Re: alloc_contig_range() with MIGRATE_MOVABLE performance regression since 4.9

[David Hildenbrand]

On 22.04.21 19:50, Florian Fainelli wrote:
> On 4/22/2021 1:56 AM, David Hildenbrand wrote:
> > On 22.04.21 09:49, Michal Hocko wrote:
> > > Cc David and Oscar who are familiar with this code as well.
> > >
> > > On Wed 21-04-21 11:36:01, Florian Fainelli wrote:
> > > > Hi all,
> > > >
> > > > I have been trying for the past few days to identify the source of a
> > > > performance regression that we are seeing with the 5.4 kernel but not
> > > > with the 4.9 kernel on ARM64. Testing something newer like 5.10 is a bit
> > > > challenging at the moment but will happen eventually.
> > > >
> > > > What we are seeing is a ~3x increase in the time needed for
> > > > alloc_contig_range() to allocate 1GB in blocks of 2MB pages. The system
> > > > is idle at the time and there are no other contenders for memory other
> > > > than the user-space programs already started (DHCP client, shell, etc.).
> >
> > Hi,
> >
> > If you can easily reproduce it might be worth to just try bisecting;
> > that could be faster than manually poking around in the code.
> >
> > Also, it would be worth having a look at the state of upstream Linux.
> > Upstream Linux developers tend to not care about minor performance
> > regressions on oldish kernels.
>
> This is a big pain point here and I cannot agree more, but until we
> bridge that gap, this is not exactly easy to do for me unfortunately and
> neither is bisection :/
>
> > There has been work on improving exactly the situation you are
> > describing -- a "fail fast" / "no retry" mode for alloc_contig_range().
> > Maybe it tackles exactly this issue.
> >
> > https://lkml.kernel.org/r/20210121175502.274391-3-minchan@xxxxxxxxxx
> >
> > Minchan is already on cc.
>
> This patch does not appear to be helping, in fact, I had locally applied
> this patch from way back when:
>
> https://lkml.org/lkml/2014/5/28/113
>
> which would effectively do this unconditionally. Let me see if I can
> showcase this problem a x86 virtual machine operating in similar
> conditions to ours.

How exactly are you allocating these 2MiB blocks?

Via CMA->alloc_contig_range() or via alloc_contig_range() directly? I 
assume via CMA.

For

https://lkml.kernel.org/r/20210121175502.274391-3-minchan@xxxxxxxxxx

to do its work you'll have to pass  __GFP_NORETRY to 
alloc_contig_range(). This requires CMA adaptions, from where we call 
alloc_contig_range().

--
Thanks,

David / dhildenb