Re: [RFC PATCH 2/2] arm64: Implement vmalloc based thread_info allocator

[Minchan Kim <minchan@kernel.org>]

On Mon, May 25, 2015 at 07:01:33PM +0900, Jungseok Lee wrote:
> On May 25, 2015, at 2:49 AM, Arnd Bergmann wrote:
> > On Monday 25 May 2015 01:02:20 Jungseok Lee wrote:
> >> Fork-routine sometimes fails to get a physically contiguous region for
> >> thread_info on 4KB page system although free memory is enough. That is,
> >> a physically contiguous region, which is currently 16KB, is not available
> >> since system memory is fragmented.
> >> 
> >> This patch tries to solve the problem as allocating thread_info memory
> >> from vmalloc space, not 1:1 mapping one. The downside is one additional
> >> page allocation in case of vmalloc. However, vmalloc space is large enough,
> >> around 240GB, under a combination of 39-bit VA and 4KB page. Thus, it is
> >> not a big tradeoff for fork-routine service.
> > 
> > vmalloc has a rather large runtime cost. I'd argue that failing to allocate
> > thread_info structures means something has gone very wrong.
> 
> That is why the feature is marked "N" by default.
> I focused on fork-routine stability rather than performance.

If VM has trouble with order-2 allocation, your system would be
trouble soon although fork at the moment manages to be successful
because such small high-order(ex, order <= PAGE_ALLOC_COSTLY_ORDER)
allocation is common in the kernel so VM should handle it smoothly.
If VM didn't, it means we should fix VM itself, not a specific
allocation site. Fork is one of victim by that.

> 
> Could you give me an idea how to evaluate performance degradation?
> Running some benchmarks would be helpful, but I would like to try to
> gather data based on meaningful methodology.
> 
> > Can you describe the scenario that leads to fragmentation this bad?
> 
> Android, but I could not describe an exact reproduction procedure step
> by step since it's behaved and reproduced randomly. As reading the following
> thread from mm mailing list, a similar symptom is observed on other systems. 
> 
> https://lkml.org/lkml/2015/4/28/59
> 
> Although I do not know the details of a system mentioned in the thread,
> even order-2 page allocation is not smoothly operated due to fragmentation on
> low memory system.

What Joonsoo have tackle is generic fragmentation problem, not *a* fork fail,
which is more right approach to handle small high-order allocation problem.

> 
> I think the point is *low memory system*. 64-bit kernel is usually a feasible
> option when system memory is enough, but 64-bit kernel and low memory system
> combo is not unusual in case of ARM64.
> 
> > Could the stack size be reduced to 8KB perhaps?
> 
> I guess probably not.
> 
> A commit, 845ad05e, says that 8KB is not enough to cover SpecWeb benchmark.
> The stack size is 16KB on x86_64. I am not sure whether all applications,
> which work fine on x86_64 machine, run very well on ARM64 with 8KB stack size.
> 
> Best Regards
> Jungseok Lee
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-- 
Kind regards,
Minchan Kim

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