Re: [PATCH v9 1/5] drm: Add a sharable drm page-pool implementation

[John Stultz]

On Tue, Jul 6, 2021 at 2:15 PM Daniel Vetter <daniel@xxxxxxxx> wrote:
>
> On Tue, Jul 6, 2021 at 11:04 PM John Stultz <john.stultz@xxxxxxxxxx> wrote:
> > On Wed, Jun 30, 2021 at 11:52 PM Christian König
> > <christian.koenig@xxxxxxx> wrote:
> > >
> > > Am 01.07.21 um 00:24 schrieb John Stultz:
> > > > On Wed, Jun 30, 2021 at 2:10 AM Christian König
> > > > <christian.koenig@xxxxxxx> wrote:
> > > >> Am 30.06.21 um 03:34 schrieb John Stultz:
> > > >>> +static unsigned long page_pool_size; /* max size of the pool */
> > > >>> +
> > > >>> +MODULE_PARM_DESC(page_pool_size, "Number of pages in the drm page pool");
> > > >>> +module_param(page_pool_size, ulong, 0644);
> > > >>> +
> > > >>> +static atomic_long_t nr_managed_pages;
> > > >>> +
> > > >>> +static struct mutex shrinker_lock;
> > > >>> +static struct list_head shrinker_list;
> > > >>> +static struct shrinker mm_shrinker;
> > > >>> +
> > > >>> +/**
> > > >>> + * drm_page_pool_set_max - Sets maximum size of all pools
> > > >>> + *
> > > >>> + * Sets the maximum number of pages allows in all pools.
> > > >>> + * This can only be set once, and the first caller wins.
> > > >>> + */
> > > >>> +void drm_page_pool_set_max(unsigned long max)
> > > >>> +{
> > > >>> +     if (!page_pool_size)
> > > >>> +             page_pool_size = max;
> > > >>> +}
> > > >>> +
> > > >>> +/**
> > > >>> + * drm_page_pool_get_max - Maximum size of all pools
> > > >>> + *
> > > >>> + * Return the maximum number of pages allows in all pools
> > > >>> + */
> > > >>> +unsigned long drm_page_pool_get_max(void)
> > > >>> +{
> > > >>> +     return page_pool_size;
> > > >>> +}
> > > >> Well in general I don't think it is a good idea to have getters/setters
> > > >> for one line functionality, similar applies to locking/unlocking the
> > > >> mutex below.
> > > >>
> > > >> Then in this specific case what those functions do is to aid
> > > >> initializing the general pool manager and that in turn should absolutely
> > > >> not be exposed.
> > > >>
> > > >> The TTM pool manager exposes this as function because initializing the
> > > >> pool manager is done in one part of the module and calculating the
> > > >> default value for the pages in another one. But that is not something I
> > > >> would like to see here.
> > > > So, I guess I'm not quite clear on what you'd like to see...
> > > >
> > > > Part of what I'm balancing here is the TTM subsystem normally sets a
> > > > global max size, whereas the old ION pool didn't have caps (instead
> > > > just relying on the shrinker when needed).
> > > > So I'm trying to come up with a solution that can serve both uses. So
> > > > I've got this drm_page_pool_set_max() function to optionally set the
> > > > maximum value, which is called in the TTM initialization path or set
> > > > the boot argument. But for systems that use the dmabuf system heap,
> > > > but don't use TTM, no global limit is enforced.
> > >
> > > Yeah, exactly that's what I'm trying to prevent.
> > >
> > > See if we have the same functionality used by different use cases we
> > > should not have different behavior depending on what drivers are loaded.
> > >
> > > Is it a problem if we restrict the ION pool to 50% of system memory as
> > > well? If yes than I would rather drop the limit from TTM and only rely
> > > on the shrinker there as well.
> >
> > Would having the default value as a config option (still overridable
> > via boot argument) be an acceptable solution?
>
> We're also trying to get ttm over to the shrinker model, and a first
> cut of that even landed, but didn't really work out yet. So maybe just
> aiming for the shrinker? I do agree this should be consistent across
> the board, otherwise we're just sharing code but not actually sharing
> functionality, which is a recipe for disaster because one side will
> end up breaking the other side's use-case.

Fair enough, maybe it would be best to remove the default limit, but
leave the logic so it can still be set via the boot argument?

thanks
-john