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SubjectRe: [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.

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.


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