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Date:   Mon, 4 Jun 2018 08:27:37 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Qing Huang <qing.huang@oracle.com>
Cc:     Eric Dumazet <eric.dumazet@gmail.com>,
        David Miller <davem@davemloft.net>, tariqt@mellanox.com,
        haakon.bugge@oracle.com, yanjun.zhu@oracle.com,
        netdev@vger.kernel.org, linux-rdma@vger.kernel.org,
        linux-kernel@vger.kernel.org, gi-oh.kim@profitbricks.com,
        "santosh.shilimkar@oracle.com" <santosh.shilimkar@oracle.com>,
        Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH V4] mlx4_core: allocate ICM memory in page size chunks
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On Fri 01-06-18 15:05:26, Qing Huang wrote:
> 
> 
> On 6/1/2018 12:31 AM, Michal Hocko wrote:
> > On Thu 31-05-18 19:04:46, Qing Huang wrote:
> > > 
> > > On 5/31/2018 2:10 AM, Michal Hocko wrote:
> > > > On Thu 31-05-18 10:55:32, Michal Hocko wrote:
> > > > > On Thu 31-05-18 04:35:31, Eric Dumazet wrote:
> > > > [...]
> > > > > > I merely copied/pasted from alloc_skb_with_frags() :/
> > > > > I will have a look at it. Thanks!
> > > > OK, so this is an example of an incremental development ;).
> > > > 
> > > > __GFP_NORETRY was added by ed98df3361f0 ("net: use __GFP_NORETRY for
> > > > high order allocations") to prevent from OOM killer. Yet this was
> > > > not enough because fb05e7a89f50 ("net: don't wait for order-3 page
> > > > allocation") didn't want an excessive reclaim for non-costly orders
> > > > so it made it completely NOWAIT while it preserved __GFP_NORETRY in
> > > > place which is now redundant. Should I send a patch?
> > > > 
> > > Just curious, how about GFP_ATOMIC flag? Would it work in a similar fashion?
> > > We experimented
> > > with it a bit in the past but it seemed to cause other issue in our tests.
> > > :-)
> > GFP_ATOMIC is a non-sleeping (aka no reclaim) context with an access to
> > memory reserves. So the risk is that you deplete those reserves and
> > cause issues to other subsystems which need them as well.
> > 
> > > By the way, we didn't encounter any OOM killer events. It seemed that the
> > > mlx4_alloc_icm() triggered slowpath.
> > > We still had about 2GB free memory while it was highly fragmented.
> > The compaction was able to make a reasonable forward progress for you.
> > But considering mlx4_alloc_icm is called with GFP_KERNEL resp. GFP_HIGHUSER
> > then the OOM killer is clearly possible as long as the order is lower
> > than 4.
> 
> The allocation was 256KB so the order was much higher than 4. The compaction
> seemed to be the root
> cause for our problem. It took too long to finish its work while putting
> mlx4_alloc_icm to sleep in a heavily
> fragmented memory situation . Will NORETRY flag avoid the compaction ops and
> fail the 256KB allocation
> immediately so mlx4_alloc_icm can enter adjustable lower order allocation
> code path quickly?

Costly orders should only perform a light compaction attempt unless
__GFP_RETRY_MAY_FAIL is used IIRC. CCing Vlastimil. So __GFP_NORETRY
shouldn't make any difference.
-- 
Michal Hocko
SUSE Labs