--- name: "sqlhist" section: "1" platform: "linux" package: "libtracefs" version: "1.8.4-1" release: "Arch Linux 2026-11-01" platforms: ["linux"] url: "https://new.carta.tech/sqlhist" --- # sqlhist(1) Tool that uses SQL language to create / show creation of tracefs histograms and synthetic events. ## 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)](/sqlhist/1.md) 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)](/tracefs_sql/3.md). 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)](/tracefs_sql/3.md) 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)](/trace-cmd/1.md). - **-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** - **-B** *instance* For simple statements that only produce a histogram, the instance given here will be where the histogram will be created. This is ignored for full synthetic event creation, as sythetic events have a global affect on all tracing instances, where as, histograms only affect a single instance. ## EXAMPLES Create the sqlhist executable: ``` 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: ``` $ 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. ``` $ ./sqlhist -t /tmp/tracing/ 'SELECT CAST(call_site as SYM-OFFSET), bytes_req, CAST(bytes_alloc AS _COUNTER_) FROM kmalloc' ``` Produces the output: ``` 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: ``` # 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)](/trace-cmd/1.md), wake up latency can be recorded by creating a synthetic event by attaching the *sched\_waking* and the *sched\_switch* events. ``` # 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 -0 [002] dNh4 23454.902246: sched_wakeup: comm=cyclictest pid=12272 prio=120 target_cpu=002 -0 [005] ...1 23454.902246: cpu_idle: state=4294967295 cpu_id=5 -0 [007] d..1 23454.902246: cpu_idle: state=0 cpu_id=7 -0 [002] dNh1 23454.902247: hrtimer_expire_exit: hrtimer=0000000037956dc2 -0 [005] d..1 23454.902248: cpu_idle: state=0 cpu_id=5 -0 [002] dNh1 23454.902248: write_msr: 6e0, value 4866ce957272 -0 [006] ...1 23454.902248: cpu_idle: state=4294967295 cpu_id=6 -0 [002] dNh1 23454.902249: local_timer_exit: vector=236 -0 [006] d..1 23454.902250: cpu_idle: state=0 cpu_id=6 -0 [002] .N.1 23454.902250: cpu_idle: state=4294967295 cpu_id=2 -0 [002] dN.1 23454.902251: rcu_utilization: Start context switch -0 [002] dN.1 23454.902252: rcu_utilization: End context switch -0 [001] ...1 23454.902252: cpu_idle: state=4294967295 cpu_id=1 -0 [002] dN.3 23454.902253: prandom_u32: ret=3692516021 -0 [001] d..1 23454.902254: cpu_idle: state=0 cpu_id=1 -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 -0 [002] d..4 23454.902256: wakeup_lat: next_comm=cyclictest lat=17 -0 [002] d..5 23454.902258: => 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: ``` '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: ``` 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. ``` -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**. ``` # trace-cmd extract -s # trace-cmd report --cpu 2 | tail -30 -0 [002] 23454.902238: prandom_u32: ret=1633425088 -0 [002] 23454.902239: sched_wakeup: cyclictest:12275 [19] CPU:002 -0 [002] 23454.902241: hrtimer_expire_exit: hrtimer=0xffffbbd68286fe60 -0 [002] 23454.902241: hrtimer_cancel: hrtimer=0xffffbbd6826efe70 -0 [002] 23454.902242: hrtimer_expire_entry: hrtimer=0xffffbbd6826efe70 now=23455294430750 function=hrtimer_wakeup/0x0 -0 [002] 23454.902243: sched_waking: comm=cyclictest pid=12272 prio=120 target_cpu=002 -0 [002] 23454.902244: prandom_u32: ret=1102749734 -0 [002] 23454.902246: sched_wakeup: cyclictest:12272 [120] CPU:002 -0 [002] 23454.902247: hrtimer_expire_exit: hrtimer=0xffffbbd6826efe70 -0 [002] 23454.902248: write_msr: 6e0, value 4866ce957272 -0 [002] 23454.902249: local_timer_exit: vector=236 -0 [002] 23454.902250: cpu_idle: state=4294967295 cpu_id=2 -0 [002] 23454.902251: rcu_utilization: Start context switch -0 [002] 23454.902252: rcu_utilization: End context switch -0 [002] 23454.902253: prandom_u32: ret=3692516021 -0 [002] 23454.902254: sched_switch: swapper/2:0 [120] R ==> cyclictest:12275 [19] -0 [002] 23454.902256: wakeup_lat: next_comm=cyclictest lat=17 -0 [002] 23454.902258: kernel_stack: => 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)](/trace-cmd/1.md), [tracefs\_sql(3)](/tracefs_sql/3.md) ## AUTHOR Written by Steven Rostedt, \<**rostedt@goodmis.org**\[1\]\> ## 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). ## NOTES - 1\. rostedt@goodmis.org mailto:rostedt@goodmis.org