Re: page allocator bug in 3.16?

From: Alex Deucher
Date: Thu Sep 25 2014 - 16:33:15 EST

On Thu, Sep 25, 2014 at 2:55 PM, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote:
> After several days uptime with a 3.16 kernel (generally running
> Thunderbird, emacs, kernel builds, several Chrome tabs on multiple
> desktop workspaces) I've been seeing some really extreme slowdowns.
> Mostly the slowdowns are associated with gpu-related tasks, like
> opening new emacs windows, switching workspaces, laughing at internet
> gifs, etc. Because this x86_64 desktop is nouveau-based, I didn't pursue
> it right away -- 3.15 is the first time suspend has worked reliably.
> This week I started looking into what the slowdown was and discovered
> it's happening during dma allocation through swiotlb (the cpus can do
> intel iommu but I don't use it because it's not the default for most users).
> I'm still working on a bisection but each step takes 8+ hours to
> validate and even then I'm no longer sure I still have the 'bad'
> commit in the bisection. [edit: yup, I started over]
> I just discovered a smattering of these in my logs and only on 3.16-rc+ kernels:
> Sep 25 07:57:59 thor kernel: [28786.001300] alloc_contig_range test_pages_isolated(2bf560, 2bf562) failed
> This dual-Xeon box has 10GB and sysrq Show Memory isn't showing heavy
> fragmentation [1].
> Besides Mel's page allocator changes in 3.16, another suspect commit is:
> commit b13b1d2d8692b437203de7a404c6b809d2cc4d99
> Author: Shaohua Li <shli@xxxxxxxxxx>
> Date: Tue Apr 8 15:58:09 2014 +0800
> x86/mm: In the PTE swapout page reclaim case clear the accessed bit instead of flushing the TLB
> Specifically, this statement:
> It could cause incorrect page aging and the (mistaken) reclaim of
> hot pages, but the chance of that should be relatively low.
> I'm wondering if this could cause worse-case behavior with TTM? I'm
> testing a revert of this on mainline 3.16-final now, with no results yet.
> Thoughts?

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