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Subject[PATCH 00/13] kdump, vmcore: support mmap() on /proc/vmcore
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Currently, read to /proc/vmcore is done by read_oldmem() that uses
ioremap/iounmap per a single page. For example, if memory is 1GB,
ioremap/iounmap is called (1GB / 4KB)-times, that is, 262144
times. This causes big performance degradation.

To address the issue, this patch implements mmap() on /proc/vmcore to
improve read performance. My simple benchmark shows the improvement
from 200 [MiB/sec] to over 50.0 [GiB/sec].

Benchmark
=========
= Machine spec
- CPU: Intel(R) Xeon(R) CPU E7- 4820 @ 2.00GHz (4 sockets, 8 cores) (*)
- memory: 32GB
- kernel: 3.8-rc6 with this patch
- vmcore size: 31.7GB

(*) only 1 cpu is used in the 2nd kernel now.

= Benchmark Case

1) copy /proc/vmcore *WITHOUT* mmap() on /proc/vmcore

$ time dd bs=4096 if=/proc/vmcore of=/dev/null
8307246+1 records in
8307246+1 records out
real 2m 31.50s
user 0m 1.06s
sys 2m 27.60s

So performance is 214.26 [MiB/sec].

2) copy /proc/vmcore with mmap()

I ran the next command and recorded real time:

$ for n in $(seq 1 15) ; do \
> time copyvmcore2 --blocksize=$((4096 * (1 << (n - 1)))) /proc/vmcore /dev/null \
> done
where copyvmcore2 is an ad-hoc test tool that read data from
/proc/vmcore via mmap() in given block-size unit and write them to
some file.

| n | map size | time | page table | performance |
| | | (sec) | | [GiB/sec] |
|----+----------+-------+------------+-------------|
| 1 | 4 KiB | 78.35 | 8 iB | 0.40 |
| 2 | 8 KiB | 45.29 | 16 iB | 0.70 |
| 3 | 16 KiB | 23.82 | 32 iB | 1.33 |
| 4 | 32 KiB | 12.90 | 64 iB | 2.46 |
| 5 | 64 KiB | 6.13 | 128 iB | 5.17 |
| 6 | 128 KiB | 3.26 | 256 iB | 9.72 |
| 7 | 256 KiB | 1.86 | 512 iB | 17.04 |
| 8 | 512 KiB | 1.13 | 1 KiB | 28.04 |
| 9 | 1 MiB | 0.77 | 2 KiB | 41.16 |
| 10 | 2 MiB | 0.58 | 4 KiB | 54.64 |
| 11 | 4 MiB | 0.50 | 8 KiB | 63.38 |
| 12 | 8 MiB | 0.46 | 16 KiB | 68.89 |
| 13 | 16 MiB | 0.44 | 32 KiB | 72.02 |
| 14 | 32 MiB | 0.44 | 64 KiB | 72.02 |
| 15 | 64 MiB | 0.45 | 128 KiB | 70.42 |

3) copy /proc/vmcore with mmap() on /dev/oldmem

I posted another patch series for mmap() on /dev/oldmem a few weeks ago.
See: https://lkml.org/lkml/2013/2/3/431

Next is the table shown on the post showing the benchmark.

| n | map size | time | page table | performance |
| | | (sec) | | [GiB/sec] |
|----+----------+-------+------------+-------------|
| 1 | 4 KiB | 41.86 | 8 iB | 0.76 |
| 2 | 8 KiB | 25.43 | 16 iB | 1.25 |
| 3 | 16 KiB | 13.28 | 32 iB | 2.39 |
| 4 | 32 KiB | 7.20 | 64 iB | 4.40 |
| 5 | 64 KiB | 3.45 | 128 iB | 9.19 |
| 6 | 128 KiB | 1.82 | 256 iB | 17.42 |
| 7 | 256 KiB | 1.03 | 512 iB | 30.78 |
| 8 | 512 KiB | 0.61 | 1K iB | 51.97 |
| 9 | 1 MiB | 0.41 | 2K iB | 77.32 |
| 10 | 2 MiB | 0.32 | 4K iB | 99.06 |
| 11 | 4 MiB | 0.27 | 8K iB | 117.41 |
| 12 | 8 MiB | 0.24 | 16 KiB | 132.08 |
| 13 | 16 MiB | 0.23 | 32 KiB | 137.83 |
| 14 | 32 MiB | 0.22 | 64 KiB | 144.09 |
| 15 | 64 MiB | 0.22 | 128 KiB | 144.09 |

= Discussion

- For small map size, we can see performance degradation on mmap()
case due to many page table modification and TLB flushes similarly
to read_oldmem() case. But for large map size we can see the
improved performance.

Each application need to choose appropreate map size for their
preferable performance.

- mmap() on /dev/oldmem appears better than that on /proc/vmcore. But
actual processing does not only copying but also IO work. This
difference is not a problem.

- Both mmap() case shows drastically better performance than previous
RFC patch set's about 2.5 [GiB/sec] that maps all dump target memory
in kernel direct mapping address space. This is because there's no
longer memcpy() from kernel-space to user-space.

Design
======
= Support Range

- mmap() on /proc/vmcore is supported on ELF64 interface only. ELF32
interface is used only if dump target size is less than 4GB. Then,
the existing interface is enough in performance.

= Change of /proc/vmcore format

For mmap()'s page-size boundary requirement, /proc/vmcore changed its
own shape and now put its objects in page-size boundary.

- Allocate buffer for ELF headers in page-size boundary.
=> See [PATCH 01/13].

- Note objects scattered on old memory are copied in a single
page-size aligned buffer on 2nd kernel, and it is remapped to
user-space.
=> See [PATCH 09/13].

- The head and/or tail pages of memroy chunks are also copied on 2nd
kernel if either of their ends is not page-size aligned. See
=> See [PATCH 12/13].

= 32-bit PAE limitation

- On 32-bit PAE limitation, mmap_vmcore() can handle upto 16TB memory
only since remap_pfn_range()'s third argument, pfn, has 32-bit
length only, defined as unsigned long type.

TODO
====
- fix makedumpfile to use mmap() on /proc/vmcore and benchmark it to
confirm whether we can see enough performance improvement.

Test
====
Done on x86-64, x86-32 both with 1GB and over 4GB memory environments.

---
HATAYAMA Daisuke (13):
vmcore: introduce mmap_vmcore()
vmcore: copy non page-size aligned head and tail pages in 2nd kernel
vmcore: count holes generated by round-up operation for vmcore size
vmcore: round-up offset of vmcore object in page-size boundary
vmcore: copy ELF note segments in buffer on 2nd kernel
vmcore: remove unused helper function
vmcore: modify read_vmcore() to read buffer on 2nd kernel
vmcore: modify vmcore clean-up function to free buffer on 2nd kernel
vmcore: modify ELF32 code according to new type
vmcore: introduce types for objects copied in 2nd kernel
vmcore: fill unused part of buffer for ELF headers with 0
vmcore: round up buffer size of ELF headers by PAGE_SIZE
vmcore: allocate buffer for ELF headers on page-size alignment


fs/proc/vmcore.c | 408 +++++++++++++++++++++++++++++++++++------------
include/linux/proc_fs.h | 11 +
2 files changed, 313 insertions(+), 106 deletions(-)
--

Thanks.
HATAYAMA, Daisuke


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