tracing/histogram: Document hist trigger variables

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

Link: https://lkml.kernel.org/r/20211029183339.3216491-4-kaleshsingh@google.com
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 985390c..31a4632 100644 (file)
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -5606,6 +5606,7 @@ 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"
@@ -5617,6 +5618,16 @@ 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"