On Fri, 2014-01-03 at 17:47 -0500, Debabrata Banerjee wrote:
> >> On Thu, 2014-01-02 at 16:56 -0800, Eric Dumazet wrote:
> >>
> >> Hmm... it looks like I missed __GFP_NORETRY
> >>
> >>
> >>
> >> diff --git a/net/core/sock.c b/net/core/sock.c
> >> index 5393b4b719d7..5f42a4d70cb2 100644
> >> --- a/net/core/sock.c
> >> +++ b/net/core/sock.c
> >> @@ -1872,7 +1872,7 @@ bool skb_page_frag_refill(unsigned int sz, struct page_frag *pfrag, gfp_t prio)
> >> gfp_t gfp = prio;
> >>
> >> if (order)
> >> - gfp |= __GFP_COMP | __GFP_NOWARN;
> >> + gfp |= __GFP_COMP | __GFP_NOWARN | __GFP_NORETRY;
> >> pfrag->page = alloc_pages(gfp, order);
> >> if (likely(pfrag->page)) {
> >> pfrag->offset = 0;
> >>
> >>
> >>
>
> There is another patch needed (looks like good stable fixes):
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 06e72d3..d42d48c 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -378,7 +378,7 @@ refill:
> gfp_t gfp = gfp_mask;
>
> if (order)
> - gfp |= __GFP_COMP | __GFP_NOWARN;
> + gfp |= __GFP_COMP | __GFP_NOWARN |
> __GFP_NORETRY;
> nc->frag.page = alloc_pages(gfp, order);
> if (likely(nc->frag.page))
> break;
>
This is in GFP_ATOMIC cases, I dont think it can ever start compaction.
> This reduces the really pathological compact/reclaim behavior but
> doesn't fix it. Actually it still really quite bad because the whole
> thing loops until it gets to order-0 so it's effectively trying the
> allocation 4 times anyway. I typically see non-zero order allocations
> very rarely without these two pieces of code. I hotpatched a running
> system to get results from this quickly. Even setting the max order to
> order-1 I still see bad behavior. If anything this behavior should be
> conditional until this is ironed out.
>
> Performance data: http://pastebin.ubuntu.com/6687527/
It seems that you shoot the messenger : If memory is fragmented, then
one order-1 allocation is going to start compaction.
It can be a simple fork().
If your workload never fork(), then yes, you never needed compaction.
It doesn't really matter to say that which memory allocation triggered
compaction, which is a normal step in mm layer.
If you believe its badly done, you should ask to mm guys to fix/improve
it, not netdev...
We are not trying to optimize the kernel behavior for hosts in deep
memory pressure.
Using order-3 pages in TCP stack improves performance for 99% of the
hosts, there might be something wrong on your side ?
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