Re: [PATCH net] net: avoid 32 x truesize under-estimation for tiny skbs

[Alexander Duyck <alexander.duyck@gmail.com>]

On Wed, Jan 13, 2021 at 8:20 AM Eric Dumazet <eric.dumazet@gmail.com> wrote:
>
> From: Eric Dumazet <edumazet@google.com>
>
> Both virtio net and napi_get_frags() allocate skbs
> with a very small skb->head
>
> While using page fragments instead of a kmalloc backed skb->head might give
> a small performance improvement in some cases, there is a huge risk of
> under estimating memory usage.
>
> For both GOOD_COPY_LEN and GRO_MAX_HEAD, we can fit at least 32 allocations
> per page (order-3 page in x86), or even 64 on PowerPC
>
> We have been tracking OOM issues on GKE hosts hitting tcp_mem limits
> but consuming far more memory for TCP buffers than instructed in tcp_mem[2]
>
> Even if we force napi_alloc_skb() to only use order-0 pages, the issue
> would still be there on arches with PAGE_SIZE >= 32768
>
> This patch makes sure that small skb head are kmalloc backed, so that
> other objects in the slab page can be reused instead of being held as long
> as skbs are sitting in socket queues.
>
> Note that we might in the future use the sk_buff napi cache,
> instead of going through a more expensive __alloc_skb()
>
> Another idea would be to use separate page sizes depending
> on the allocated length (to never have more than 4 frags per page)
>
> I would like to thank Greg Thelen for his precious help on this matter,
> analysing crash dumps is always a time consuming task.
>
> Fixes: fd11a83dd363 ("net: Pull out core bits of __netdev_alloc_skb and add __napi_alloc_skb")
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Cc: Alexander Duyck <alexanderduyck@fb.com>
> Cc: Paolo Abeni <pabeni@redhat.com>
> Cc: Michael S. Tsirkin <mst@redhat.com>
> Cc: Greg Thelen <gthelen@google.com>
> ---
>  net/core/skbuff.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 7626a33cce590e530f36167bd096026916131897..3a8f55a43e6964344df464a27b9b1faa0eb804f3 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -501,13 +501,17 @@ EXPORT_SYMBOL(__netdev_alloc_skb);
>  struct sk_buff *__napi_alloc_skb(struct napi_struct *napi, unsigned int len,
>                                  gfp_t gfp_mask)
>  {
> -       struct napi_alloc_cache *nc = this_cpu_ptr(&napi_alloc_cache);
> +       struct napi_alloc_cache *nc;
>         struct sk_buff *skb;
>         void *data;
>
>         len += NET_SKB_PAD + NET_IP_ALIGN;
>
> -       if ((len > SKB_WITH_OVERHEAD(PAGE_SIZE)) ||
> +       /* If requested length is either too small or too big,
> +        * we use kmalloc() for skb->head allocation.
> +        */
> +       if (len <= SKB_WITH_OVERHEAD(1024) ||
> +           len > SKB_WITH_OVERHEAD(PAGE_SIZE) ||
>             (gfp_mask & (__GFP_DIRECT_RECLAIM | GFP_DMA))) {
>                 skb = __alloc_skb(len, gfp_mask, SKB_ALLOC_RX, NUMA_NO_NODE);
>                 if (!skb)
> @@ -515,6 +519,7 @@ struct sk_buff *__napi_alloc_skb(struct napi_struct *napi, unsigned int len,
>                 goto skb_success;
>         }
>
> +       nc = this_cpu_ptr(&napi_alloc_cache);
>         len += SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
>         len = SKB_DATA_ALIGN(len);
>

The fix here looks good to me.
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>

I think at some point in the future we may need to follow up and do a
rework of a bunch of this code. One thing I am wondering is if we
should look at doing some sort of memory accounting per napi_struct.
Maybe it is something we could work on tying into the page pool work
that Jesper did earlier.