linux-kernel - Re: [PATCH v4 6/6] drm/drm-file: Show finer-grained BO sizes in drm_show_memory

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Date:   Tue, 12 Sep 2023 19:14:35 -0700
From:   Rob Clark <robdclark@...il.com>
To:     Boris Brezillon <boris.brezillon@...labora.com>
Cc:     Adrián Larumbe <adrian.larumbe@...labora.com>,
        maarten.lankhorst@...ux.intel.com, mripard@...nel.org,
        tzimmermann@...e.de, airlied@...il.com, daniel@...ll.ch,
        quic_abhinavk@...cinc.com, dmitry.baryshkov@...aro.org,
        sean@...rly.run, marijn.suijten@...ainline.org, robh@...nel.org,
        steven.price@....com, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        healych@...zon.com, kernel@...labora.com,
        freedreno@...ts.freedesktop.org,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>
Subject: Re: [PATCH v4 6/6] drm/drm-file: Show finer-grained BO sizes in drm_show_memory_stats

On Tue, Sep 12, 2023 at 6:46 PM Rob Clark <robdclark@...il.com> wrote:
>
> On Tue, Sep 12, 2023 at 2:32 AM Boris Brezillon
> <boris.brezillon@...labora.com> wrote:
> >
> > On Tue, 12 Sep 2023 09:37:00 +0100
> > Adrián Larumbe <adrian.larumbe@...labora.com> wrote:
> >
> > > The current implementation will try to pick the highest available size
> > > display unit as soon as the BO size exceeds that of the previous
> > > multiplier. That can lead to loss of precision in BO's whose size is
> > > not a multiple of a MiB.
> > >
> > > Fix it by changing the unit selection criteria.
> > >
> > > For much bigger BO's, their size will naturally be aligned on something
> > > bigger than a 4 KiB page, so in practice it is very unlikely their display
> > > unit would default to KiB.
> >
> > Let's wait for Rob's opinion on this.
>
> This would mean that if you have SZ_1G + SZ_1K worth of buffers, you'd
> report the result in KiB.. which seems like overkill to me, esp given
> that the result is just a snapshot in time of a figure that
> realistically is dynamic.
>
> Maybe if you have SZ_1G+SZ_1K worth of buffers you should report the
> result with more precision than GiB, but more than MiB seems a bit
> overkill.
>
> BR,
> -R
>
> > >
> > > Signed-off-by: Adrián Larumbe <adrian.larumbe@...labora.com>
> > > ---
> > >  drivers/gpu/drm/drm_file.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/gpu/drm/drm_file.c b/drivers/gpu/drm/drm_file.c
> > > index 762965e3d503..bf7d2fe46bfa 100644
> > > --- a/drivers/gpu/drm/drm_file.c
> > > +++ b/drivers/gpu/drm/drm_file.c
> > > @@ -879,7 +879,7 @@ static void print_size(struct drm_printer *p, const char *stat,
> > >       unsigned u;
> > >
> > >       for (u = 0; u < ARRAY_SIZE(units) - 1; u++) {
> > > -             if (sz < SZ_1K)

btw, I was thinking more along the lines of:

   if (sz < 10*SZ_1K)

(or perhaps maybe 100*SZ_1K)

I mean, any visualization tool is going to scale the y axis based on
the order of magnitude.. and if I'm looking at the fdinfo with my
eyeballs I don't want to count the # of digits manually to do the
conversion in my head.  The difference btwn 4 or 5 or maybe 6 digits
is easy enough to eyeball, but more than that is too much for my
eyesight, and I'm not seeing how it is useful ;-)

But if someone really has a valid use case for having precision in 1KB
then I'm willing to be overruled.  But I'm not a fan of the earlier
approach of different drivers reporting results differently, the whole
point of fdinfo was to have some standardized reporting.

BR,
-R

> > > +             if (sz & (SZ_1K - 1))
> > >                       break;
> > >               sz = div_u64(sz, SZ_1K);
> > >       }
> >