netdev - Re: [PATCH net-next v2 1/4] net: allow > 0 order atomic page alloc in skb_page_frag

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Date:	Wed, 8 Jan 2014 16:54:05 -0500
From:	Debabrata Banerjee <dbavatar@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	Michael Dalton <mwdalton@...gle.com>,
	"David S. Miller" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Jason Wang <jasowang@...hat.com>,
	virtualization@...ts.linux-foundation.org,
	Joshua Hunt <johunt@...mai.com>, jbaron@...mai.com
Subject: Re: [PATCH net-next v2 1/4] net: allow > 0 order atomic page alloc in skb_page_frag_refill

On Wed, Jan 8, 2014 at 2:46 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Wed, 2014-01-08 at 21:18 +0200, Michael S. Tsirkin wrote:
>> On Wed, Jan 08, 2014 at 10:26:03AM -0800, Eric Dumazet wrote:
>> > On Wed, 2014-01-08 at 20:08 +0200, Michael S. Tsirkin wrote:
>> >
>> > > Eric said we also need a patch to add __GFP_NORETRY, right?
>> > > Probably before this one in series.
>> >
>> > Nope, this __GFP_NORETRY has nothing to do with this.
>> >
>> > I am not yet convinced we want it.
>> >
>> > This needs mm guys advice, as its a tradeoff for mm layer more than
>> > networking...
>>
>> Well maybe Cc linux-mm then?
>
> Well, I do not care of people mlocking the memory and complaining that
> compaction does not work.
>
> If these people care, they should contact mm guys, eventually.
>
> Really this is an issue that has nothing to do with this patch set.
>

Actually I have more data on this:

1. __GFP_NORETRY really does help and should go into stable tree.

2. You may want to consider GFP_NOKSWAPD, because even in the
GFP_ATOMIC case you are waking up kswapd to do reclaims on a
continuous basis even when you don't enter direct reclaim.

3. mlocking memory had very little to do with it, that was a
red-herring. I tested out the problem scenario with no mlocks. You
simply need memory pressure from page_cache, and mm ends up constantly
reclaiming and trying to keep another 1-2GB free on our systems (8GB
phys ~4GB left for kernel, ~3GB optimally used for page_cache).

4. I think perhaps using a kmem_cache allocation for this buffer is
the right way to make this work. I am experimenting with a patch to do
this.

-Debabrata

-Debabrata
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