Re: [BUG] Page allocation failures with newest kernels

[Marcin Wojtas <mw@semihalf.com>]

Hi Mel,

Thanks for posting patch. I tested it on LKv4.4.8. Despite
"mode:0x2284020" shows that __GFP_ATOMIC is now not stripped, the
issue remains:
http://pastebin.com/DmezUJSc

Best regards,
Marcin

2016-06-09 20:13 GMT+02:00 Marcin Wojtas <mw@semihalf.com>:
> Hi Mel,
>
> My last email got cut in half.
>
> 2016-06-08 12:09 GMT+02:00 Mel Gorman <mgorman@techsingularity.net>:
>> On Tue, Jun 07, 2016 at 07:36:57PM +0200, Marcin Wojtas wrote:
>>> Hi Mel,
>>>
>>>
>>>
>>> 2016-06-03 14:36 GMT+02:00 Mel Gorman <mgorman@techsingularity.net>:
>>> > On Fri, Jun 03, 2016 at 01:57:06PM +0200, Marcin Wojtas wrote:
>>> >> >> For the record: the newest kernel I was able to reproduce the dumps
>>> >> >> was v4.6: http://pastebin.com/ekDdACn5. I've just checked v4.7-rc1,
>>> >> >> which comprise a lot (mainly yours) changes in mm, and I'm wondering
>>> >> >> if there may be a spot fix or rather a series of improvements. I'm
>>> >> >> looking forward to your opinion and would be grateful for any advice.
>>> >> >>
>>> >> >
>>> >> > I don't believe we want to reintroduce the reserve to cope with CMA. One
>>> >> > option would be to widen the gap between low and min watermark by the
>>> >> > size of the CMA region. The effect would be to wake kswapd earlier which
>>> >> > matters considering the context of the failing allocation was
>>> >> > GFP_ATOMIC.
>>> >>
>>> >> Of course my intention is not reintroducing anything that's gone
>>> >> forever, but just to find out way to overcome current issues. Do you
>>> >> mean increasing CMA size?
>>> >
>>> > No. There is a gap between the low and min watermarks. At the low point,
>>> > kswapd is woken up and at the min point allocation requests either
>>> > either direct reclaim or fail if they are atomic. What I'm suggesting
>>> > is that you adjust the low watermark and add the size of the CMA area
>>> > to it so that kswapd is woken earlier. The watermarks are calculated in
>>> > __setup_per_zone_wmarks
>>> >
>>>
>>> I printed all zones' settings, whose watermarks are configured within
>>> __setup_per_zone_wmarks(). There are three DMA, Normal and Movable -
>>> only first one's watermarks have non-zero values. Increasing DMA min
>>> watermark didn't help. I also played with increasing
>>
>> Patch?
>>
>
> I played with increasing min_free_kbytes from ~2600 to 16000. It
> resulted in shifting watermarks levels in __setup_per_zone_wmarks(),
> however only for zone DMA. Normal and Movable remained at 0. No
> progress with avoiding page alloc failures - a gap between 'free' and
> 'free_cma' was huge, so I don't think that CMA itself would be a root
> cause.
>
>> Did you establish why GFP_ATOMIC (assuming that's the failing site) had
>> not specified __GFP_ATOMIC at the time of the allocation failure?
>>
>
> Yes. It happens in new_slab() in following lines:
> return allocate_slab(s, flags & (GFP_RECLAIM_MASK | GFP_CONSTRAINT_MASK), node);
> I added "| GFP_ATOMIC" and in such case I got same dumps but with one
> bit set more in gfp_mask, so I don't think it's an issue.
>
> Latest patches in v4.7-rc1 seem to boost page alloc performance enough
> to avoid problems observed between v4.2 and v4.6. Hence before
> rebasing from v4.4 to another LTS >v4.7 in future, we decided as a WA
> to return to using MIGRATE_RESERVE + adding fix for
> early_page_nid_uninitialised(). Now operation seems stable on all our
> SoC's during the tests.
>
> Best regards,
> Marcin

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