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SubjectRE: [PATCH v2] mm: show free pages per each migrate type
Date

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> Date: Fri, 10 Apr 2015 19:50:07 -0700
> From: rientjes@google.com
> To: neilzhang1123@hotmail.com
> CC: linux-mm@kvack.org; linux-kernel@vger.kernel.org; akpm@linux-foundation.org
> Subject: Re: [PATCH v2] mm: show free pages per each migrate type
>
> On Thu, 9 Apr 2015, Neil Zhang wrote:
>
>> show detailed free pages per each migrate type in show_free_areas.
>>
>> After apply this patch, the log printed out will be changed from
>>
>> [ 558.212844@0] Normal: 218*4kB (UEMC) 207*8kB (UEMC) 126*16kB (UEMC) 21*32kB (UC) 5*64kB (C) 3*128kB (C) 1*256kB (C) 1*512kB (C) 0*1024kB 0*2048kB 1*4096kB (R) = 10784kB
>> [ 558.227840@0] HighMem: 3*4kB (UMR) 3*8kB (UMR) 2*16kB (UM) 3*32kB (UMR) 0*64kB 1*128kB (M) 1*256kB (R) 0*512kB 0*1024kB 0*2048kB 0*4096kB = 548kB
>>
>> to
>>
>> [ 806.506450@1] Normal: 8969*4kB 4370*8kB 2*16kB 3*32kB 2*64kB 3*128kB 3*256kB 1*512kB 0*1024kB 1*2048kB 0*4096kB = 74804kB
>> [ 806.517456@1] orders: 0 1 2 3 4 5 6 7 8 9 10
>> [ 806.527077@1] Unmovable: 8287 4370 0 0 0 0 0 0 0 0 0
>> [ 806.536699@1] Reclaimable: 681 0 0 0 0 0 0 0 0 0 0
>> [ 806.546321@1] Movable: 1 0 0 0 0 0 0 0 0 0 0
>> [ 806.555942@1] Reserve: 0 0 2 3 2 3 3 1 0 1 0
>> [ 806.565564@1] CMA: 0 0 0 0 0 0 0 0 0 0 0
>> [ 806.575187@1] Isolate: 0 0 0 0 0 0 0 0 0 0 0
>> [ 806.584810@1] HighMem: 80*4kB 15*8kB 0*16kB 0*32kB 0*64kB 0*128kB 0*256kB 0*512kB 0*1024kB 0*2048kB 0*4096kB = 440kB
>> [ 806.595383@1] orders: 0 1 2 3 4 5 6 7 8 9 10
>> [ 806.605004@1] Unmovable: 12 0 0 0 0 0 0 0 0 0 0
>> [ 806.614626@1] Reclaimable: 0 0 0 0 0 0 0 0 0 0 0
>> [ 806.624248@1] Movable: 11 15 0 0 0 0 0 0 0 0 0
>> [ 806.633869@1] Reserve: 57 0 0 0 0 0 0 0 0 0 0
>> [ 806.643491@1] CMA: 0 0 0 0 0 0 0 0 0 0 0
>> [ 806.653113@1] Isolate: 0 0 0 0 0 0 0 0 0 0 0
>>
>> Signed-off-by: Neil Zhang <neilzhang1123@hotmail.com>
>
> Sorry, this is just way too verbose. This output is emitted to the kernel
> log on oom kill and since we lack a notification mechanism on system oom,
> the _only_ way for userspace to detect oom kills that have occurred is by
> scraping the kernel log. This is exactly what we do, and we have missed
> oom kill events because they scroll from the ring buffer due to excessive
> output such as this, which is why output was limited with the
> show_free_areas() filter in the first place. Just because oom kill output
> is much less than it has been in the past, for precisely this reason,
> doesn't mean we can make it excessive again.
>

Just like you said, OOM kill is much less than before, but we still need to analyze it when 
it happens on a mobile device. It can give more detailed info for us when debugging.
 
Besides OOM kill, we also can check the memory usages in runtime by echo 'm' to sysRq.
It can help us to  find out code defect sometimes, for example, we even found that the NR_FREE_CMA
memory was not align with the total CMA pages in the free list showed by this patch.

> So nack on this patch, and if we really need to have this information (I
> don't know your motivation for adding it since you list none in your
> changelog), then we need to consider an oom verbosity sysctl or, better,
> an actual system oom notification to userspace based on eventfd() without
> requiring memcg.

Best Regards,
Neil Zhang

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