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

On Tue, Jan 08, 2013 at 06:32:29PM -0800, Eric Dumazet wrote:

On Tue, 2013-01-08 at 18:14 -0800, Eric Dumazet wrote:

quoted

On Tue, 2013-01-08 at 23:23 +0000, Eric Wong wrote:

quoted

Mel Gorman [off-list ref] wrote:

quoted

Please try the following patch. However, even if it works the benefit of
capture may be so marginal that partially reverting it and simplifying
compaction.c is the better decision.

I already got my VM stuck on this one.  I had two twosleepy instances,
2774 was the one that got stuck (also confirmed by watching top).

Btw, have you been able to reproduce this on your end?

I think the easiest reproduction on my 2-core VM is by running 2
twosleepy processes and doing the following to dirty a lot of pages:

Given the persistent sk_stream_wait_memory() traces I suspect a plain
TCP bug, triggered by some extra wait somewhere.

Please mm guys don't spend too much time right now, I'll try to
reproduce the problem.

Don't be confused by sk_stream_wait_memory() name.
A thread is stuck here because TCP stack is failing to wake it.

Hmm, it seems sk_filter() can return -ENOMEM because skb has the
pfmemalloc() set.

The skb should not have pfmemalloc set in most cases, particularly after
cfd19c5a (mm: only set page->pfmemalloc when ALLOC_NO_WATERMARKS was used)
but the capture patch also failed to clear pfmemalloc properly so it could
be set in error.

-- 
Mel Gorman
SUSE Labs