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tracing: Add variable reference handling to hist triggers

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Add the necessary infrastructure to allow the variables defined on one
event to be referenced in another.  This allows variables set by a
previous event to be referenced and used in expressions combining the
variable values saved by that previous event and the event fields of
the current event.  For example, here's how a latency can be
calculated and saved into yet another variable named 'wakeup_lat':

    # echo 'hist:keys=pid,prio:ts0=common_timestamp ...
    # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp-$ts0 ...

In the first event, the event's timetamp is saved into the variable
ts0.  In the next line, ts0 is subtracted from the second event's
timestamp to produce the latency.

Further users of variable references will be described in subsequent
patches, such as for instance how the 'wakeup_lat' variable above can
be displayed in a latency histogram.

Link: http://lkml.kernel.org/r/b1d3e6975374e34d501ff417c20189c3f9b2c7b8.1516069914.git.tom.zanussi@linux.intel.com

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
This commit is contained in:
Tom Zanussi
2018-01-15 20:51:56 -06:00
committed by Steven Rostedt (VMware)
parent 19a9facd0f
commit 067fe038e7
4 changed files with 656 additions and 16 deletions
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2
kernel/trace/trace.c
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@@ -7783,6 +7783,7 @@ static int instance_mkdir(const char *name)
INIT_LIST_HEAD(&tr->systems);
INIT_LIST_HEAD(&tr->events);
INIT_LIST_HEAD(&tr->hist_vars);
if (allocate_trace_buffers(tr, trace_buf_size) < 0)
goto out_free_tr;
@@ -8533,6 +8534,7 @@ __init static int tracer_alloc_buffers(void)
INIT_LIST_HEAD(&global_trace.systems);
INIT_LIST_HEAD(&global_trace.events);
INIT_LIST_HEAD(&global_trace.hist_vars);
list_add(&global_trace.list, &ftrace_trace_arrays);
apply_trace_boot_options();