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SubjectRe: Hugepages mixed with stacks in process address space
(cc linux-mm).

And thanks.

On Tue, 04 Sep 2018 17:08:34 +0200 Jacek Tomaka <Jacek.Tomaka@poczta.fm> wrote:

> Hello,
>
> I was trying to track down the performance differences of one of my applications
> between running it on kernel used in Centos 7.4 and the latest 4.x version.
> On 4.x kernels its performance depended on the run and the variability
> was more than 30%.
>
> Bisecting showed that my issue was introduced by :
> fd8526ad14c182605e42b64646344b95befd9f94 :x86/mm: Implement ASLR for
> hugetlb mappings
>
> But it was not the ASLR aspect of that commit that created the issue but the
> change from bottom-up to top-down unmapped area lookup when allocating
> huge pages.
>
> After that change, the huge page allocations could become intertwined with
> stacks. Before, the stacks and huge pages were on the other side of the process
> address space.
>
> The machine i am seeing it on is Knights Landing 7250, with 68 cores x 4
> hyper-threads.
>
> My application spawns 272 threads and each thread allocates its memory - a
> couple of 2MB huge pages and does some computation, dominated by memory
> accesses.
>
> My theory is that because KNL has 8-way 2MB TLB, when the huge pages are
> exactly 8 pages apart they collide. And this is where the variability comes from,
> if the stacks come in between, they increase chances of them colliding.
>
> I do realise that the application is (I am ) doing a few things dubiously: it
> allocates memory on each thread and each huge page separately. But i thought
> you might want to know about this behaviour change.
>
> When i allocate all my memory before i start threads, the problem goes away.
>
> /proc/PID/maps:
> After change:
> 7f5e06a00000-7f5e06c00000 rw-p 00000000 00:0f 31809 /anon_hugepage (deleted)
> 7f5e06c00000-7f5e06e00000 rw-p 00000000 00:0f 29767 /anon_hugepage (deleted)
> 7f5e06e00000-7f5e07000000 rw-p 00000000 00:0f 30787 /anon_hugepage (deleted)
> 7f5e07000000-7f5e07200000 rw-p 00000000 00:0f 30786 /anon_hugepage (deleted)
> 7f5e07200000-7f5e07400000 rw-p 00000000 00:0f 28744 /anon_hugepage (deleted)
> 7f5e075ff000-7f5e07600000 ---p 00000000 00:00 0
> 7f5e07600000-7f5e07e00000 rw-p 00000000 00:00 0
> 7f5e07e00000-7f5e08000000 rw-p 00000000 00:0f 30785 /anon_hugepage (deleted)
> 7f5e08000000-7f5e08021000 rw-p 00000000 00:00 0
> 7f5e08021000-7f5e0c000000 ---p 00000000 00:00 0
> 7f5e0c000000-7f5e0c021000 rw-p 00000000 00:00 0
> 7f5e0c021000-7f5e10000000 ---p 00000000 00:00 0
> 7f5e10000000-7f5e10021000 rw-p 00000000 00:00 0
> 7f5e10021000-7f5e14000000 ---p 00000000 00:00 0
> 7f5e14200000-7f5e14400000 rw-p 00000000 00:0f 29765 /anon_hugepage (deleted)
> 7f5e14400000-7f5e14600000 rw-p 00000000 00:0f 28743 /anon_hugepage (deleted)
> 7f5e14600000-7f5e14800000 rw-p 00000000 00:0f 29764 /anon_hugepage (deleted)
> (...)
>
> Before change:
> 2aaaaac00000-2aaaaae00000 rw-p 00000000 00:0f 25582 /anon_hugepage (deleted)
> 2aaaaae00000-2aaaab000000 rw-p 00000000 00:0f 25583 /anon_hugepage (deleted)
> 2aaaab000000-2aaaab200000 rw-p 00000000 00:0f 25584 /anon_hugepage (deleted)
> 2aaaab200000-2aaaab400000 rw-p 00000000 00:0f 25585 /anon_hugepage (deleted)
> 2aaaab400000-2aaaab600000 rw-p 00000000 00:0f 25601 /anon_hugepage (deleted)
> 2aaaab600000-2aaaab800000 rw-p 00000000 00:0f 25599 /anon_hugepage (deleted)
> 2aaaab800000-2aaaaba00000 rw-p 00000000 00:0f 25602 /anon_hugepage (deleted)
> 2aaaaba00000-2aaaabc00000 rw-p 00000000 00:0f 26652 /anon_hugepage (deleted)
> (...)
> 7fc4f0021000-7fc4f4000000 ---p 00000000 00:00 0
> 7fc4f4000000-7fc4f4021000 rw-p 00000000 00:00 0
> 7fc4f4021000-7fc4f8000000 ---p 00000000 00:00 0
> 7fc4f8000000-7fc4f8021000 rw-p 00000000 00:00 0
> 7fc4f8021000-7fc4fc000000 ---p 00000000 00:00 0
> 7fc4fc000000-7fc4fc021000 rw-p 00000000 00:00 0
> 7fc4fc021000-7fc500000000 ---p 00000000 00:00 0
> 7fc500000000-7fc500021000 rw-p 00000000 00:00 0
> 7fc500021000-7fc504000000 ---p 00000000 00:00 0
> 7fc504000000-7fc504021000 rw-p 00000000 00:00 0
> 7fc504021000-7fc508000000 ---p 00000000 00:00 0
> 7fc508000000-7fc508021000 rw-p 00000000 00:00 0
> 7fc508021000-7fc50c000000 ---p 00000000 00:00 0
> (...)
>
> I was wondering if this intertwined stacks and hugepages is an expected
> feature of ASLR? If not, maybe mmap's MAP_STACK flag could finally start
> to be used by the kernel to keep all the stacks together in process address
> space?
>
> Or should users just not allocate huge pages on separate threads?
>
> MAP_STACK could also be used to mark a VMA as a mapping for stack,
> (if there are flags left) to re-implement:
> 65376df582174ffcec9e6471bf5b0dd79ba05e4a proc: revert /proc/<pid>/maps [stack:TID] annotation
> correctly, as having these pieces of information in place would greatly
> simplify my investigation.
>
> Regards.
> Jacek Tomaka

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