On Fri, 2011-04-29 at 13:02 -0500, James Bottomley wrote:

On Fri, 2011-04-29 at 11:31 -0500, James Bottomley wrote:

quoted

On Fri, 2011-04-29 at 10:37 -0500, James Bottomley wrote:

quoted

On Fri, 2011-04-29 at 12:23 +0200, Sedat Dilek wrote:

quoted

But as I see these RCU (CPU) stalls, the patch from [1] might be worth a try.
First, I have seen negative effects on my UP-system was when playing
with linux-next [2].
It was not clear what the origin was and the the side-effects were
somehow "bizarre".
The issue could be easily reproduced by tar-ing the kernel build-dir
to an external USB-hdd.
The issue kept RCU and TIP folks really busy.
Before stepping 4 weeks in the dark, give it a try and let me know in
case of success.

Well, it's highly unlikely because that's a 2.6.39 artifact and the bug
showed up in 2.6.38 ... I tried it just in case with no effect, so we
know it isn't the cause.

Actually, I tell a lie: it does't stop kswapd spinning on PREEMPT, but
it does seem to prevent non-PREEMPT from locking up totally (at least it
survives three back to back untar runs).

It's probable it alters the memory pin conditions that cause the spin,
so it's masking the problem rather than fixing it.

Confirmed ... it's just harder to reproduce with the hrtimers init fix.
The problem definitely still exists (I had to load up the system more
before doing the tar).

This time I've caught kswapd in mem_cgroup_shrink_node_zone.  sysrq-w
doesn't complete for an unknown reason

As a follow on to this, there's a shrink_zone(0, ...) in the cgroup
path.  This causes it to scan all memory exhaustively (generating quite
a lot of work).  The comment above it implies it's some type of hack for
cgroup accounting, but reducing it to DEF_PRIORITY makes the hang go
away (verified on both 2.6.39-rc4 and 2.6.38.4).  Note that I still get
soft lockups in kswapd0, but they no longer hang the box.  I'll prepare
a patch for the next round of debate.

James


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