On Thu, 2 Apr 2020 16:19:20 +0900
Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
> On Wed, 1 Apr 2020 11:04:01 -0400
> Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> > On Wed, 1 Apr 2020 10:21:12 -0400
> > Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> >
> > > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> > > index 6519b7afc499..7f1466253ca8 100644
> > > --- a/kernel/trace/trace.c
> > > +++ b/kernel/trace/trace.c
> > > @@ -3487,6 +3487,14 @@ struct trace_entry *trace_find_next_entry(struct trace_iterator *iter,
> > > */
> > > if (iter->ent && iter->ent != iter->temp) {
> > > if (!iter->temp || iter->temp_size < iter->ent_size) {
> > > + /*
> > > + * This function is only used to add markers between
> > > + * events that are far apart (see trace_print_lat_context()),
> > > + * but if this is called in an atomic context (like NMIs)
> > > + * we can't call kmalloc(), thus just return NULL.
> > > + */
> > > + if (in_atomic() || irqs_disabled())
> > > + return NULL;
> > > kfree(iter->temp);
> > > iter->temp = kmalloc(iter->ent_size, GFP_KERNEL);
> > > if (!iter->temp)
> >
> > Peter informed me on IRC not to use in_atomic() as it doesn't catch
> > spin_locks when CONFIG_PREEMPT is not defined.
> >
> > As the issue is just with ftrace_dump(), I'll have it use a static buffer
> > instead of 128 bytes. Which should be big enough for most events, and if
> > not, then it will just miss the markers.
>
>
> That sounds good, but the below patch seems to do different thing.
> Does it just makes trace_find_next_entry() always fail if it is
> called from ftrace_dump()?
Bah! I send my emails on a different machine than I create the patches on.
Below was my first iteration, then I decided to at least try to print some,
changed it, but never copied the new version over, and ended up sending the
last one.
Here's the actual patch!
-- Steve
From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>
Subject: [PATCH] tracing: Do not allocate buffer in trace_find_next_entry() in
atomic
When dumping out the trace data in latency format, a check is made to peek
at the next event to compare its timestamp to the current one, and if the
delta is of a greater size, it will add a marker showing so. But to do this,
it needs to save the current event otherwise peeking at the next event will
remove the current event. To save the event, a temp buffer is used, and if
the event is bigger than the temp buffer, the temp buffer is freed and a
bigger buffer is allocated.
This allocation is a problem when called in atomic context. The only way
this gets called via atomic context is via ftrace_dump(). Thus, use a static
buffer of 128 bytes (which covers most events), and if the event is bigger
than that, simply return NULL. The callers of trace_find_next_entry() need
to handle a NULL case, as that's what would happen if the allocation failed.
Link: https://lore.kernel.org/r/20200326091256.GR11705@shao2-debian
Fixes: ff895103a84ab ("tracing: Save off entry when peeking at next entry")
Reported-by: kernel test robot <rong.a.chen@xxxxxxxxx>
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
---
kernel/trace/trace.c | 20 +++++++++++++++++++-
1 file changed, 19 insertions(+), 1 deletion(-)
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 6519b7afc499..4b7bbfe7f809 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -3472,6 +3472,9 @@ __find_next_entry(struct trace_iterator *iter, int *ent_cpu,
return next;
}
+#define STATIC_TEMP_BUF_SIZE 128
+static char static_temp_buf[STATIC_TEMP_BUF_SIZE];
+
/* Find the next real entry, without updating the iterator itself */
struct trace_entry *trace_find_next_entry(struct trace_iterator *iter,
int *ent_cpu, u64 *ent_ts)
@@ -3480,13 +3483,26 @@ struct trace_entry *trace_find_next_entry(struct trace_iterator *iter,
int ent_size = iter->ent_size;
struct trace_entry *entry;
+ /*
+ * If called from ftrace_dump(), then the iter->temp buffer
+ * will be the static_temp_buf and not created from kmalloc.
+ * If the entry size is greater than the buffer, we can
+ * not save it. Just return NULL in that case. This is only
+ * used to add markers when two consecutive events' time
+ * stamps have a large delta. See trace_print_lat_context()
+ */
+ if (iter->temp == static_temp_buf &&
+ STATIC_TEMP_BUF_SIZE < ent_size)
+ return NULL;
+
/*
* The __find_next_entry() may call peek_next_entry(), which may
* call ring_buffer_peek() that may make the contents of iter->ent
* undefined. Need to copy iter->ent now.
*/
if (iter->ent && iter->ent != iter->temp) {
- if (!iter->temp || iter->temp_size < iter->ent_size) {
+ if ((!iter->temp || iter->temp_size < iter->ent_size) &&
+ !WARN_ON_ONCE(iter->temp == static_temp_buf)) {
kfree(iter->temp);
iter->temp = kmalloc(iter->ent_size, GFP_KERNEL);
if (!iter->temp)
@@ -9203,6 +9219,8 @@ void ftrace_dump(enum ftrace_dump_mode oops_dump_mode)
/* Simulate the iterator */
trace_init_global_iter(&iter);
+ /* Can not use kmalloc for iter.temp */
+ iter.temp = static_temp_buf;
for_each_tracing_cpu(cpu) {
atomic_inc(&per_cpu_ptr(iter.array_buffer->data, cpu)->disabled);
--
2.20.1
