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From: Ian Rogers <irogers@google.com>
Date: Thu, 29 Feb 2024 16:19:11 -0800
Message-ID: <CAP-5=fUGTZXQv28+v-t1mhFyWb9BuxRzCD+N4WTU0=hTqv+xLA@mail.gmail.com>
Subject: Re: [PATCH v1 4/6] perf threads: Move threads to its own files
To: David Laight <David.Laight@aculab.com>
Cc: Namhyung Kim <namhyung@kernel.org>, Peter Zijlstra <peterz@infradead.org>, 
	Ingo Molnar <mingo@redhat.com>, Arnaldo Carvalho de Melo <acme@kernel.org>, Mark Rutland <mark.rutland@arm.com>, 
	Alexander Shishkin <alexander.shishkin@linux.intel.com>, Jiri Olsa <jolsa@kernel.org>, 
	Adrian Hunter <adrian.hunter@intel.com>, Oliver Upton <oliver.upton@linux.dev>, 
	Yang Jihong <yangjihong1@huawei.com>, 
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, 
	"linux-perf-users@vger.kernel.org" <linux-perf-users@vger.kernel.org>, 
	"bpf@vger.kernel.org" <bpf@vger.kernel.org>
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On Thu, Feb 29, 2024 at 1:59=E2=80=AFPM David Laight <David.Laight@aculab.c=
om> wrote:
>
> From: Ian Rogers
> > Sent: 27 February 2024 07:24
> >
> > On Mon, Feb 26, 2024 at 11:07=E2=80=AFPM Namhyung Kim <namhyung@kernel.=
org> wrote:
> > >
> > > On Tue, Feb 13, 2024 at 10:37=E2=80=AFPM Ian Rogers <irogers@google.c=
om> 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 =3D 0;
> > -
> > -       for (int i =3D 0; i < THREADS__TABLE_SIZE; i++)
> > -               nr +=3D machine->threads[i].nr;
> > -
> > -       return nr;
> > -}
> >
> > The new code is in threads.c:
> > +size_t threads__nr(struct threads *threads)
> > +{
> > +       size_t nr =3D 0;
> > +
> > +       for (int i =3D 0; i < THREADS__TABLE_SIZE; i++) {
> > +               struct threads_table_entry *table =3D &threads->table[i=
];
> > +
> > +               down_read(&table->lock);
> > +               nr +=3D 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 +=3D 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
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1=
 1PT, UK
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