = &machine->threads[i];
> > -               for (nd = rb_first_cached(&threads->entries); nd;
> > -                    nd = rb_next(nd)) {
> > +               for (nd = rb_first_cached(&table->entries); nd; nd = rb_next(nd)) {
> >                         struct thread_rb_node *trb = rb_entry(nd, struct thread_rb_node, rb_node);
> > +                       int rc = fn(trb->thread, data);
> >
> > -                       rc = fn(trb->thread, priv);
> >                         if (rc != 0)
> >                                 return rc;
> >                 }
> >         }
> > -       return rc;
> > +       return 0;
>
> Don't we need locking in this function?

I thought there was a deadlock, but I was either mistaken or now it is
resolved. I'll add in the read lock as you say.

Thanks,
Ian


> Thanks,
> Namhyung
>
>
> > +
> > +}
> > +
> > +int machine__for_each_thread(struct machine *machine,
> > +                            int (*fn)(struct thread *thread, void *p),
> > +                            void *priv)
> > +{
> > +       return threads__for_each_thread(&machine->threads, fn, priv);
> >  }
> >