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Date: Thu, 22 Mar 2018 09:22:31 -0700
From: Alexander Duyck <alexander.duyck@...il.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: Matthew Wilcox <mawilcox@...rosoft.com>,
Netdev <netdev@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
Jesper Dangaard Brouer <brouer@...hat.com>,
Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH v2 6/8] page_frag_cache: Use a mask instead of offset
On Thu, Mar 22, 2018 at 8:31 AM, Matthew Wilcox <willy@...radead.org> wrote:
> From: Matthew Wilcox <mawilcox@...rosoft.com>
>
> By combining 'va' and 'offset' into 'addr' and using a mask instead,
> we can save a compare-and-branch in the fast-path of the allocator.
> This removes 4 instructions on x86 (both 32 and 64 bit).
What is the point of renaming "va"? I'm seeing a lot of unneeded
renaming in these patches that doesn't really seem needed and is just
making things harder to review.
> We can avoid storing the mask at all if we know that we're only allocating
> a single page. This shrinks page_frag_cache from 12 to 8 bytes on 32-bit
> CONFIG_BASE_SMALL build.
>
> Signed-off-by: Matthew Wilcox <mawilcox@...rosoft.com>
So I am not really a fan of CONFIG_BASE_SMALL in general, so
advertising gains in size is just going back down the reducing size at
the expense of performance train of thought.
Do we know for certain that a higher order page is always aligned to
the size of the higher order page itself? That is one thing I have
never been certain about. I know for example there are head and tail
pages so I was never certain if it was possible to create a higher
order page that is not aligned to to the size of the page itself.
If we can get away with making that assumption then yes I would say
this is probably an optimization, though I think we would be better
off without the CONFIG_BASE_SMALL bits.
> ---
> include/linux/mm_types.h | 12 +++++++-----
> mm/page_alloc.c | 40 +++++++++++++++-------------------------
> 2 files changed, 22 insertions(+), 30 deletions(-)
>
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 0defff9e3c0e..ebe93edec752 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -225,12 +225,9 @@ struct page {
> #define PFC_MEMALLOC (1U << 31)
>
> struct page_frag_cache {
> - void * va;
> + void *addr;
> #if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
> - __u16 offset;
> - __u16 size;
> -#else
> - __u32 offset;
> + unsigned int mask;
So this is just an akward layout. You now have essentially:
#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
#else
unsigned int mask;
#endif
> #endif
> /* we maintain a pagecount bias, so that we dont dirty cache line
> * containing page->_refcount every time we allocate a fragment.
> @@ -239,6 +236,11 @@ struct page_frag_cache {
> };
>
> #define page_frag_cache_pfmemalloc(pfc) ((pfc)->pagecnt_bias & PFC_MEMALLOC)
> +#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
> +#define page_frag_cache_mask(pfc) (pfc)->mask
> +#else
> +#define page_frag_cache_mask(pfc) (~PAGE_MASK)
> +#endif
>
> typedef unsigned long vm_flags_t;
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 5a2e3e293079..d15a5348a8e4 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4336,22 +4336,19 @@ EXPORT_SYMBOL(free_pages);
> * drivers to provide a backing region of memory for use as either an
> * sk_buff->head, or to be used in the "frags" portion of skb_shared_info.
> */
> -static struct page *__page_frag_cache_refill(struct page_frag_cache *pfc,
> +static void *__page_frag_cache_refill(struct page_frag_cache *pfc,
> gfp_t gfp_mask)
> {
> unsigned int size = PAGE_SIZE;
> struct page *page = NULL;
> - struct page *old = pfc->va ? virt_to_page(pfc->va) : NULL;
> + struct page *old = pfc->addr ? virt_to_head_page(pfc->addr) : NULL;
> gfp_t gfp = gfp_mask;
> unsigned int pagecnt_bias = pfc->pagecnt_bias & ~PFC_MEMALLOC;
>
> /* If all allocations have been freed, we can reuse this page */
> if (old && page_ref_sub_and_test(old, pagecnt_bias)) {
> page = old;
> -#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
> - /* if size can vary use size else just use PAGE_SIZE */
> - size = pfc->size;
> -#endif
> + size = page_frag_cache_mask(pfc) + 1;
> /* Page count is 0, we can safely set it */
> set_page_count(page, size);
> goto reset;
> @@ -4364,27 +4361,24 @@ static struct page *__page_frag_cache_refill(struct page_frag_cache *pfc,
> PAGE_FRAG_CACHE_MAX_ORDER);PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
> if (page)
> size = PAGE_FRAG_CACHE_MAX_SIZE;
> - pfc->size = size;
> + pfc->mask = size - 1;
> #endif
> if (unlikely(!page))
> page = alloc_pages_node(NUMA_NO_NODE, gfp, 0);
> if (!page) {
> - pfc->va = NULL;
> + pfc->addr = NULL;
> return NULL;
> }
>
> - pfc->va = page_address(page);
> -
> /* Using atomic_set() would break get_page_unless_zero() users. */
> page_ref_add(page, size - 1);
You could just use the pfc->mask here instead of size - 1 just to
avoid having to do the subtraction more than once assuming the
compiler doesn't optimize it.
> reset:
> - /* reset page count bias and offset to start of new frag */
> pfc->pagecnt_bias = size;
> if (page_is_pfmemalloc(page))
> pfc->pagecnt_bias |= PFC_MEMALLOC;
> - pfc->offset = size;
> + pfc->addr = page_address(page) + size;
>
> - return page;
> + return pfc->addr;
> }
>
> void __page_frag_cache_drain(struct page *page, unsigned int count)
> @@ -4405,24 +4399,20 @@ EXPORT_SYMBOL(__page_frag_cache_drain);
> void *page_frag_alloc(struct page_frag_cache *pfc,
> unsigned int size, gfp_t gfp_mask)
> {
> - struct page *page;
> - int offset;
> + void *addr = pfc->addr;
> + unsigned int offset = (unsigned long)addr & page_frag_cache_mask(pfc);
>
> - if (unlikely(!pfc->va)) {
> -refill:
> - page = __page_frag_cache_refill(pfc, gfp_mask);
> - if (!page)
> + if (unlikely(offset < size)) {
> + addr = __page_frag_cache_refill(pfc, gfp_mask);
> + if (!addr)
> return NULL;
> }
>
> - offset = pfc->offset - size;
> - if (unlikely(offset < 0))
> - goto refill;
> -
> + addr -= size;
> + pfc->addr = addr;
> pfc->pagecnt_bias--;
> - pfc->offset = offset;
>
> - return pfc->va + offset;
> + return addr;
> }
> EXPORT_SYMBOL(page_frag_alloc);
>
> --
> 2.16.2
>
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