Re: [PATCH] mm: make allocation counters per-order

[Mel Gorman]

On Thu, Jul 06, 2017 at 02:04:31PM +0100, Roman Gushchin wrote:
> High-order allocations are obviously more costly, and it's very useful
> to know how many of them happens, if there are any issues
> (or suspicions) with memory fragmentation.
> 
> This commit changes existing per-zone allocation counters to be
> per-zone per-order. These counters are displayed using a new
> procfs interface (similar to /proc/buddyinfo):
> 
> $ cat /proc/allocinfo
>      DMA          0          0          0          0          0 \
>        0          0          0          0          0          0
>    DMA32          3          0          1          0          0 \
>        0          0          0          0          0          0
>   Normal    4997056      23594      10902      23686        931 \
>       23        122        786         17          1          0
>  Movable          0          0          0          0          0 \
>        0          0          0          0          0          0
>   Device          0          0          0          0          0 \
>        0          0          0          0          0          0
> 
> The existing vmstat interface remains untouched*, and still shows
> the total number of single page allocations, so high-order allocations
> are represented as a corresponding number of order-0 allocations.
> 
> $ cat /proc/vmstat | grep alloc
> pgalloc_dma 0
> pgalloc_dma32 7
> pgalloc_normal 5461660
> pgalloc_movable 0
> pgalloc_device 0
> 
> * I've added device zone for consistency with other zones,
> and to avoid messy exclusion of this zone in the code.
> 

The alloc counter updates are themselves a surprisingly heavy cost to
the allocation path and this makes it worse for a debugging case that is
relatively rare. I'm extremely reluctant for such a patch to be added
given that the tracepoints can be used to assemble such a monitor even
if it means running a userspace daemon to keep track of it. Would such a
solution be suitable? Failing that if this is a severe issue, would it be
possible to at least make this a compile-time or static tracepoint option?
That way, only people that really need it have to take the penalty.

-- 
Mel Gorman
SUSE Labs