tracing/ftrace: implement a set_flag callback for tracers

Impact: give a way to send specific messages to tracers

The current implementation of tracing uses some flags to control the
output of general tracers. But we have no way to implement custom
flags handling for a specific tracer. This patch proposes a new
callback for the struct tracer which called set_flag and a structure
that represents a 32 bits variable flag.

A tracer can implement a struct tracer_flags on which it puts the
initial value of the flag integer. Than it can place a range of flags
with their name and their flag mask on the flag integer. The structure
that implement a single flag is called struct tracer_opt.

These custom flags will be available through the trace_options file
like the general tracing flags. Changing their value is done like the
other general flags. For example if you have a flag that calls "foo",
you can activate it by writing "foo" or "nofoo" on trace_options.

Note that the set_flag callback is optional and is only needed if you
want the flags changing to be signaled to your tracer and let it to
accept or refuse their assignment.

V2: Some arrangements in coding style....

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 86 insertions, 6 deletions

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 2596b5a968c4..9531fddcfb8d 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -43,6 +43,20 @@
 unsigned long __read_mostly     tracing_max_latency = (cycle_t)ULONG_MAX;
 unsigned long __read_mostly     tracing_thresh;
 
+/* For tracers that don't implement custom flags */
+static struct tracer_opt dummy_tracer_opt[] = {
+        { }
+};
+
+static struct tracer_flags dummy_tracer_flags = {
+        .val = 0,
+        .opts = dummy_tracer_opt
+};
+
+static int dummy_set_flag(u32 old_flags, u32 bit, int set)
+{
+        return 0;
+}
 
 /*
  * Kill all tracing for good (never come back).
@@ -529,6 +543,14 @@ int register_tracer(struct tracer *type)
                 }
         }
 
+        if (!type->set_flag)
+                type->set_flag = &dummy_set_flag;
+        if (!type->flags)
+                type->flags = &dummy_tracer_flags;
+        else
+                if (!type->flags->opts)
+                        type->flags->opts = dummy_tracer_opt;
+
 #ifdef CONFIG_FTRACE_STARTUP_TEST
         if (type->selftest) {
                 struct tracer *saved_tracer = current_trace;
@@ -2426,10 +2448,13 @@ static ssize_t
 tracing_trace_options_read(struct file *filp, char __user *ubuf,
                        size_t cnt, loff_t *ppos)
 {
+        int i;
         char *buf;
         int r = 0;
         int len = 0;
-        int i;
+        u32 tracer_flags = current_trace->flags->val;
+        struct tracer_opt *trace_opts = current_trace->flags->opts;
+
 
         /* calulate max size */
         for (i = 0; trace_options[i]; i++) {
@@ -2437,6 +2462,15 @@ tracing_trace_options_read(struct file *filp, char __user *ubuf,
                 len += 3; /* "no" and space */
         }
 
+        /*
+         * Increase the size with names of options specific
+         * of the current tracer.
+         */
+        for (i = 0; trace_opts[i].name; i++) {
+                len += strlen(trace_opts[i].name);
+                len += 3; /* "no" and space */
+        }
+
         /* +2 for \n and \0 */
         buf = kmalloc(len + 2, GFP_KERNEL);
         if (!buf)
@@ -2449,6 +2483,15 @@ tracing_trace_options_read(struct file *filp, char __user *ubuf,
                         r += sprintf(buf + r, "no%s ", trace_options[i]);
         }
 
+        for (i = 0; trace_opts[i].name; i++) {
+                if (tracer_flags & trace_opts[i].bit)
+                        r += sprintf(buf + r, "%s ",
+                                trace_opts[i].name);
+                else
+                        r += sprintf(buf + r, "no%s ",
+                                trace_opts[i].name);
+        }
+
         r += sprintf(buf + r, "\n");
         WARN_ON(r >= len + 2);
 
@@ -2459,6 +2502,40 @@ tracing_trace_options_read(struct file *filp, char __user *ubuf,
         return r;
 }
 
+/* Try to assign a tracer specific option */
+static int set_tracer_option(struct tracer *trace, char *cmp, int neg)
+{
+        struct tracer_flags *trace_flags = trace->flags;
+        struct tracer_opt *opts = NULL;
+        int ret = 0, i = 0;
+        int len;
+
+        for (i = 0; trace_flags->opts[i].name; i++) {
+                opts = &trace_flags->opts[i];
+                len = strlen(opts->name);
+
+                if (strncmp(cmp, opts->name, len) == 0) {
+                        ret = trace->set_flag(trace_flags->val,
+                                opts->bit, !neg);
+                        break;
+                }
+        }
+        /* Not found */
+        if (!trace_flags->opts[i].name)
+                return -EINVAL;
+
+        /* Refused to handle */
+        if (ret)
+                return ret;
+
+        if (neg)
+                trace_flags->val &= ~opts->bit;
+        else
+                trace_flags->val |= opts->bit;
+
+        return 0;
+}
+
 static ssize_t
 tracing_trace_options_write(struct file *filp, const char __user *ubuf,
                         size_t cnt, loff_t *ppos)
@@ -2466,6 +2543,7 @@ tracing_trace_options_write(struct file *filp, const char __user *ubuf,
         char buf[64];
         char *cmp = buf;
         int neg = 0;
+        int ret;
         int i;
 
         if (cnt >= sizeof(buf))
@@ -2492,11 +2570,13 @@ tracing_trace_options_write(struct file *filp, const char __user *ubuf,
                         break;
                 }
         }
-        /*
-         * If no option could be set, return an error:
-         */
-        if (!trace_options[i])
-                return -EINVAL;
+
+        /* If no option could be set, test the specific tracer options */
+        if (!trace_options[i]) {
+                ret = set_tracer_option(current_trace, cmp, neg);
+                if (ret)
+                        return ret;
+        }
 
         filp->f_pos += cnt;