[PATCH v3 3/4] tracing/histogram: Document hist trigger variables

From: Kalesh Singh
Date: Fri Oct 29 2021 - 14:34:26 EST


Update the tracefs README to describe how hist trigger variables
can be created.

Signed-off-by: Kalesh Singh <kaleshsingh@xxxxxxxxxx>
Acked-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
---

Changes in v2:
- Add Masami's Acked-by.

kernel/trace/trace.c | 11 +++++++++++
1 file changed, 11 insertions(+)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index bc677cd64224..c41b3786401d 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -5628,6 +5628,7 @@ static const char readme_msg[] =
#ifdef CONFIG_HIST_TRIGGERS
" hist trigger\t- If set, event hits are aggregated into a hash table\n"
"\t Format: hist:keys=<field1[,field2,...]>\n"
+ "\t [:<var1>=<field|var_ref|numeric_literal>[,<var2>=...]]\n"
"\t [:values=<field1[,field2,...]>]\n"
"\t [:sort=<field1[,field2,...]>]\n"
"\t [:size=#entries]\n"
@@ -5639,6 +5640,16 @@ static const char readme_msg[] =
"\t common_timestamp - to record current timestamp\n"
"\t common_cpu - to record the CPU the event happened on\n"
"\n"
+ "\t A hist trigger variable can be:\n"
+ "\t - a reference to a field e.g. x=current_timestamp,\n"
+ "\t - a reference to another variable e.g. y=$x,\n"
+ "\t - a numeric literal: e.g. ms_per_sec=1000,\n"
+ "\t - an arithmetic expression: e.g. time_secs=current_timestamp/1000\n"
+ "\n"
+ "\t hist trigger aritmethic expressions support addition(+), subtraction(-),\n"
+ "\t multiplication(*) and division(/) operators. An operand can be either a\n"
+ "\t variable reference, field or numeric literal.\n"
+ "\n"
"\t When a matching event is hit, an entry is added to a hash\n"
"\t table using the key(s) and value(s) named, and the value of a\n"
"\t sum called 'hitcount' is incremented. Keys and values\n"
--
2.33.1.1089.g2158813163f-goog