Re: ppoll() stuck on POLLIN while TCP peer is sending

[Eric Dumazet <eric.dumazet@gmail.com>]

On Thu, 2013-01-10 at 19:42 +0000, Mel Gorman wrote:

> Thanks Eric, it's much appreciated. However, I'm still very much in favour
> of a partial revert as in retrospect the implementation of capture took the
> wrong approach. Could you confirm the following patch works for you?
> It's should functionally have the same effect as the first revert and
> there are only minor changes from the last revert prototype I sent you
> but there is no harm in being sure.
> 
> ---8<---
> mm: compaction: Partially revert capture of suitable high-order page
> 
> Eric Wong reported on 3.7 and 3.8-rc2 that ppoll() got stuck when waiting
> for POLLIN on a local TCP socket. It was easier to trigger if there was disk
> IO and dirty pages at the same time and he bisected it to commit 1fb3f8ca
> "mm: compaction: capture a suitable high-order page immediately when it
> is made available".
> 
> The intention of that patch was to improve high-order allocations under
> memory pressure after changes made to reclaim in 3.6 drastically hurt
> THP allocations but the approach was flawed. For Eric, the problem was
> that page->pfmemalloc was not being cleared for captured pages leading to
> a poor interaction with swap-over-NFS support causing the packets to be
> dropped. However, I identified a few more problems with the patch including
> the fact that it can increase contention on zone->lock in some cases which
> could result in async direct compaction being aborted early.
> 
> In retrospect the capture patch took the wrong approach. What it should
> have done is mark the pageblock being migrated as MIGRATE_ISOLATE if it
> was allocating for THP and avoided races that way. While the patch was
> showing to improve allocation success rates at the time, the benefit is
> marginal given the relative complexity and it should be revisited from
> scratch in the context of the other reclaim-related changes that have taken
> place since the patch was first written and tested. This patch partially
> reverts commit 1fb3f8ca "mm: compaction: capture a suitable high-order
> page immediately when it is made available".
> 
> Reported-by: Eric Wong <normalperson@yhbt.net>
> Cc: stable@vger.kernel.org
> Signed-off-by: Mel Gorman <mgorman@suse.de>
> ---

It seems to solve the problem on my kvm testbed

(512 MB of ram, 2 vcpus)

Tested-by: Eric Dumazet <edumazet@google.com>


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