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Date: Fri, 16 May 2014 17:54:06 +0100
From: Wei Liu <wei.liu2@...rix.com>
To: Zoltan Kiss <zoltan.kiss@...rix.com>
CC: Wei Liu <wei.liu2@...rix.com>,
Eric Dumazet <eric.dumazet@...il.com>,
<netdev@...r.kernel.org>, <xen-devel@...ts.xen.org>,
David Vrabel <david.vrabel@...rix.com>,
Konrad Wilk <konrad.wilk@...cle.com>,
Boris Ostrovsky <boris.ostrovsky@...cle.com>,
Stefan Bader <stefan.bader@...onical.com>
Subject: Re: [PATCH net-next] xen-netfront: try linearizing SKB if it
occupies too many slots
On Fri, May 16, 2014 at 05:29:05PM +0100, Zoltan Kiss wrote:
> On 16/05/14 16:34, Wei Liu wrote:
> > On Fri, May 16, 2014 at 08:22:19AM -0700, Eric Dumazet wrote:
> >> On Fri, 2014-05-16 at 15:36 +0100, Wei Liu wrote:
> >>> On Fri, May 16, 2014 at 07:21:08AM -0700, Eric Dumazet wrote:
> >>>> On Fri, 2014-05-16 at 14:11 +0100, Wei Liu wrote:
> >>>>
> >>>>> It's not that common to trigger this, I only saw a few reports. In fact
> >>>>> Stefan's report is the first one that comes with a method to reproduce
> >>>>> it.
> >>>>>
> >>>>> I tested with redis-benchmark on a guest with 256MB RAM and only saw a
> >>>>> few "failed to linearize", never saw a single one with 1GB guest.
> >>>>
> >>>> Well, I am just saying. This is asking order-5 allocations, and yes,
> >>>> this is going to fail after few days of uptime, no matter what you try.
> >>>>
> >>>
> >>> Hmm... I see what you mean -- memory fragmentation leads to allocation
> >>> failure. Thanks.
> >>
> >> In the mean time, have you tried to lower gso_max_size ?
> >>
> >> Setting it witk netif_set_gso_max_size() to something like 56000 might
> >> avoid the problem.
> >>
> >> (Not sure if it is applicable in your case)
> >>
> >
> > It works, at least in this Redis testcase. Could you explain a bit where
> > this 56000 magic number comes from? :-)
> >
> > Presumably I can derive it from some constant in core network code?
>
> I guess it just makes more unlikely to have packets with problematic layout. But the following packet would still fail:
> linear buffer : 80 bytes, on 2 pages
> 17 frags, 80 bytes each, each spanning over page boundary.
>
Presumably max GSO size affects packet layout, that's what I was trying
to figure out.
> I just had an idea: a modified version of xenvif_handle_frag_list function from netback would be useful for us here. It recreates the frags array on fully utilized 4k pages. Plus we can use pskb_expand_head to reduce the page number on the linear buffer (although it might not work, see my comment in the patch)
> The worst case linear buffer then spans N+1 pages, and has N*PAGE_SIZE+1 bytes. Then the frags after this coalescing should have 16*PAGE_SIZE - (N*PAGE_SIZE+2) bytes at most, which is 16-N pages. Altogether that's 16+1 page, which should definitely fit!
> This is what I mean:
>
> 8<--------------
> diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
> index 158b5e6..b1133d6 100644
> --- a/drivers/net/xen-netfront.c
> +++ b/drivers/net/xen-netfront.c
> @@ -544,6 +544,73 @@ static int xennet_count_skb_frag_slots(struct sk_buff *skb)
> return pages;
> }
>
> +int xenvif_reduce_pages(struct sk_buff *skb, int target)
> +{
> + unsigned int offset = skb_headlen(skb);
> + skb_frag_t frags[MAX_SKB_FRAGS];
> + int newfrags, oldfrags;
> + unsigned int pages, optimal;
> +
> + BUG_ON(!target);
> +
> + pages = DIV_ROUND_UP(offset_in_page(skb->data) + skb_headlen(skb), PAGE_SIZE);
> + optimal = DIV_ROUND_UP(skb_headlen(skb), PAGE_SIZE);
> + if (pages - optimal) {
> + int err;
> +/* FIXME: we should check if pskb_expand_head really allocates on page boundary,
> + * otherwise we can still have suboptimal page layout */
> + if (unlikely(err = pskb_expand_head(skb, 0, 0, GFP_ATOMIC)))
I'm a bit lost. What do you expect from the call to pskb_expand_head?
I'm sorry I cannot see immediate result from the comment of
pskb_expand_head. If you call with nhead and ntail equal to 0 it creates
identical copy, but I don't see guarantee on page alignment. Did I miss
something?
> + return err;
> + target -= pages - optimal;
> + if (!target)
> + return 0;
> + }
> +
> + /* Subtract frags size, we will correct it later */
> + skb->truesize -= skb->data_len;
> +
> + /* Create a brand new frags array and coalesce there */
> + for (newfrags = 0; offset < skb->len; newfrags++) {
> + struct page *page;
> + unsigned int len;
> +
> + BUG_ON(newfrags >= MAX_SKB_FRAGS);
> + page = alloc_page(GFP_ATOMIC);
And the ammount of memory allocation is a bit overkill I think (though
it's still better than the order-5 allocation in skb_linearize). Can you
not just memmove all paged data to first few frags and release other
frags?
Anyway, this method might still work, just a bit overkill IMHO.
Wei.
> + if (!page) {
> + int j;
> + skb->truesize += skb->data_len;
> + for (j = 0; j < newfrags; j++)
> + put_page(frags[j].page.p);
> + return -ENOMEM;
> + }
> +
> + if (offset + PAGE_SIZE < skb->len)
> + len = PAGE_SIZE;
> + else
> + len = skb->len - offset;
> + if (skb_copy_bits(skb, offset, page_address(page), len))
> + BUG();
> +
> + offset += len;
> + frags[newfrags].page.p = page;
> + frags[newfrags].page_offset = 0;
> + skb_frag_size_set(&frags[newfrags], len);
> + }
> +
> + /* Drop the original buffers */
> + for (oldfrags = 0; oldfrags < skb_shinfo(skb)->nr_frags; oldfrags++)
> + skb_frag_unref(skb, oldfrags);
> +
> + /* Swap the new frags array with the old one */
> + memcpy(skb_shinfo(skb)->frags,
> + frags,
> + newfrags * sizeof(skb_frag_t));
> + skb_shinfo(skb)->nr_frags = newfrags;
> + /* Correct truesize */
> + skb->truesize += newfrags * PAGE_SIZE;
> + return 0;
> +}
> +
> static int xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
> {
> unsigned short id;
> @@ -573,11 +640,21 @@ static int xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
> slots = DIV_ROUND_UP(offset + len, PAGE_SIZE) +
> xennet_count_skb_frag_slots(skb);
> if (unlikely(slots > MAX_SKB_FRAGS + 1)) {
> - net_alert_ratelimited(
> - "xennet: skb rides the rocket: %d slots\n", slots);
> - goto drop;
> + if (unlikely(xenvif_reduce_pages(skb, slots - MAX_SKB_FRAGS - 1))) {
> + net_alert_ratelimited(
> + "xennet: couldn't reduce slot number from %d\n", slots);
> + goto drop;
> + }
> + slots = DIV_ROUND_UP(offset_in_page(data) + len, PAGE_SIZE) +
> + xennet_count_skb_frag_slots(skb);
> + if (unlikely(slots > MAX_SKB_FRAGS + 1)) {
> + net_alert_ratelimited(
> + "xennet: slot reduction doesn't work, slots: %d\n", slots);
> + goto drop;
> + }
> }
>
> +
> spin_lock_irqsave(&np->tx_lock, flags);
>
> if (unlikely(!netif_carrier_ok(dev) ||
--
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