Messages in this thread | | | Date | Mon, 23 Oct 2023 14:43:10 +0300 | From | Ilias Apalodimas <> | Subject | Re: [PATCH net-next v12 1/5] page_pool: unify frag_count handling in page_pool_is_last_frag() |
| |
Hi Yunsheng,
[...]
> + * 1. 'n == 1': no need to actually overwrite it. > + * 2. 'n != 1': overwrite it with one, which is the rare case > + * for pp_frag_count draining. > * > - * The main advantage to doing this is that an atomic_read is > - * generally a much cheaper operation than an atomic update, > - * especially when dealing with a page that may be partitioned > - * into only 2 or 3 pieces. > + * The main advantage to doing this is that not only we avoid a atomic > + * update, as an atomic_read is generally a much cheaper operation than > + * an atomic update, especially when dealing with a page that may be > + * partitioned into only 2 or 3 pieces; but also unify the pp_frag_count > + * handling by ensuring all pages have partitioned into only 1 piece > + * initially, and only overwrite it when the page is partitioned into > + * more than one piece. > */ > - if (atomic_long_read(&page->pp_frag_count) == nr) > + if (atomic_long_read(&page->pp_frag_count) == nr) { > + /* As we have ensured nr is always one for constant case using > + * the BUILD_BUG_ON(), only need to handle the non-constant case > + * here for pp_frag_count draining, which is a rare case. > + */ > + BUILD_BUG_ON(__builtin_constant_p(nr) && nr != 1); > + if (!__builtin_constant_p(nr)) > + atomic_long_set(&page->pp_frag_count, 1);
Aren't we changing the behaviour of the current code here? IIRC is atomic_long_read(&page->pp_frag_count) == nr we never updated the atomic pp_frag_count and the reasoning was that the next caller can set it properly.
> + > return 0; > + } > > ret = atomic_long_sub_return(nr, &page->pp_frag_count); > WARN_ON(ret < 0); > + > + /* We are the last user here too, reset pp_frag_count back to 1 to > + * ensure all pages have been partitioned into 1 piece initially, > + * this should be the rare case when the last two fragment users call > + * page_pool_defrag_page() currently. > + */ > + if (unlikely(!ret)) > + atomic_long_set(&page->pp_frag_count, 1); > + > return ret; > } >
[....]
Thanks /Ilias
| |