Re: [PATCH 2/2] mm/page_ref: add tracepoint to track down page reference manipulation

From: Michal Nazarewicz
Date: Tue Nov 10 2015 - 11:02:53 EST


On Mon, Nov 09 2015, Joonsoo Kim wrote:
> CMA allocation should be guaranteed to succeed by definition,

Uh? Thatâs a peculiar statement. Which is to say that itâs not true.

> but,
> unfortunately, it would be failed sometimes. It is hard to track down
> the problem, because it is related to page reference manipulation and
> we don't have any facility to analyze it.
>
> This patch adds tracepoints to track down page reference manipulation.
> With it, we can find exact reason of failure and can fix the problem.
> Following is an example of tracepoint output.
>
> <...>-9018 [004] 92.678375: page_ref_set: pfn=0x17ac9 flags=0x0 count=1 mapcount=0 mapping=(nil) mt=4 val=1
> <...>-9018 [004] 92.678378: kernel_stack:
> => get_page_from_freelist (ffffffff81176659)
> => __alloc_pages_nodemask (ffffffff81176d22)
> => alloc_pages_vma (ffffffff811bf675)
> => handle_mm_fault (ffffffff8119e693)
> => __do_page_fault (ffffffff810631ea)
> => trace_do_page_fault (ffffffff81063543)
> => do_async_page_fault (ffffffff8105c40a)
> => async_page_fault (ffffffff817581d8)
> [snip]
> <...>-9018 [004] 92.678379: page_ref_mod: pfn=0x17ac9 flags=0x40048 count=2 mapcount=1 mapping=0xffff880015a78dc1 mt=4 val=1
> [snip]
> ...
> ...
> <...>-9131 [001] 93.174468: test_pages_isolated: start_pfn=0x17800 end_pfn=0x17c00 fin_pfn=0x17ac9 ret=fail
> [snip]
> <...>-9018 [004] 93.174843: page_ref_mod_and_test: pfn=0x17ac9 flags=0x40068 count=0 mapcount=0 mapping=0xffff880015a78dc1 mt=4 val=-1 ret=1
> => release_pages (ffffffff8117c9e4)
> => free_pages_and_swap_cache (ffffffff811b0697)
> => tlb_flush_mmu_free (ffffffff81199616)
> => tlb_finish_mmu (ffffffff8119a62c)
> => exit_mmap (ffffffff811a53f7)
> => mmput (ffffffff81073f47)
> => do_exit (ffffffff810794e9)
> => do_group_exit (ffffffff81079def)
> => SyS_exit_group (ffffffff81079e74)
> => entry_SYSCALL_64_fastpath (ffffffff817560b6)
>
> This output shows that problem comes from exit path. In exit path,
> to improve performance, pages are not freed immediately. They are gathered
> and processed by batch. During this process, migration cannot be possible
> and CMA allocation is failed. This problem is hard to find without this
> page reference tracepoint facility.
>
> Enabling this feature bloat kernel text 20 KB in my configuration.
>
> text data bss dec hex filename
> 12041272 2223424 1507328 15772024 f0a978 vmlinux_disabled
> 12064844 2225920 1507328 15798092 f10f4c vmlinux_enabled
>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

Acked-by: Michal Nazarewicz <mina86@xxxxxxxxxx>

> ---
> include/trace/events/page_ref.h | 128 ++++++++++++++++++++++++++++++++++++++++

I havenât really looked at the above file though.

--
Best regards, _ _
.o. | Liege of Serenely Enlightened Majesty of o' \,=./ `o
..o | Computer Science, ããã âmina86â ãããããã (o o)
ooo +--<mpn@xxxxxxxxxx>--<xmpp:mina86@xxxxxxxxxx>-----ooO--(_)--Ooo--
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