Re: [PATCH 0/3] OOM detection rework v4

[Joonsoo Kim]

2016-03-03 18:26 GMT+09:00 Michal Hocko <mhocko@xxxxxxxxxx>:
> On Wed 02-03-16 23:34:21, Joonsoo Kim wrote:
>> 2016-03-02 23:06 GMT+09:00 Michal Hocko <mhocko@xxxxxxxxxx>:
>> > On Wed 02-03-16 22:32:09, Joonsoo Kim wrote:
>> >> 2016-03-02 18:50 GMT+09:00 Michal Hocko <mhocko@xxxxxxxxxx>:
>> >> > On Wed 02-03-16 11:19:54, Joonsoo Kim wrote:
>> >> >> On Mon, Feb 29, 2016 at 10:02:13PM +0100, Michal Hocko wrote:
>> >> > [...]
>> >> >> > > + /*
>> >> >> > > +  * OK, so the watermak check has failed. Make sure we do all the
>> >> >> > > +  * retries for !costly high order requests and hope that multiple
>> >> >> > > +  * runs of compaction will generate some high order ones for us.
>> >> >> > > +  *
>> >> >> > > +  * XXX: ideally we should teach the compaction to try _really_ hard
>> >> >> > > +  * if we are in the retry path - something like priority 0 for the
>> >> >> > > +  * reclaim
>> >> >> > > +  */
>> >> >> > > + if (order && order <= PAGE_ALLOC_COSTLY_ORDER)
>> >> >> > > +         return true;
>> >> >> > > +
>> >> >> > >   return false;
>> >> >>
>> >> >> This seems not a proper fix. Checking watermark with high order has
>> >> >> another meaning that there is high order page or not. This isn't
>> >> >> what we want here.
>> >> >
>> >> > Why not? Why should we retry the reclaim if we do not have >=order page
>> >> > available? Reclaim itself doesn't guarantee any of the freed pages will
>> >> > form the requested order. The ordering on the LRU lists is pretty much
>> >> > random wrt. pfn ordering. On the other hand if we have a page available
>> >> > which is just hidden by watermarks then it makes perfect sense to retry
>> >> > and free even order-0 pages.
>> >>
>> >> If we have >= order page available, we would not reach here. We would
>> >> just allocate it.
>> >
>> > not really, we can still be under the low watermark. Note that the
>>
>> you mean min watermark?
>
> ohh, right...
>
>> > target for the should_reclaim_retry watermark check includes also the
>> > reclaimable memory.
>>
>> I guess that usual case for high order allocation failure has enough freepage.
>
> Not sure I understand you mean here but I wouldn't be surprised if high
> order failed even with enough free pages. And that is exactly why I am
> claiming that reclaiming more pages is no free ticket to high order
> pages.

I didn't say that it's free ticket. OOM kill would be the most expensive ticket
that we have. Why do you want to kill something? It also doesn't guarantee
to make high order pages. It is just another way of reclaiming memory. What is
the difference between plain reclaim and OOM kill? Why do we use OOM kill
in this case?

> [...]
>> >> I just did quick review to your patches so maybe I am wrong.
>> >> Am I missing something?
>> >
>> > The core idea behind should_reclaim_retry is to check whether the
>> > reclaiming all the pages would help to get over the watermark and there
>> > is at least one >= order page. Then it really makes sense to retry. As
>>
>> How you can judge that reclaiming all the pages would help to check
>> there is at least one >= order page?
>
> Again, not sure I understand you here. __zone_watermark_ok checks both
> wmark and an available page of the sufficient order. While increased
> free_pages (which includes reclaimable pages as well) will tell us
> whether we have a chance to get over the min wmark, the order check will
> tell us we have something to allocate from after we reach the min wmark.

Again, your assumption would be different with mine. My assumption is that
high order allocation problem happens due to fragmentation rather than
low free memory. In this case, there is no high order page. Even if you can
reclaim 1TB and add this counter to freepage counter, high order page
counter will not be changed and watermark check would fail. So, high order
allocation will not go through retry logic. This is what you want?

>> > the compaction has already was performed before this is called we should
>> > have created some high order pages already. The decay guarantees that we
>>
>> Not really. Compaction could fail.
>
> Yes it could have failed. But what is the point to retry endlessly then?

I didn't say we should retry endlessly.

> [...]
>> >> At least, reset no_progress_loops when did_some_progress. High
>> >> order allocation up to PAGE_ALLOC_COSTLY_ORDER is as important
>> >> as order 0. And, reclaim something would increase probability of
>> >> compaction success.
>> >
>> > This is something I still do not understand. Why would reclaiming
>> > random order-0 pages help compaction? Could you clarify this please?
>>
>> I just can tell simple version. Please check the link from me on another reply.
>> Compaction could scan more range of memory if we have more freepage.
>> This is due to algorithm limitation. Anyway, so, reclaiming random
>> order-0 pages helps compaction.
>
> I will have a look at that code but this just doesn't make any sense.
> The compaction should be reshuffling pages, this shouldn't be a function
> of free memory.

Please refer the link I mentioned before. There is a reason why more free
memory would help compaction success. Compaction doesn't work
like as random reshuffling. It has an algorithm to reduce system overall
fragmentation so there is limitation.

>> >> Why do we limit retry as 16 times with no evidence of potential
>> >> impossibility of making high order page?
>> >
>> > If we tried to compact 16 times without any progress then this sounds
>> > like a sufficient evidence to me. Well, this number is somehow arbitrary
>> > but the main point is to limit it to _some_ number, if we can show that
>> > a larger value would work better then we can update it of course.
>>
>> My arguing is for your band aid patch.
>> My point is that why retry count for order-0 is reset if there is some progress,
>> but, retry counter for order up to costly isn't reset even if there is
>> some progress
>
> Because we know that order-0 requests have chance to proceed if we keep
> reclaiming order-0 pages while this is not true for order > 0. If we did
> reset the no_progress_loops for order > 0 && order <= PAGE_ALLOC_COSTLY_ORDER
> then we would be back to the zone_reclaimable heuristic. Why? Because
> order-0 reclaim progress will keep !costly in the reclaim loop while
> compaction still might not make any progress. So we either have to fail
> when __zone_watermark_ok fails for the order (which turned out to be
> too easy to trigger) or have the fixed amount of retries regardless the
> watermark check result. We cannot relax both unless we have other
> measures in place.

As mentioned before, OOM kill also doesn't guarantee to make high order page.
Reclaim more memory as much as possible makes more sense to me.
Timing of OOM kill for order-0 is reasonable because there is not enough
freeable page. But, it's not reasonable to kill something when we have
much reclaimable memory like as your current implementation.

Thanks.