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zircon-guest/third_party/linux/refs/heads/linux-7.3.y/./tools/perf/Documentation/perf-c2c.txt
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perf-c2c(1)
===========
NAME
----
perf-c2c - Shared Data C2C/HITM Analyzer.
SYNOPSIS
--------
[verse]
'perf c2c record' [<options>] <command>
'perf c2c record' [<options>] \-- [<record command options>] <command>
'perf c2c report' [<options>]
DESCRIPTION
-----------
C2C stands for Cache To Cache.
The perf c2c tool provides means for Shared Data C2C/HITM analysis. It allows
you to track down the cacheline contentions.
On Intel, the tool is based on load latency and precise store facility events
provided by Intel CPUs. On PowerPC, the tool uses random instruction sampling
with thresholding feature. On AMD, the tool uses IBS op pmu (due to hardware
limitations, perf c2c is not supported on Zen3 cpus). On Arm64 it uses SPE to
sample load and store operations, therefore hardware and kernel support is
required. See linkperf:perf-arm-spe[1] for a setup guide. Due to the
statistical nature of Arm SPE sampling, not every memory operation will be
sampled.
These events provide:
- memory address of the access
- type of the access (load and store details)
- latency (in cycles) of the load access
The c2c tool provide means to record this data and report back access details
for cachelines with highest contention - highest number of HITM accesses.
The basic workflow with this tool follows the standard record/report phase.
User uses the record command to record events data and report command to
display it.
RECORD OPTIONS
--------------
-e::
--event=::
Select the PMU event. Use 'perf c2c record -e list'
to list available events.
-v::
--verbose::
Be more verbose (show counter open errors, etc).
-l::
--ldlat::
Configure mem-loads latency. Supported on Intel, Arm64 and some AMD
processors. Ignored on other archs.
On supported AMD processors:
- /sys/bus/event_source/devices/ibs_op/caps/ldlat file contains '1'.
- Supported latency values are 128 to 2048 (both inclusive).
- Latency value which is a multiple of 128 incurs a little less profiling
overhead compared to other values.
- Load latency filtering is disabled by default.
-k::
--all-kernel::
Configure all used events to run in kernel space.
-u::
--all-user::
Configure all used events to run in user space.
REPORT OPTIONS
--------------
-k::
--vmlinux=<file>::
vmlinux pathname
-v::
--verbose::
Be more verbose (show counter open errors, etc).
-i::
--input::
Specify the input file to process.
-N::
--node-info::
Show extra node info in report (see NODE INFO section)
-c::
--coalesce::
Specify sorting fields for single cacheline display.
Following fields are available: tid,pid,iaddr,dso
(see COALESCE)
-g::
--call-graph::
Setup callchains parameters.
Please refer to perf-report man page for details.
--stdio::
Force the stdio output (see STDIO OUTPUT)
--stats::
Display only statistic tables and force stdio mode.
--full-symbols::
Display full length of symbols.
--no-source::
Do not display Source:Line column.
--show-all::
Show all captured HITM lines, with no regard to HITM % 0.0005 limit.
-f::
--force::
Don't do ownership validation.
-d::
--display::
Switch to HITM type (rmt, lcl) or peer snooping type (peer) to display
and sort on. Total HITMs (tot) as default, except Arm64 uses peer mode
as default.
--stitch-lbr::
Show callgraph with stitched LBRs, which may have more complete
callgraph. The perf.data file must have been obtained using
perf c2c record --call-graph lbr.
Disabled by default. In common cases with call stack overflows,
it can recreate better call stacks than the default lbr call stack
output. But this approach is not foolproof. There can be cases
where it creates incorrect call stacks from incorrect matches.
The known limitations include exception handing such as
setjmp/longjmp will have calls/returns not match.
--double-cl::
Group the detection of shared cacheline events into double cacheline
granularity. Some architectures have an Adjacent Cacheline Prefetch
feature, which causes cacheline sharing to behave like the cacheline
size is doubled.
-M::
--disassembler-style=::
Set disassembler style for objdump.
--objdump=<path>::
Path to objdump binary.
C2C RECORD
----------
The perf c2c record command setup options related to HITM cacheline analysis
and calls standard perf record command.
Following perf record options are configured by default:
(check perf record man page for details)
-W,-d,--phys-data,--sample-cpu
The following table lists the events monitored on different architectures.
Unless specified otherwise with the -e option, the tool will select the
default events.
+--------+---------------+-----------------+--------------------------------------------------------------------------------+
| Arch | Configuration | Options | Events |
+--------+---------------+-----------------+--------------------------------------------------------------------------------+
| Intel | Default | -e ldlat-loads | cpu/mem-loads,ldlat=30/P |
| | | -e ldlat-stores | cpu/mem-stores/P |
| |---------------+-----------------+--------------------------------------------------------------------------------+
| | Load only | -e ldlat-loads | cpu/mem-loads,ldlat=30/P |
| |---------------+-----------------+--------------------------------------------------------------------------------+
| | Store only | -e ldlat-stores | cpu/mem-stores/P |
+--------+---------------+-----------------+--------------------------------------------------------------------------------+
| Intel | Default | -e ldlat-loads | {cpu/mem-loads-aux/,cpu/mem-loads,ldlat=30/}:P |
| with | | -e ldlat-stores | cpu/mem-stores/P |
| AUX |--------------+------------------+--------------------------------------------------------------------------------+
| | Load only | -e ldlat-loads | {cpu/mem-loads-aux/,cpu/mem-loads,ldlat=30/}:P |
| |---------------+-----------------+--------------------------------------------------------------------------------+
| | Store only | -e ldlat-stores | cpu/mem-stores/P |
+--------+---------------+-----------------+--------------------------------------------------------------------------------+
| AMD | Default | -e mem-ldst | ibs_op// (without latency support) |
| | | | ibs_op/ldlat=30/ (with latency support) |
+--------+---------------+-----------------+--------------------------------------------------------------------------------+
| PowerPC| Default | -e ldlat-loads | cpu/mem-loads/ |
| | | -e ldlat-stores | cpu/mem-stores/ |
| |---------------+-----------------+--------------------------------------------------------------------------------+
| | Load only | -e ldlat-loads | cpu/mem-loads/ |
| |---------------+-----------------+--------------------------------------------------------------------------------+
| | Store only | -e ldlat-stores | cpu/mem-stores/ |
+--------+---------------+-----------------+--------------------------------------------------------------------------------+
| Arm | Default | -e spe-ldst | arm_spe_0/ts_enable=1,pa_enable=1,load_filter=1,store_filter=1,min_latency=30/ |
| SPE |---------------+-----------------+--------------------------------------------------------------------------------+
| | Load only | -e spe-load | arm_spe_0/ts_enable=1,pa_enable=1,load_filter=1,min_latency=30/ |
| |---------------+-----------------+--------------------------------------------------------------------------------+
| | Store only | -e spe-store | arm_spe_0/ts_enable=1,pa_enable=1,store_filter=1/ |
+--------+---------------+-----------------+--------------------------------------------------------------------------------+
User can pass any 'perf record' option behind '--' mark, like (to enable
callchains and system wide monitoring):
$ perf c2c record -- -g -a
Please check RECORD OPTIONS section for specific c2c record options.
C2C REPORT
----------
The perf c2c report command displays shared data analysis. It comes in two
display modes: stdio and tui (default).
The report command workflow is following:
- sort all the data based on the cacheline address
- store access details for each cacheline
- sort all cachelines based on user settings
- display data
In general perf report output consist of 2 basic views:
1) most expensive cachelines list
2) offsets details for each cacheline
For each cacheline in the 1) list we display following data:
(Both stdio and TUI modes follow the same fields output)
Index
- zero based index to identify the cacheline
Cacheline
- cacheline address (hex number)
Rmt/Lcl Hitm (Display with HITM types)
- cacheline percentage of all Remote/Local HITM accesses
Peer Snoop (Display with peer type)
- cacheline percentage of all peer accesses
LLC Load Hitm - Total, LclHitm, RmtHitm (For display with HITM types)
- count of Total/Local/Remote load HITMs
Load Peer - Total, Local, Remote (For display with peer type)
- count of Total/Local/Remote load from peer cache or DRAM
Total records
- sum of all cachelines accesses
Total loads
- sum of all load accesses
Total stores
- sum of all store accesses
Store Reference - L1Hit, L1Miss, N/A
L1Hit - store accesses that hit L1
L1Miss - store accesses that missed L1
N/A - store accesses with memory level is not available
Core Load Hit - FB, L1, L2
- count of load hits in FB (Fill Buffer), L1 and L2 cache
LLC Load Hit - LlcHit, LclHitm
- count of LLC load accesses, includes LLC hits and LLC HITMs
RMT Load Hit - RmtHit, RmtHitm
- count of remote load accesses, includes remote hits and remote HITMs;
on Arm neoverse cores, RmtHit is used to account remote accesses,
includes remote DRAM or any upward cache level in remote node
Load Dram - Lcl, Rmt
- count of local and remote DRAM accesses
For each offset in the 2) list we display following data:
HITM - Rmt, Lcl (Display with HITM types)
- % of Remote/Local HITM accesses for given offset within cacheline
Peer Snoop - Rmt, Lcl (Display with peer type)
- % of Remote/Local peer accesses for given offset within cacheline
Store Refs - L1 Hit, L1 Miss, N/A
- % of store accesses that hit L1, missed L1 and N/A (no available) memory
level for given offset within cacheline
Data address - Offset
- offset address
Pid
- pid of the process responsible for the accesses
Tid
- tid of the process responsible for the accesses
Code address
- code address responsible for the accesses
cycles - rmt hitm, lcl hitm, load (Display with HITM types)
- sum of cycles for given accesses - Remote/Local HITM and generic load
cycles - rmt peer, lcl peer, load (Display with peer type)
- sum of cycles for given accesses - Remote/Local peer load and generic load
cpu cnt
- number of cpus that participated on the access
Symbol
- code symbol related to the 'Code address' value
Shared Object
- shared object name related to the 'Code address' value
Source:Line
- source information related to the 'Code address' value
Node
- nodes participating on the access (see NODE INFO section)
NODE INFO
---------
The 'Node' field displays nodes that accesses given cacheline
offset. Its output comes in 3 flavors:
- node IDs separated by ','
- node IDs with stats for each ID, in following format:
Node{cpus %hitms %stores} (Display with HITM types)
Node{cpus %peers %stores} (Display with peer type)
- node IDs with list of affected CPUs in following format:
Node{cpu list}
User can switch between above flavors with -N option or
use 'n' key to interactively switch in TUI mode.
COALESCE
--------
User can specify how to sort offsets for cacheline.
Following fields are available and governs the final
output fields set for cacheline offsets output:
tid - coalesced by process TIDs
pid - coalesced by process PIDs
iaddr - coalesced by code address, following fields are displayed:
Code address, Code symbol, Shared Object, Source line
dso - coalesced by shared object
By default the coalescing is setup with 'pid,iaddr'.
STDIO OUTPUT
------------
The stdio output displays data on standard output.
Following tables are displayed:
Trace Event Information
- overall statistics of memory accesses
Global Shared Cache Line Event Information
- overall statistics on shared cachelines
Shared Data Cache Line Table
- list of most expensive cachelines
Shared Cache Line Distribution Pareto
- list of all accessed offsets for each cacheline
TUI OUTPUT
----------
The TUI output provides interactive interface to navigate
through cachelines list and to display offset details.
Pressing the 'TAB' key in the cacheline view switches to the function
view. The function view shows a three-level hierarchy of the symbolized
entries retained in the cacheline view, organized around functions rather
than cachelines. Levels 1 and 2 normally show function names, while level 3
shows cacheline addresses. Lower levels are indented beneath their parents.
Verbose mode also includes code addresses in function rows, and code addresses
remain available in the per-cacheline detail view ('d').
The function view requires `iaddr` in the cacheline coalescing fields. If
`--coalesce` omits it, TAB reports that the view is unavailable rather than
attributing already-coalesced samples to an arbitrary function.
Level 1: the read-side function, sorted by Cycles % (estimated load
cycles: HITM, peer-snoop and other-load cycles)
Level 2: the functions sampled writing the shared lines read by the
level-1 function, sorted by store count. This can be the same
function when it has both read and write samples
Level 3: the specific cachelines shared by the reader/writer pair
The Cycles % value is the function's share of event-provided load
latency/weight estimates from cacheline-detail entries retained in the
current view. It can include non-HITM and non-peer loads coalesced into
entries that pass the C2C filter, so it is not a pure contention-cycle
percentage. The share is relative to the functions and entries retained
for the current report and is not comparable across recordings or different
`--coalesce` settings.
The store count on a level-1 row is the number of sampled stores by writers
shown in the function view into the cachelines that function reads, including
stores from the same function. It decomposes into the level-2 writer rows;
each level-2 count in turn decomposes into that writer's stores on its level-3
cachelines. A level-3 count is therefore not the cacheline's total store
count. The level-1 value is not the number of stores made by the reader and
is not additive across level-1 rows: two functions reading the same line each
carry the stores into that line.
Each function aggregates all of its code addresses into a single entry,
and a level-2 writer aggregates all of its shared cachelines, so a
reader/writer pair is a single row with its total shown -- there is no
need to sum a writer's traffic across cachelines by hand.
In the function view the 'd' key opens the detail view of the selected
level-3 cacheline, 'e'/'+' expands or collapses the current entry, and 'TAB',
'ESC', 'q' or Ctrl-C returns to the cacheline view.
For example, with the first two read-side functions collapsed and
dequeue_pushable_task expanded to show the functions writing the lines it
reads -- two of which are further expanded to their individual cachelines:
Shared Data Functions Table (19 entries, sorted on Cycles %)
Cycles Store
% count Function / Contending function / Cacheline
----------------------------------------------------------------------
+ 35.67% 876 + [k] cpupri_set
+ 24.31% 424 + [k] pull_rt_task
- 16.53% 555 - [k] dequeue_pushable_task
145 - [k] pull_rt_task
145 0xff2d0082809da080
139 - [k] enqueue_pushable_task
70 0xff2d00a2071f9640
69 0xff2d0082809da000
Here dequeue_pushable_task pays 16.53% of the estimated read-side load-cycle
cost. Its store count decomposes into its level-2 writers, and each writer's
count decomposes into its level-3 cachelines: pull_rt_task's 145 stores fall
on a single line, while enqueue_pushable_task's 139 stores split across two
lines (70 and 69). A writer can be the same function as the reader when it
has both read and write samples; after cacheline coalescing and
function-level grouping, the view cannot distinguish same-thread accesses
from different threads running the same function.
For details please refer to the help window by pressing '?' key.
CREDITS
-------
Although Don Zickus, Dick Fowles and Joe Mario worked together
to get this implemented, we got lots of early help from Arnaldo
Carvalho de Melo, Stephane Eranian, Jiri Olsa and Andi Kleen.
C2C BLOG
--------
Check Joe's blog on c2c tool for detailed use case explanation:
https://joemario.github.io/blog/2016/09/01/c2c-blog/
SEE ALSO
--------
linkperf:perf-record[1], linkperf:perf-mem[1], linkperf:perf-arm-spe[1]