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SubjectRe: [PATCH 4/4] tracing, page-allocator: Add a postprocessing script for page-allocator-related ftrace events
3
* Andrew Morton <akpm@linux-foundation.org> wrote:

> > This patch adds a simple post-processing script for the
> > page-allocator-related trace events. It can be used to give an
> > indication of who the most allocator-intensive processes are and
> > how often the zone lock was taken during the tracing period.
> > Example output looks like
> >
> > find-2840
> > o pages allocd = 1877
> > o pages allocd under lock = 1817
> > o pages freed directly = 9
> > o pcpu refills = 1078
> > o migrate fallbacks = 48
> > - fragmentation causing = 48
> > - severe = 46
> > - moderate = 2
> > - changed migratetype = 7
>
> The usual way of accumulating and presenting such measurements is
> via /proc/vmstat. How do we justify adding a completely new and
> different way of doing something which we already do?

/proc/vmstat has a couple of technical and usage disadvantages:

- it is pretty coarse - all-of-system, nothing else

- expensive to read (have to read the full file with all fields)

- has to be polled, has no notion for events

- it does not offer sampling of workloads

- it does not allow the separation of workloads: you cannot measure
just a single workload, you cannot measure just a single process,
nor a single CPU.

Incidentally there's an upstream kernel instrumentation and
statistics framework that solves all the above disadvantages of
/proc/vmstat:

- it is finegrained: per task or per workload or per cpu or full system

- cheap to read - the counts can be accessed individually

- is event based, can be poll()ed

- offers sampling of workloads, of any subset of these values

- it allows easy separation of workloads

All that is needed are the patches form Mel and Rik and it's
plug-and-play.

Let me demonstrate these features in action (i've applied the
patches for testing to -tip):

First, discovery/enumeration of available counters can be done via
'perf list':

titan:~> perf list
[...]
kmem:kmalloc [Tracepoint event]
kmem:kmem_cache_alloc [Tracepoint event]
kmem:kmalloc_node [Tracepoint event]
kmem:kmem_cache_alloc_node [Tracepoint event]
kmem:kfree [Tracepoint event]
kmem:kmem_cache_free [Tracepoint event]
kmem:mm_page_free_direct [Tracepoint event]
kmem:mm_pagevec_free [Tracepoint event]
kmem:mm_page_alloc [Tracepoint event]
kmem:mm_page_alloc_zone_locked [Tracepoint event]
kmem:mm_page_pcpu_drain [Tracepoint event]
kmem:mm_page_alloc_extfrag [Tracepoint event]

Then any (or all) of the above event sources can be activated and
measured. For example the page alloc/free properties of a 'hackbench
run' are:

titan:~> perf stat -e kmem:mm_page_pcpu_drain -e kmem:mm_page_alloc
-e kmem:mm_pagevec_free -e kmem:mm_page_free_direct ./hackbench 10
Time: 0.575

Performance counter stats for './hackbench 10':

13857 kmem:mm_page_pcpu_drain
27576 kmem:mm_page_alloc
6025 kmem:mm_pagevec_free
20934 kmem:mm_page_free_direct

0.613972165 seconds time elapsed

You can observe the statistical properties as well, by using the
'repeat the workload N times' feature of perf stat:

titan:~> perf stat --repeat 5 -e kmem:mm_page_pcpu_drain -e
kmem:mm_page_alloc -e kmem:mm_pagevec_free -e
kmem:mm_page_free_direct ./hackbench 10
Time: 0.627
Time: 0.644
Time: 0.564
Time: 0.559
Time: 0.626

Performance counter stats for './hackbench 10' (5 runs):

12920 kmem:mm_page_pcpu_drain ( +- 3.359% )
25035 kmem:mm_page_alloc ( +- 3.783% )
6104 kmem:mm_pagevec_free ( +- 0.934% )
18376 kmem:mm_page_free_direct ( +- 4.941% )

0.643954516 seconds time elapsed ( +- 2.363% )

Furthermore, these tracepoints can be used to sample the workload as
well. For example the page allocations done by a 'git gc' can be
captured the following way:

titan:~/git> perf record -f -e kmem:mm_page_alloc -c 1 ./git gc
Counting objects: 1148, done.
Delta compression using up to 2 threads.
Compressing objects: 100% (450/450), done.
Writing objects: 100% (1148/1148), done.
Total 1148 (delta 690), reused 1148 (delta 690)
[ perf record: Captured and wrote 0.267 MB perf.data (~11679 samples) ]

To check which functions generated page allocations:

titan:~/git> perf report
# Samples: 10646
#
# Overhead Command Shared Object
# ........ ............... ..........................
#
23.57% git-repack /lib64/libc-2.5.so
21.81% git /lib64/libc-2.5.so
14.59% git ./git
11.79% git-repack ./git
7.12% git /lib64/ld-2.5.so
3.16% git-repack /lib64/libpthread-2.5.so
2.09% git-repack /bin/bash
1.97% rm /lib64/libc-2.5.so
1.39% mv /lib64/ld-2.5.so
1.37% mv /lib64/libc-2.5.so
1.12% git-repack /lib64/ld-2.5.so
0.95% rm /lib64/ld-2.5.so
0.90% git-update-serv /lib64/libc-2.5.so
0.73% git-update-serv /lib64/ld-2.5.so
0.68% perf /lib64/libpthread-2.5.so
0.64% git-repack /usr/lib64/libz.so.1.2.3
Or to see it on a more finegrained level:

titan:~/git> perf report --sort comm,dso,symbol
# Samples: 10646
#
# Overhead Command Shared Object Symbol
# ........ ............... .......................... ......
#
9.35% git-repack ./git [.] insert_obj_hash
9.12% git ./git [.] insert_obj_hash
7.31% git /lib64/libc-2.5.so [.] memcpy
6.34% git-repack /lib64/libc-2.5.so [.] _int_malloc
6.24% git-repack /lib64/libc-2.5.so [.] memcpy
5.82% git-repack /lib64/libc-2.5.so [.] __GI___fork
5.47% git /lib64/libc-2.5.so [.] _int_malloc
2.99% git /lib64/libc-2.5.so [.] memset
Furthermore, call-graph sampling can be done too, of page
allocations - to see precisely what kind of page allocations there
are:

titan:~/git> perf record -f -g -e kmem:mm_page_alloc -c 1 ./git gc
Counting objects: 1148, done.
Delta compression using up to 2 threads.
Compressing objects: 100% (450/450), done.
Writing objects: 100% (1148/1148), done.
Total 1148 (delta 690), reused 1148 (delta 690)
[ perf record: Captured and wrote 0.963 MB perf.data (~42069 samples) ]

titan:~/git> perf report -g
# Samples: 10686
#
# Overhead Command Shared Object
# ........ ............... ..........................
#
23.25% git-repack /lib64/libc-2.5.so
|
|--50.00%-- _int_free
|
|--37.50%-- __GI___fork
| make_child
|
|--12.50%-- ptmalloc_unlock_all2
| make_child
|
--6.25%-- __GI_strcpy
21.61% git /lib64/libc-2.5.so
|
|--30.00%-- __GI_read
| |
| --83.33%-- git_config_from_file
| git_config
| |
[...]
Or you can observe the whole system's page allocations for 10
seconds:

titan:~/git> perf stat -a -e kmem:mm_page_pcpu_drain -e
kmem:mm_page_alloc -e kmem:mm_pagevec_free -e
kmem:mm_page_free_direct sleep 10

Performance counter stats for 'sleep 10':

171585 kmem:mm_page_pcpu_drain
322114 kmem:mm_page_alloc
73623 kmem:mm_pagevec_free
254115 kmem:mm_page_free_direct

10.000591410 seconds time elapsed

Or observe how fluctuating the page allocations are, via statistical
analysis done over ten 1-second intervals:

titan:~/git> perf stat --repeat 10 -a -e kmem:mm_page_pcpu_drain -e
kmem:mm_page_alloc -e kmem:mm_pagevec_free -e
kmem:mm_page_free_direct sleep 1

Performance counter stats for 'sleep 1' (10 runs):

17254 kmem:mm_page_pcpu_drain ( +- 3.709% )
34394 kmem:mm_page_alloc ( +- 4.617% )
7509 kmem:mm_pagevec_free ( +- 4.820% )
25653 kmem:mm_page_free_direct ( +- 3.672% )

1.058135029 seconds time elapsed ( +- 3.089% )

Or you can annotate the recorded 'git gc' run on a per symbol basis
and check which instructions/source-code generated page allocations:

titan:~/git> perf annotate __GI___fork
------------------------------------------------
Percent | Source code & Disassembly of libc-2.5.so
------------------------------------------------
:
:
: Disassembly of section .plt:
: Disassembly of section .text:
:
: 00000031a2e95560 <__fork>:
[...]
0.00 : 31a2e95602: b8 38 00 00 00 mov $0x38,%eax
0.00 : 31a2e95607: 0f 05 syscall
83.42 : 31a2e95609: 48 3d 00 f0 ff ff cmp $0xfffffffffffff000,%rax
0.00 : 31a2e9560f: 0f 87 4d 01 00 00 ja 31a2e95762 <__fork+0x202>
0.00 : 31a2e95615: 85 c0 test %eax,%eax
( this shows that 83.42% of __GI___fork's page allocations come from
the 0x38 system call it performs. )

etc. etc. - a lot more is possible. I could list a dozen of
other different usecases straight away - neither of which is
possible via /proc/vmstat.

/proc/vmstat is not in the same league really, in terms of
expressive power of system analysis and performance
analysis.

All that the above results needed were those new tracepoints
in include/tracing/events/kmem.h.

Ingo


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