[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <45347CA3.9020904@yahoo.com.au>
Date: Tue, 17 Oct 2006 16:48:03 +1000
From: Nick Piggin <nickpiggin@...oo.com.au>
To: "Martin J. Bligh" <mbligh@...gle.com>
CC: Andrew Morton <akpm@...l.org>, LKML <linux-kernel@...r.kernel.org>,
Linux Memory Management <linux-mm@...ck.org>
Subject: Re: [PATCH] Fix bug in try_to_free_pages and balance_pgdat when they
fail to reclaim pages
Martin J. Bligh wrote:
> Nick Piggin wrote:
>
>> Martin Bligh wrote:
>>
>>> The same bug is contained in both try_to_free_pages and balance_pgdat.
>>> On reclaiming the requisite number of pages we correctly set
>>> prev_priority back to DEF_PRIORITY.
>>
>>
>>
>> AFAIKS, we set prev_priority to the priority at which the zone was
>> deemed to require no more reclaiming, not DEF_PRIORITY.
>
>
> Well, it's zone->temp_priority, which was set to DEF_PRIORITY at the
> top of the function, though I suppose something else might have
> changed it since.
Yes.
>
>>> However, we ALSO do this even
>>> if we loop over all priorities and fail to reclaim.
>>
>>
>>
>> If that happens, shouldn't prev_priority be set to 0?
>
>
> Yes, but it's not. We fall off the bottom of the loop, and set it
> back to temp_priority. At best, the code is unclear.
But temp_priority should be set to 0 at that point.
> I suppose shrink_zones() might in theory knock temp_priority down
> as it goes, so it might come out right. But given that it's a global
> (per zone), not per-reclaimer, I fail to see how that's really safe.
> Supposing someone else has just started reclaim, and is still at
> prio 12?
But your loops are not exactly per reclaimer either. Granted there
is a large race window in the current code, but this patch isn't the
way to fix that particular problem.
> Moreover, whilst try_to_free_pages calls shrink_zones, balance_pgdat
> does not. Nothing else I can see sets temp_priority.
balance_pgdat.
>
> > I don't agree the patch is correct.
>
> You think it's doing something wrong? Or just unnecessary?
Unnecesary and indicates something else is broken if you are seeing
problems here.
> I'm inclined to think the whole concept of temp_priority and
> prev_priority are pretty broken. This may not fix the whole thing,
> but it seems to me to make it better than it was before.
I think it is broken too. I liked my split active lists, but at that point
vmscan.c was in don't-touch mode.
>> We saw problems with this before releasing SLES10 too. See
>> zone_is_near_oom and other changesets from around that era. I would
>> like to know what workload was prevented from going OOM with these
>> changes, but zone_is_near_oom didn't help -- it must have been very
>> marginal (or there may indeed be a bug somewhere).
>
>
> Google production workload. Multiple reclaimers operating - one is
> down to priority 0 on the reclaim, but distress is still set to 0,
> thanks to prev_priority being borked. Hence we don't reclaim mapped
> pages, the reclaim fails, OOM killer kicks in.
OK, so it sounds like temp_priority is being overwritten by the
race. I'd consider throwing out temp_priority completely, and just
going with adjusting prev_priority as we go.
> Forward ported from an earlier version of 2.6 ... but I don't see
> why we need extra heuristics here, it seems like a clear and fairly
> simple bug. We're in deep crap with reclaim, and we go set the
> global indicator back to "oh no, everything's fine". Not a good plan.
All that reclaim_mapped code is pretty arbitrary anyway. What is needed
is the zone_is_near_oom so we can decouple all those heuristics from
the OOM decision. So do you still see the problem on upstream kernel
without your patches applied?
--
Send instant messages to your online friends http://au.messenger.yahoo.com
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
Powered by blists - more mailing lists