Re: [PATCH v1 4/6] perf threads: Move threads to its own files

[Ian Rogers]

On Thu, Feb 29, 2024 at 1:59 PM David Laight <David.Laight@xxxxxxxxxx> wrote:
>
> From: Ian Rogers
> > Sent: 27 February 2024 07:24
> >
> > On Mon, Feb 26, 2024 at 11:07 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
> > >
> > > On Tue, Feb 13, 2024 at 10:37 PM Ian Rogers <irogers@xxxxxxxxxx> wrote:
> > > >
> > > > Move threads out of machine and move thread_rb_node into the 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.
> > >
> > > Doesn't look like a simple code move.  Can we split the locking
> > > change from the move to make the reviewer's life a bit easier? :)
> >
> > Not sure I follow. Take threads_nr as an example.
> >
> > The old code is in machine.c, so:
> > -static size_t machine__threads_nr(const struct machine *machine)
> > -{
> > -       size_t nr = 0;
> > -
> > -       for (int i = 0; i < THREADS__TABLE_SIZE; i++)
> > -               nr += machine->threads[i].nr;
> > -
> > -       return nr;
> > -}
> >
> > The new code is in threads.c:
> > +size_t threads__nr(struct threads *threads)
> > +{
> > +       size_t nr = 0;
> > +
> > +       for (int i = 0; i < THREADS__TABLE_SIZE; i++) {
> > +               struct threads_table_entry *table = &threads->table[i];
> > +
> > +               down_read(&table->lock);
> > +               nr += table->nr;
> > +               up_read(&table->lock);
> > +       }
> > +       return nr;
> > +}
> >
> > So it is a copy paste from one file to the other. The only difference
> > is that the old code failed to take a lock when reading "nr" so the
> > locking is added. I wanted to make sure all the functions in threads.c
> > were properly correct wrt locking, semaphore creation and destruction,
> > etc.  We could have a broken threads.c and fix it in the next change,
> > but given that's a bug it could make bisection more difficult.
> > Ultimately I thought the locking changes were small enough to not
> > warrant being on their own compared to the advantages of having a sane
> > threads abstraction.
>
> The lock is pretty much entirely pointless.
> All it really does is slow the code down.
> The most you could want is:
>         nr += READ_ONCE(table->nr);
> to avoid any hypothetical data tearing.

Completely agreed, but as this is user space code I'm unclear on
thread sanitizer support for READ_ONCE. The code is also only called
in debug statements. The migration to a hashmap in the later patches
means the complexity of taking the lock is more justified, although
we're using libbpf's hashmap that has a size variable.

Thanks,
Ian

>         David
>
> -
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