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Subject[RFC PATCH v1 0/3] kdump, vmcore: Map vmcore memory in direct mapping region
Currently, kdump reads the 1st kernel's memory, called old memory in
the source code, using ioremap per a single page. This causes big
performance degradation since page tables modification and tlb flush
happen each time the single page is read.

This issue turned out from Cliff's kernel-space filtering work.

To avoid calling ioremap, we map a whole 1st kernel's memory targeted
as vmcore regions in direct mapping table. By this we got big
performance improvement. See the following simple benchmark.

Machine spec:

| CPU | Intel(R) Xeon(R) CPU E7- 4820 @ 2.00GHz (4 sockets, 8 cores) (*) |
| Memory | 32 GB |
| Kernel | 3.7 vanilla and with this patch set |

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


I executed the following commands on the 2nd kernel and recorded real

$ time dd bs=$((4096 * n)) if=/proc/vmcore of=/dev/null

[3.7 vanilla]

| block size | time | performance |
| [KB] | | [MB/sec] |
| 4 | 5m 46.97s | 93.56 |
| 8 | 4m 20.68s | 124.52 |
| 16 | 3m 37.85s | 149.01 |

[3.7 with this patch]

| block size | time | performance |
| [KB] | | [GB/sec] |
| 4 | 17.59s | 1.85 |
| 8 | 14.73s | 2.20 |
| 16 | 14.26s | 2.28 |
| 32 | 13.38s | 2.43 |
| 64 | 12.77s | 2.54 |
| 128 | 12.41s | 2.62 |
| 256 | 12.50s | 2.60 |
| 512 | 12.37s | 2.62 |
| 1024 | 12.30s | 2.65 |
| 2048 | 12.29s | 2.64 |
| 4096 | 12.32s | 2.63 |

[perf bench]

I also did perf bench mem memcpy -o on the 2nd kenrel like:

# /var/crash/perf bench mem memcpy -o -l 128MB
# Running mem/memcpy benchmark...
# Copying 128MB Bytes ...

2.854337 GB/Sec (with prefault)

Several trials stably showed around 2.85 [GB/Sec].


* Why direct mapping region

I chose direct mapping region because this address space has 64TB
length to cover a whole physical memory while vmlloc-and-ioremap
region has 16TB only. For some particular machine with huge memory,
the latter is already problematic.

In the near future, machine with more than 64TB could occur, but
then direct mapping space would also be extended to follow.

* Memory consumption issue on the 2nd kenrel

Typical reserved memory size for the 2nd kerne is 512MB. But if
mapping tera-byte memory with 4kB pages, page table size amounts to
more than giga bytes.

But direct mapping region is mapped using 1GB and 2MB pages. By
this, memory consumption for page table is minimamized in most

Boot debug message tells you how each map is mapped:

vmcore: [oldmem 0000000027000000-000000002708afff]
vmcore: [oldmem 0000000000100000-0000000026ffffff]
vmcore: [oldmem 0000000037000000-000000007b00cfff]
vmcore: [oldmem 0000000100000000-000000087fffffff]
[mem 0x27000000-0x2708afff] page 4k
[mem 0x00100000-0x001fffff] page 4k
[mem 0x00200000-0x26ffffff] page 2M
[mem 0x37000000-0x7affffff] page 2M
[mem 0x7b000000-0x7b00cfff] page 4k
[mem 0x100000000-0x87fffffff] page 1G

where each [oldmem <start>-<end>] is mapped region and I omited some
other messages.


* Use of init_memory_mapping

init_memory_mapping is used to map memory in direct mapping region
both in boot time and memory hot-plug codes. This should be used
here too, but just as I explain in the patch description, I faced
some page-fault related bugs after it was called in the 2nd kernel
boot. This means page table mapping is not done correctly.

As a workaround, I wrote the code constructing page table from
scratch just like Cliff's patch, and it works well aparently now.

But ideally it's necessary to know why init_memory_mapping doesn't
work well. I continue to debug this. Sugestion around this is very
helpful. This issue comes purely from lack of my familiality around
here (^^;

* Benchmark of Cliff's kernel-space filtering

He has attempted kernel-space filtering of makedumpfile for
performance improvement. I noticed the ioremap issue through his
this work.

I now think bad performance is mainly caused by the ioremap issue. I
don't know how much filtering performance is improved by doing it in
kernel-space. I guess there's just a similar improvement just like
increasing block size just as the above benchmark.

Anyway, we need first to compare kernel-space filtering with
user-space one.

Note that this work is orthogonal to kernel-space filtering, can be
proceeded separately.


HATAYAMA Daisuke (3):
vmcore: read vmcore through direct mapping region
vmcore: map vmcore memory in direct mapping region
vmcore: Add function to merge memory mapping of vmcore

fs/proc/vmcore.c | 420 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
1 files changed, 419 insertions(+), 1 deletions(-)



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