[Devel] [PATCH rh7] ms/net: don't wait for order-3 page allocation

[Alexey Kuznetsov]

Hello!

BTW, check this a few lines above:

        gfp_head = gfp_mask;
        if (gfp_head & __GFP_WAIT)
                gfp_head |= __GFP_REPEAT;

What is this??


On Tue, Feb 13, 2018 at 5:21 PM, Andrey Ryabinin
<aryabinin at virtuozzo.com> wrote:
> From: Shaohua Li <shli at fb.com>
>
> We saw excessive direct memory compaction triggered by skb_page_frag_refill.
> This causes performance issues and add latency. Commit 5640f7685831e0
> introduces the order-3 allocation. According to the changelog, the order-3
> allocation isn't a must-have but to improve performance. But direct memory
> compaction has high overhead. The benefit of order-3 allocation can't
> compensate the overhead of direct memory compaction.
>
> This patch makes the order-3 page allocation atomic. If there is no memory
> pressure and memory isn't fragmented, the alloction will still success, so we
> don't sacrifice the order-3 benefit here. If the atomic allocation fails,
> direct memory compaction will not be triggered, skb_page_frag_refill will
> fallback to order-0 immediately, hence the direct memory compaction overhead is
> avoided. In the allocation failure case, kswapd is waken up and doing
> compaction, so chances are allocation could success next time.
>
> alloc_skb_with_frags is the same.
>
> The mellanox driver does similar thing, if this is accepted, we must fix
> the driver too.
>
> V3: fix the same issue in alloc_skb_with_frags as pointed out by Eric
> V2: make the changelog clearer
>
> Cc: Eric Dumazet <edumazet at google.com>
> Cc: Chris Mason <clm at fb.com>
> Cc: Debabrata Banerjee <dbavatar at gmail.com>
> Signed-off-by: Shaohua Li <shli at fb.com>
> Acked-by: Eric Dumazet <edumazet at google.com>
> Signed-off-by: David S. Miller <davem at davemloft.net>
>
> https://jira.sw.ru/browse/PSBM-81488
> (cherry picked from commit fb05e7a89f500cfc06ae277bdc911b281928995d)
> Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
> ---
>  net/core/skbuff.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 4147d7219589..dca9d1d9676b 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -4360,7 +4360,7 @@ struct sk_buff *alloc_skb_with_frags(unsigned long header_len,
>
>                 while (order) {
>                         if (npages >= 1 << order) {
> -                               page = alloc_pages(gfp_mask |
> +                               page = alloc_pages((gfp_mask & ~__GFP_WAIT) |
>                                                    __GFP_COMP |
>                                                    __GFP_NOWARN |
>                                                    __GFP_NORETRY,
> --
> 2.13.6
>