Re: [PATCH] drm/ttm/pool: Revert to clear-on-alloc to honor TTM_TT_FLAG_ZERO_ALLOC

[Nirmoy Das <nirmoy.das@xxxxxxxxx>]

Hi Christian,

On 6/21/2024 4:54 PM, Christian König wrote:
> Am 20.06.24 um 18:01 schrieb Nirmoy Das:
> > Currently ttm pool is not honoring TTM_TT_FLAG_ZERO_ALLOC flag and
> > clearing pages on free. It does help with allocation latency but 
> > clearing
> > happens even if drm driver doesn't passes the flag. If clear on free
> > is needed then a new flag can be added for that purpose.
>
> Mhm, thinking more about it that will most likely get push back from 
> others as well.


Agreed, it is diverting a lot from a known behavior.

> How about the attached patch? We just skip clearing pages when the 
> driver set the ZERO_ALLOC flag again before freeing them.
>
> Maybe rename the flag or add a new one for that, but in general that 
> looks like the option with the least impact.


I would prefer a few flag (TTM_TT_FLAG_CLEARED_ALLOC ?) which driver can 
set before freeing. I can resend the patch if you are

fine with it.


Regards,

Nirmoy


> Regards,
> Christian.
>
> > Cc: Christian Koenig <christian.koenig@xxxxxxx>
> > Cc: "Thomas Hellström" <thomas.hellstrom@xxxxxxxxxxxxxxx>
> > Cc: Matthew Auld <matthew.auld@xxxxxxxxx>
> > Signed-off-by: Nirmoy Das <nirmoy.das@xxxxxxxxx>
> > ---
> >   drivers/gpu/drm/ttm/ttm_pool.c | 31 +++++++++++++++++--------------
> >   1 file changed, 17 insertions(+), 14 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/ttm/ttm_pool.c 
> > b/drivers/gpu/drm/ttm/ttm_pool.c
> > index 6e1fd6985ffc..cbbd722185ee 100644
> > --- a/drivers/gpu/drm/ttm/ttm_pool.c
> > +++ b/drivers/gpu/drm/ttm/ttm_pool.c
> > @@ -224,15 +224,6 @@ static void ttm_pool_unmap(struct ttm_pool 
> > *pool, dma_addr_t dma_addr,
> >   /* Give pages into a specific pool_type */
> >   static void ttm_pool_type_give(struct ttm_pool_type *pt, struct 
> > page *p)
> >   {
> > -    unsigned int i, num_pages = 1 << pt->order;
> > -
> > -    for (i = 0; i < num_pages; ++i) {
> > -        if (PageHighMem(p))
> > -            clear_highpage(p + i);
> > -        else
> > -            clear_page(page_address(p + i));
> > -    }
> > -
> >       spin_lock(&pt->lock);
> >       list_add(&p->lru, &pt->pages);
> >       spin_unlock(&pt->lock);
> > @@ -240,15 +231,26 @@ static void ttm_pool_type_give(struct 
> > ttm_pool_type *pt, struct page *p)
> >   }
> >     /* Take pages from a specific pool_type, return NULL when nothing 
> > available */
> > -static struct page *ttm_pool_type_take(struct ttm_pool_type *pt)
> > +static struct page *ttm_pool_type_take(struct ttm_pool_type *pt, 
> > bool clear)
> >   {
> >       struct page *p;
> >         spin_lock(&pt->lock);
> >       p = list_first_entry_or_null(&pt->pages, typeof(*p), lru);
> >       if (p) {
> > +        unsigned int i, num_pages = 1 << pt->order;
> > +
> >           atomic_long_sub(1 << pt->order, &allocated_pages);
> >           list_del(&p->lru);
> > +        if (clear) {
> > +            for (i = 0; i < num_pages; ++i) {
> > +                if (PageHighMem(p))
> > +                    clear_highpage(p + i);
> > +                else
> > +                    clear_page(page_address(p + i));
> > +            }
> > +        }
> > +
> >       }
> >       spin_unlock(&pt->lock);
> >   @@ -279,7 +281,7 @@ static void ttm_pool_type_fini(struct 
> > ttm_pool_type *pt)
> >       list_del(&pt->shrinker_list);
> >       spin_unlock(&shrinker_lock);
> >   -    while ((p = ttm_pool_type_take(pt)))
> > +    while ((p = ttm_pool_type_take(pt, false)))
> >           ttm_pool_free_page(pt->pool, pt->caching, pt->order, p);
> >   }
> >   @@ -330,7 +332,7 @@ static unsigned int ttm_pool_shrink(void)
> >       list_move_tail(&pt->shrinker_list, &shrinker_list);
> >       spin_unlock(&shrinker_lock);
> >   -    p = ttm_pool_type_take(pt);
> > +    p = ttm_pool_type_take(pt, false);
> >       if (p) {
> >           ttm_pool_free_page(pt->pool, pt->caching, pt->order, p);
> >           num_pages = 1 << pt->order;
> > @@ -457,10 +459,11 @@ int ttm_pool_alloc(struct ttm_pool *pool, 
> > struct ttm_tt *tt,
> >            num_pages;
> >            order = min_t(unsigned int, order, __fls(num_pages))) {
> >           struct ttm_pool_type *pt;
> > +        bool clear = tt->page_flags & TTM_TT_FLAG_ZERO_ALLOC;
> >             page_caching = tt->caching;
> >           pt = ttm_pool_select_type(pool, tt->caching, order);
> > -        p = pt ? ttm_pool_type_take(pt) : NULL;
> > +        p = pt ? ttm_pool_type_take(pt, clear) : NULL;
> >           if (p) {
> >               r = ttm_pool_apply_caching(caching, pages,
> >                              tt->caching);
> > @@ -480,7 +483,7 @@ int ttm_pool_alloc(struct ttm_pool *pool, struct 
> > ttm_tt *tt,
> >                   if (num_pages < (1 << order))
> >                       break;
> >   -                p = ttm_pool_type_take(pt);
> > +                p = ttm_pool_type_take(pt, clear);
> >               } while (p);
> >           }