tracing: Record trace_clock and recover when reboot

Record trace_clock information in the trace_scratch area and recover
the trace_clock when boot, so that reader can docode the timestamp
correctly.
Note that since most trace_clocks records the timestamp in nano-
seconds, this is not a bug. But some trace_clock, like counter and
tsc will record the counter value. Only for those trace_clock user
needs this information.

Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://lore.kernel.org/174720625803.1925039.1815089037443798944.stgit@mhiramat.tok.corp.google.com
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index cf51c30b137fba95fad20586d4c92069211da8f2..2c1764ed87b05920f5bfe9786bce6fb9a3d5ab40 100644 (file)
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -6066,6 +6066,7 @@ struct trace_mod_entry {
 };
 
 struct trace_scratch {
+       unsigned int            clock_id;
        unsigned long           text_addr;
        unsigned long           nr_entries;
        struct trace_mod_entry  entries[];
@@ -6181,6 +6182,7 @@ static void update_last_data(struct trace_array *tr)
        if (tr->scratch) {
                struct trace_scratch *tscratch = tr->scratch;
 
+               tscratch->clock_id = tr->clock_id;
                memset(tscratch->entries, 0,
                       flex_array_size(tscratch, entries, tscratch->nr_entries));
                tscratch->nr_entries = 0;
@@ -7403,6 +7405,12 @@ int tracing_set_clock(struct trace_array *tr, const char *clockstr)
        tracing_reset_online_cpus(&tr->max_buffer);
 #endif
 
+       if (tr->scratch && !(tr->flags & TRACE_ARRAY_FL_LAST_BOOT)) {
+               struct trace_scratch *tscratch = tr->scratch;
+
+               tscratch->clock_id = i;
+       }
+
        mutex_unlock(&trace_types_lock);
 
        return 0;
@@ -9628,6 +9636,15 @@ static void setup_trace_scratch(struct trace_array *tr,
 
        /* Scan modules to make text delta for modules. */
        module_for_each_mod(make_mod_delta, tr);
+
+       /* Set trace_clock as the same of the previous boot. */
+       if (tscratch->clock_id != tr->clock_id) {
+               if (tscratch->clock_id >= ARRAY_SIZE(trace_clocks) ||
+                   tracing_set_clock(tr, trace_clocks[tscratch->clock_id].name) < 0) {
+                       pr_info("the previous trace_clock info is not valid.");
+                       goto reset;
+               }
+       }
        return;
  reset:
        /* Invalid trace modules */