[PATCH 7/7] libtracefs: Make a man page for the sqlhist man page example
From: Steven Rostedt
Date: Thu Aug 12 2021 - 22:17:53 EST
From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>
As the sqlhist example in the tracefs_sql(3) man page is becoming quite
the tool to use, give it its own man page!
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
---
Documentation/Makefile | 53 +++-
Documentation/libtracefs-sql.txt | 2 +-
Documentation/libtracefs-sqlhist.txt.1 | 351 +++++++++++++++++++++++++
3 files changed, 397 insertions(+), 9 deletions(-)
create mode 100644 Documentation/libtracefs-sqlhist.txt.1
diff --git a/Documentation/Makefile b/Documentation/Makefile
index efe7b09d4e68..0c00505852b5 100644
--- a/Documentation/Makefile
+++ b/Documentation/Makefile
@@ -10,6 +10,9 @@ MAN3_TXT= \
$(wildcard libtracefs-*.txt) \
libtracefs.txt
+MAN1_TEXT= \
+ $(wildcard libtracefs-*.txt.1)
+
MAN_TXT = $(MAN3_TXT)
_MAN_XML=$(patsubst %.txt,%.xml,$(MAN_TXT))
_MAN_HTML=$(patsubst %.txt,%.html,$(MAN_TXT))
@@ -19,6 +22,15 @@ MAN_XML=$(addprefix $(OUTPUT),$(_MAN_XML))
MAN_HTML=$(addprefix $(OUTPUT),$(_MAN_HTML))
DOC_MAN3=$(addprefix $(OUTPUT),$(_DOC_MAN3))
+_MAN1_XML=$(patsubst %.txt.1,%.xml,$(MAN1_TEXT))
+_MAN1_HTML=$(patsubst %.txt.1,%.html,$(MAN1_TEXT))
+_DOC_MAN1=$(patsubst %.txt.1,%.m,$(MAN1_TEXT))
+
+MAN1_XML=$(addprefix $(OUTPUT),$(_MAN1_XML))
+MAN1_HTML=$(addprefix $(OUTPUT),$(_MAN1_HTML))
+DOC_MAN1=$(addprefix $(OUTPUT),$(_DOC_MAN1))
+
+
# Make the path relative to DESTDIR, not prefix
ifndef DESTDIR
prefix?=$(HOME)
@@ -28,6 +40,7 @@ htmldir?=$(prefix)/share/doc/libtracefs-doc
pdfdir?=$(prefix)/share/doc/libtracefs-doc
mandir?=$(prefix)/share/man
man3dir=$(mandir)/man3
+man1dir=$(mandir)/man1
ASCIIDOC=asciidoc
ASCIIDOC_EXTRA = --unsafe -f asciidoc.conf
@@ -131,12 +144,13 @@ endif
all: check-man-tools html man
-man: man3
+man: man3 man1
man3: $(DOC_MAN3)
+man1: $(DOC_MAN1)
-html: $(MAN_HTML)
+html: $(MAN_HTML) $(MAN1_HTML)
-$(MAN_HTML) $(DOC_MAN3): asciidoc.conf
+$(MAN_HTML) $(MAN1_HTML) $(DOC_MAN3) $(DOC_MAN1): asciidoc.conf
install: check-man-tools install-man install-html
@@ -148,7 +162,10 @@ endif
install-%.3: $(OUTPUT)%.3
$(Q)$(call do_install,$<,$(man3dir),644);
-do-install-man: man $(addprefix install-,$(wildcard $(OUTPUT)*.3))
+install-%.1: $(OUTPUT)%.1
+ $(Q)$(call do_install,$<,$(man1dir),644);
+
+do-install-man: man $(addprefix install-,$(wildcard $(OUTPUT)*.3)) $(addprefix install-,$(wildcard $(OUTPUT)*.1))
install-man: man
$(Q)$(MAKE) -C . do-install-man
@@ -156,17 +173,20 @@ install-man: man
install-%.txt: $(OUTPUT)%.html
$(Q)$(call do_install,$<,$(htmldir),644);
-do-install-html: html $(addprefix install-,$(wildcard *.txt))
+install-%.txt.1: $(OUTPUT)%.html
+ $(Q)$(call do_install,$<,$(htmldir),644);
+
+do-install-html: html $(addprefix install-,$(wildcard *.txt)) $(addprefix install-,$(wildcard *.txt.1))
install-html: html do-install-html
uninstall: uninstall-man uninstall-html
uninstall-man:
- $(Q)$(RM) $(addprefix $(DESTDIR)$(man3dir)/,$(DOC_MAN3))
+ $(Q)$(RM) $(addprefix $(DESTDIR)$(man3dir)/,$(DOC_MAN3)) $(addprefix $(DESTDIR)$(man1dir)/,$(DOC_MAN1))
uninstall-html:
- $(Q)$(RM) $(addprefix $(DESTDIR)$(htmldir)/,$(MAN_HTML))
+ $(Q)$(RM) $(addprefix $(DESTDIR)$(htmldir)/,$(MAN_HTML)) $(addprefix $(DESTDIR)$(htmldir)/,$(MAN1_HTML))
ifdef missing_tools
DO_INSTALL_MAN = $(warning Please install $(missing_tools) to have the man pages installed)
@@ -177,7 +197,7 @@ endif
CLEAN_FILES = \
$(MAN_XML) $(addsuffix +,$(MAN_XML)) \
$(MAN_HTML) $(addsuffix +,$(MAN_HTML)) \
- $(DOC_MAN3) *.3 *.m
+ $(DOC_MAN3) *.3 *.1 *.m
clean:
$(Q) $(RM) $(CLEAN_FILES)
@@ -188,6 +208,11 @@ $(OUTPUT)%.3 : $(OUTPUT)%.txt
$(ASCIIDOC) -b manpage -d manpage \
$(ASCIIDOC_EXTRA) -alibtracefs_version=$(TRACEFS_VERSION) -o $@+ $< && \
mv $@+ $@
+$(OUTPUT)%.1 : $(OUTPUT)%.txt.1
+ $(QUIET_ASCIIDOC)$(RM) $@+ $@ && \
+ $(ASCIIDOC) -b manpage -d manpage \
+ $(ASCIIDOC_EXTRA) -alibtracefs_version=$(TRACEFS_VERSION) -o $@+ $< && \
+ mv $@+ $@
endif
$(OUTPUT)%.m : $(OUTPUT)%.xml
@@ -201,8 +226,20 @@ $(OUTPUT)%.xml : %.txt
$(ASCIIDOC_EXTRA) -alibtracefs_version=$(TRACEFS_VERSION) -o $@+ $< && \
mv $@+ $@
+$(OUTPUT)%.xml : %.txt.1
+ $(QUIET_ASCIIDOC)$(RM) $@+ $@ && \
+ $(ASCIIDOC) -b docbook -d manpage \
+ $(ASCIIDOC_EXTRA) -alibtracefs_version=$(TRACEFS_VERSION) -o $@+ $< && \
+ mv $@+ $@
+
$(MAN_HTML): $(OUTPUT)%.html : %.txt
$(QUIET_ASCIIDOC)$(RM) $@+ $@ && \
$(ASCIIDOC) -b $(ASCIIDOC_HTML) -d manpage \
$(ASCIIDOC_EXTRA) -alibtracefs_version=$(TRACEFS_VERSION) -o $@+ $< && \
mv $@+ $@
+
+$(MAN1_HTML): $(OUTPUT)%.html : %.txt.1
+ $(QUIET_ASCIIDOC)$(RM) $@+ $@ && \
+ $(ASCIIDOC) -b $(ASCIIDOC_HTML) -d manpage \
+ $(ASCIIDOC_EXTRA) -alibtracefs_version=$(TRACEFS_VERSION) -o $@+ $< && \
+ mv $@+ $@
diff --git a/Documentation/libtracefs-sql.txt b/Documentation/libtracefs-sql.txt
index 95625157590e..e762eaea28c2 100644
--- a/Documentation/libtracefs-sql.txt
+++ b/Documentation/libtracefs-sql.txt
@@ -290,7 +290,6 @@ Then you can run the above examples:
JOIN sched_switch as end ON start.pid = end.next_pid
WHERE start.prio < 100 || end.prev_prio < 100'
--
-
EXAMPLE
-------
[source,c]
@@ -548,6 +547,7 @@ FILES
SEE ALSO
--------
+_sqlhist(1)_,
_libtracefs(3)_,
_libtraceevent(3)_,
_trace-cmd(1)_,
diff --git a/Documentation/libtracefs-sqlhist.txt.1 b/Documentation/libtracefs-sqlhist.txt.1
new file mode 100644
index 000000000000..1e94ea4cd138
--- /dev/null
+++ b/Documentation/libtracefs-sqlhist.txt.1
@@ -0,0 +1,351 @@
+SQLHIST(1)
+==========
+
+NAME
+----
+sqlhist - Tool that uses SQL language to create / show creation of tracefs histograms and synthetic events.
+
+SYNOPSIS
+--------
+*sqlhist* ['OPTIONS'] ['SQL-select-command']
+
+DESCRIPTION
+-----------
+The sqlhist(1) will take an SQL like statement to create tracefs histograms and
+synthetic events that can perform various actions for various handling of the
+data.
+
+The tracefs file system interfaces with the Linux tracing infrastructure that
+has various dynamic and static events through out the kernel. Each of these
+events can have a "histogram" attached to it, where the fields of the event
+will define the buckets of the histogram.
+
+A synthetic event is a way to attach two separate events and use the fields
+and time stamps of those events to create a new dynamic event. This new
+dynamic event is call a synthetic event. The fields of each event can have
+simple calculations done on them where, for example, the delta between
+a field of one event to a field of the other event can be taken. This also
+works for the time stamps of the events where the time delta between the
+two events can also be extracted and placed into the synthetic event.
+
+Other actions can be done from the fields of the events. A snapshot can
+be taken of the kernel ring buffer a variable used in the synthetic
+event creating hits a max, or simply changes.
+
+The commands to create histograms and synthetic events are complex and
+not easy to remember. *sqlhist* is used to convert SQL syntax into the
+commands needed to create the histogram or synthetic event.
+
+The *SQL-select-command* is a SQL string defined by *tracefs_sql*(3).
+
+Note, this must be run as root (or sudo) as interacting with the tracefs
+directory requires root privilege, unless the *-t* option is given with
+a copy of the _tracefs_ directory and its events.
+
+The *sqlhist* is a simple program where its code actual exists in the
+*tracefs_sql*(3) man page.
+
+OPTIONS
+-------
+*-n* 'name'::
+ The name of the synthetic event to create. This event can then be
+ used like any other event, and enabled via *trace-cmd*(1).
+
+*-t* 'tracefs-dir'::
+ In order to test this out as non root user, a copy of the tracefs directory
+ can be used, and passing that directory with this option will allow
+ the program to work. Obviously, *-e* will not work as non-root because
+ it will not be able to execute.
+
+ # mkdir /tmp/tracing
+ # cp -r /sys/kernel/tracing/events /tmp/tracing
+ # exit
+ $ ./sqlhist -t /tmp/tracing ...
+
+*-e*::
+ Not only display the commands to create the histogram, but also execute them.
+ This requires root privilege.
+
+*-f* 'file'::
+ Instead of reading the SQL commands from the command line, read them from
+ _file_. If _file_ is '-' then read from standard input.
+
+*-m* 'var'::
+ Do the given action when the variable _var_ hits a new maximum. This can
+ not be used with *-c*.
+
+*-c* 'var'::
+ Do the given action when the variable _var_ changes its value. This can
+ not be used with *-m*.
+
+*-s*::
+ Perform a snapshot instead of calling the synthetic event.
+
+*-T*::
+ Perform both a snapshot and trace the synthetic event.
+
+*-S* 'fields[,fields]'::
+ Save the given fields. The fields must be fields of the "end" event given
+ in the *SQL-select-command*
+
+
+EXAMPLES
+--------
+
+Create the sqlhist executable:
+
+[source, c]
+--
+ man tracefs_sql | sed -ne '/^EXAMPLE/,/FILES/ { /EXAMPLE/d ; /FILES/d ; p}' > sqlhist.c
+ gcc -o sqlhist sqlhist.c `pkg-config --cflags --libs libtracefs`
+--
+
+As described above, for testing purposes, make a copy of the event directory:
+[source, c]
+--
+ $ mkdir /tmp/tracing
+ $ sudo cp -r /sys/kernel/tracing/events /tmp/tracing/
+ $ sudo chmod -R 0644 /tmp/tracing/
+--
+
+For an example of simple histogram output using the copy of the tracefs directory.
+[source, c]
+--
+ $ ./sqlhist -t /tmp/tracing/ 'SELECT CAST(call_site as SYM-OFFSET), bytes_req, CAST(bytes_alloc AS _COUNTER_) FROM kmalloc'
+--
+
+Produces the output:
+[source, c]
+--
+ echo 'hist:keys=call_site.sym-offset,bytes_req:vals=bytes_alloc' > /sys/kernel/tracing/events/kmem/kmalloc/trigger
+--
+
+Which could be used by root:
+[source, c]
+--
+ # echo 'hist:keys=call_site.sym-offset,bytes_req:vals=bytes_alloc' > /sys/kernel/tracing/events/kmem/kmalloc/trigger
+ # cat /sys/kernel/tracing/events/kmem/kmalloc/hist
+# event histogram
+#
+# trigger info: hist:keys=call_site.sym-offset,bytes_req:vals=hitcount,bytes_alloc:sort=hitcount:size=2048 [active]
+#
+
+{ call_site: [ffffffff813f8d8a] load_elf_phdrs+0x4a/0xb0 , bytes_req: 728 } hitcount: 1 bytes_alloc: 1024
+{ call_site: [ffffffffc0c69e74] nf_ct_ext_add+0xd4/0x1d0 [nf_conntrack] , bytes_req: 128 } hitcount: 1 bytes_alloc: 128
+{ call_site: [ffffffff818355e6] dma_resv_get_fences+0xf6/0x440 , bytes_req: 8 } hitcount: 1 bytes_alloc: 8
+{ call_site: [ffffffffc06dc73f] intel_gt_get_buffer_pool+0x15f/0x290 [i915] , bytes_req: 424 } hitcount: 1 bytes_alloc: 512
+{ call_site: [ffffffff813f8d8a] load_elf_phdrs+0x4a/0xb0 , bytes_req: 616 } hitcount: 1 bytes_alloc: 1024
+{ call_site: [ffffffff8161a44c] __sg_alloc_table+0x11c/0x180 , bytes_req: 32 } hitcount: 1 bytes_alloc: 32
+{ call_site: [ffffffffc070749d] shmem_get_pages+0xad/0x5d0 [i915] , bytes_req: 16 } hitcount: 1 bytes_alloc: 16
+{ call_site: [ffffffffc07507f5] intel_framebuffer_create+0x25/0x60 [i915] , bytes_req: 408 } hitcount: 1 bytes_alloc: 512
+{ call_site: [ffffffffc06fc20f] eb_parse+0x34f/0x910 [i915] , bytes_req: 408 } hitcount: 1 bytes_alloc: 512
+{ call_site: [ffffffffc0700ebd] i915_gem_object_get_pages_internal+0x5d/0x270 [i915] , bytes_req: 16 } hitcount: 1 bytes_alloc: 16
+{ call_site: [ffffffffc0771188] intel_frontbuffer_get+0x38/0x220 [i915] , bytes_req: 400 } hitcount: 1 bytes_alloc: 512
+{ call_site: [ffffffff8161a44c] __sg_alloc_table+0x11c/0x180 , bytes_req: 128 } hitcount: 1 bytes_alloc: 128
+{ call_site: [ffffffff813f8f45] load_elf_binary+0x155/0x1680 , bytes_req: 28 } hitcount: 1 bytes_alloc: 32
+{ call_site: [ffffffffc07038c8] __assign_mmap_offset+0x208/0x3d0 [i915] , bytes_req: 288 } hitcount: 1 bytes_alloc: 512
+{ call_site: [ffffffff813737b2] alloc_bprm+0x32/0x2f0 , bytes_req: 416 } hitcount: 1 bytes_alloc: 512
+{ call_site: [ffffffff813f9027] load_elf_binary+0x237/0x1680 , bytes_req: 64 } hitcount: 1 bytes_alloc: 64
+{ call_site: [ffffffff8161a44c] __sg_alloc_table+0x11c/0x180 , bytes_req: 64 } hitcount: 1 bytes_alloc: 64
+{ call_site: [ffffffffc040ffe7] drm_vma_node_allow+0x27/0xe0 [drm] , bytes_req: 40 } hitcount: 2 bytes_alloc: 128
+{ call_site: [ffffffff813cda98] __do_sys_timerfd_create+0x58/0x1c0 , bytes_req: 336 } hitcount: 2 bytes_alloc: 1024
+{ call_site: [ffffffff818355e6] dma_resv_get_fences+0xf6/0x440 , bytes_req: 40 } hitcount: 2 bytes_alloc: 128
+{ call_site: [ffffffff8139b75a] single_open+0x2a/0xa0 , bytes_req: 32 } hitcount: 2 bytes_alloc: 64
+{ call_site: [ffffffff815df715] bio_kmalloc+0x25/0x80 , bytes_req: 136 } hitcount: 2 bytes_alloc: 384
+{ call_site: [ffffffffc071e5cd] i915_vma_work+0x1d/0x50 [i915] , bytes_req: 416 } hitcount: 3 bytes_alloc: 1536
+{ call_site: [ffffffff81390d0d] alloc_fdtable+0x4d/0x100 , bytes_req: 56 } hitcount: 3 bytes_alloc: 192
+{ call_site: [ffffffffc06ff65f] i915_gem_do_execbuffer+0x158f/0x2440 [i915] , bytes_req: 16 } hitcount: 4 bytes_alloc: 64
+{ call_site: [ffffffff8137713c] alloc_pipe_info+0x5c/0x230 , bytes_req: 384 } hitcount: 5 bytes_alloc: 2560
+{ call_site: [ffffffff813771b4] alloc_pipe_info+0xd4/0x230 , bytes_req: 640 } hitcount: 5 bytes_alloc: 5120
+{ call_site: [ffffffff81834cdb] dma_resv_list_alloc+0x1b/0x40 , bytes_req: 40 } hitcount: 6 bytes_alloc: 384
+{ call_site: [ffffffff81834cdb] dma_resv_list_alloc+0x1b/0x40 , bytes_req: 56 } hitcount: 9 bytes_alloc: 576
+{ call_site: [ffffffff8120086e] tracing_map_sort_entries+0x9e/0x3e0 , bytes_req: 24 } hitcount: 60 bytes_alloc: 1920
+
+Totals:
+ Hits: 122
+ Entries: 30
+ Dropped: 0
+--
+
+Note, although the examples use uppercase for the SQL keywords, they do not have
+to be. 'SELECT' could also be 'select' or even 'sElEcT'.
+
+By using the full SQL language, synthetic events can be made and processed.
+For example, using *sqlhist* along with *trace-cmd*(1), wake up latency can
+be recorded by creating a synthetic event by attaching the _sched_waking_
+and the _sched_switch_ events.
+
+[source, c]
+--
+ # sqlhist -n wakeup_lat -e -T -m lat 'SELECT end.next_comm AS comm, (end.TIMESTAMP_USECS - start.TIMESTAMP_USECS) AS lat FROM ' \
+ 'sched_waking AS start JOIN sched_switch AS end ON start.pid = end.next_pid WHERE end.next_prio < 100 && end.next_comm == "cyclictest"'
+ # trace-cmd start -e all -e wakeup_lat -R stacktrace
+ # cyclictest -l 1000 -p80 -i250 -a -t -q -m -d 0 -b 1000 --tracemark
+ # trace-cmd show -s | tail -30
+ <idle>-0 [002] dNh4 23454.902246: sched_wakeup: comm=cyclictest pid=12272 prio=120 target_cpu=002
+ <idle>-0 [005] ...1 23454.902246: cpu_idle: state=4294967295 cpu_id=5
+ <idle>-0 [007] d..1 23454.902246: cpu_idle: state=0 cpu_id=7
+ <idle>-0 [002] dNh1 23454.902247: hrtimer_expire_exit: hrtimer=0000000037956dc2
+ <idle>-0 [005] d..1 23454.902248: cpu_idle: state=0 cpu_id=5
+ <idle>-0 [002] dNh1 23454.902248: write_msr: 6e0, value 4866ce957272
+ <idle>-0 [006] ...1 23454.902248: cpu_idle: state=4294967295 cpu_id=6
+ <idle>-0 [002] dNh1 23454.902249: local_timer_exit: vector=236
+ <idle>-0 [006] d..1 23454.902250: cpu_idle: state=0 cpu_id=6
+ <idle>-0 [002] .N.1 23454.902250: cpu_idle: state=4294967295 cpu_id=2
+ <idle>-0 [002] dN.1 23454.902251: rcu_utilization: Start context switch
+ <idle>-0 [002] dN.1 23454.902252: rcu_utilization: End context switch
+ <idle>-0 [001] ...1 23454.902252: cpu_idle: state=4294967295 cpu_id=1
+ <idle>-0 [002] dN.3 23454.902253: prandom_u32: ret=3692516021
+ <idle>-0 [001] d..1 23454.902254: cpu_idle: state=0 cpu_id=1
+ <idle>-0 [002] d..2 23454.902254: sched_switch: prev_comm=swapper/2 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=cyclictest next_pid=12275 next_prio=19
+ <idle>-0 [002] d..4 23454.902256: wakeup_lat: next_comm=cyclictest lat=17
+ <idle>-0 [002] d..5 23454.902258: <stack trace>
+ => trace_event_raw_event_synth
+ => action_trace
+ => event_hist_trigger
+ => event_triggers_call
+ => trace_event_buffer_commit
+ => trace_event_raw_event_sched_switch
+ => __traceiter_sched_switch
+ => __schedule
+ => schedule_idle
+ => do_idle
+ => cpu_startup_entry
+ => secondary_startup_64_no_verify
+--
+
+Here's the options for *sqlhist* explained:
+
+ *-n wakeup_lat* ::
+ Name the synthetic event to use *wakeup_lat*.
+
+ *-e*::
+ Execute the commands that are printed.
+
+ *-T*::
+ Perform both a trace action and then a snapshot action (swap the buffer into the static 'snapshot' buffer).
+
+ *-m lat*::
+ Trigger the actions whenever 'lat' hits a new maximum value.
+
+Now a breakdown of the SQL statement:
+[source, c]
+--
+ 'SELECT end.next_comm AS comm, (end.TIMESTAMP_USECS - start.TIMESTAMP_USECS) AS lat FROM ' \
+ 'sched_waking AS start JOIN sched_switch AS end ON start.pid = end.next_pid WHERE end.next_prio < 100 && end.next_comm == "cyclictest"'
+--
+ *end.next_comm AS comm*::
+ Save the 'sched_switch' field *next_comm* and place it into the *comm* field of the 'wakeup_lat' synthetic event.
+
+ *(end.TIMESTAMP_USECS - start.TIMESTAMP_USECS) AS lat*::
+ Take the delta of the time stamps from the 'sched_switch' event and the 'sched_waking' event.
+ As time stamps are usually recorded in nanoseconds, *TIMESTAMP* would give the full nanosecond time stamp,
+ but here, the *TIMESTAMP_USECS* will truncate it into microseconds. The value is saved in the
+ variable *lat*, which will also be recorded in the synthetic event.
+
+ *FROM 'sched_waking' AS start JOIN sched_switch AS end ON start.pid = end.next_pid*::
+ Create the synthetic event by joining _sched_waking_ to _sched_switch_, matching
+ the _sched_waking_ 'pid' field with the _sched_switch_ 'next_pid' field.
+ Also make *start* an alias for _sched_waking_ and *end* an alias for _sched_switch_
+ which then an use *start* and *end* as a subsitute for _sched_waking_ and _sched_switch_
+ respectively through out the rest of the SQL statement.
+
+ *WHERE end.next_prio < 100 && end.next_comm == "cyclictest"*::
+ Filter the logic where it executes only if _sched_waking_ 'next_prio' field
+ is less than 100. (Note, in the Kernel, priorities are inverse, and the real-time
+ priorities are represented from 0-100 where 0 is the highest priority).
+ Also only trace when the 'next_comm' (the task scheduling in) of the _sched_switch_
+ event has the name "cyclictest".
+
+For the *trace-cmd*(3) command:
+[source, c]
+--
+ trace-cmd start -e all -e wakeup_lat -R stacktrace
+--
+
+ *trace-cmd start*::
+ Enables tracing (does not record to a file).
+
+ *-e all*::
+ Enable all events
+
+ *-e wakeup_lat -R stacktrace*::
+ have the "wakeup_lat" event (our synthetic event) enable the *stacktrace* trigger, were
+ for every instance of the "wakeup_lat" event, a kernel stack trace will be recorded
+ in the ring buffer.
+
+After calling *cyclictest* (a real-time tool to measure wakeup latency), read the snapshot
+buffer.
+
+ *trace-cmd show -s*::
+ *trace-cmd show* reads the kernel ring buffer, and the *-s* option will read the *snapshot*
+ buffer instead of the normal one.
+
+[source, c]
+--
+ <idle>-0 [002] d..4 23454.902256: wakeup_lat: next_comm=cyclictest lat=17
+--
+ We see on the "wakeup_lat" event happened on CPU 2, with a wake up latency 17 microseconds.
+
+This can be extracted into a *trace.dat* file that *trace-cmd*(3) can read and do further
+analysis, as well as *kernelshark*.
+
+[source, c]
+--
+ # trace-cmd extract -s
+ # trace-cmd report --cpu 2 | tail -30
+ <idle>-0 [002] 23454.902238: prandom_u32: ret=1633425088
+ <idle>-0 [002] 23454.902239: sched_wakeup: cyclictest:12275 [19] CPU:002
+ <idle>-0 [002] 23454.902241: hrtimer_expire_exit: hrtimer=0xffffbbd68286fe60
+ <idle>-0 [002] 23454.902241: hrtimer_cancel: hrtimer=0xffffbbd6826efe70
+ <idle>-0 [002] 23454.902242: hrtimer_expire_entry: hrtimer=0xffffbbd6826efe70 now=23455294430750 function=hrtimer_wakeup/0x0
+ <idle>-0 [002] 23454.902243: sched_waking: comm=cyclictest pid=12272 prio=120 target_cpu=002
+ <idle>-0 [002] 23454.902244: prandom_u32: ret=1102749734
+ <idle>-0 [002] 23454.902246: sched_wakeup: cyclictest:12272 [120] CPU:002
+ <idle>-0 [002] 23454.902247: hrtimer_expire_exit: hrtimer=0xffffbbd6826efe70
+ <idle>-0 [002] 23454.902248: write_msr: 6e0, value 4866ce957272
+ <idle>-0 [002] 23454.902249: local_timer_exit: vector=236
+ <idle>-0 [002] 23454.902250: cpu_idle: state=4294967295 cpu_id=2
+ <idle>-0 [002] 23454.902251: rcu_utilization: Start context switch
+ <idle>-0 [002] 23454.902252: rcu_utilization: End context switch
+ <idle>-0 [002] 23454.902253: prandom_u32: ret=3692516021
+ <idle>-0 [002] 23454.902254: sched_switch: swapper/2:0 [120] R ==> cyclictest:12275 [19]
+ <idle>-0 [002] 23454.902256: wakeup_lat: next_comm=cyclictest lat=17
+ <idle>-0 [002] 23454.902258: kernel_stack: <stack trace >
+=> trace_event_raw_event_synth (ffffffff8121a0db)
+=> action_trace (ffffffff8121e9fb)
+=> event_hist_trigger (ffffffff8121ca8d)
+=> event_triggers_call (ffffffff81216c72)
+=> trace_event_buffer_commit (ffffffff811f7618)
+=> trace_event_raw_event_sched_switch (ffffffff8110fda4)
+=> __traceiter_sched_switch (ffffffff8110d449)
+=> __schedule (ffffffff81c02002)
+=> schedule_idle (ffffffff81c02c86)
+=> do_idle (ffffffff8111e898)
+=> cpu_startup_entry (ffffffff8111eba9)
+=> secondary_startup_64_no_verify (ffffffff81000107)
+--
+
+BUGS
+----
+
+As *sqlhist* is just example code from a man page, it is guaranteed to contain
+lots of bugs. For one thing, not all error paths are covered properly.
+
+SEE ALSO
+--------
+trace-cmd(1), tracefs_sql(3)
+
+AUTHOR
+------
+Written by Steven Rostedt, <rostedt@xxxxxxxxxxx>
+
+RESOURCES
+---------
+https://git.kernel.org/pub/scm/utils/trace-cmd/trace-cmd.git/
+
+COPYING
+-------
+Copyright \(C) 2021 , Inc. Free use of this software is granted under
+the terms of the GNU Public License (GPL).
+
--
2.30.2