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SubjectRe: [PATCH 1/3] ftrace: add logic to record overruns
On Mon, 21 Apr 2008 17:09:36 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> This patch sets up the infrastructure to record overruns of the tracing
> buffer.
>
> Signed-off-by: Steven Rostedt <srostedt@redhat.com>
> ---
> kernel/trace/trace.c | 16 +++++++++++-----
> kernel/trace/trace.h | 6 +++++-
> 2 files changed, 16 insertions(+), 6 deletions(-)
>
> Index: linux-sched-devel.git/kernel/trace/trace.c
> ===================================================================
> --- linux-sched-devel.git.orig/kernel/trace/trace.c 2008-04-21 13:47:30.000000000 -0400
> +++ linux-sched-devel.git/kernel/trace/trace.c 2008-04-21 14:39:43.000000000 -0400
> @@ -612,6 +612,7 @@ void unregister_tracer(struct tracer *ty
> void tracing_reset(struct trace_array_cpu *data)
> {
> data->trace_idx = 0;
> + data->overrun = 0;
> data->trace_head = data->trace_tail = head_page(data);
> data->trace_head_idx = 0;
> data->trace_tail_idx = 0;
> @@ -753,6 +754,7 @@ tracing_get_trace_entry(struct trace_arr
> if (data->trace_head == data->trace_tail &&
> idx_next == data->trace_tail_idx) {
> /* overrun */
> + data->overrun++;
> data->trace_tail_idx++;
> if (data->trace_tail_idx >= ENTRIES_PER_PAGE) {
> data->trace_tail =
...
> @@ -2510,6 +2511,11 @@ tracing_read_pipe(struct file *filp, cha
> for_each_cpu_mask(cpu, mask) {
> data = iter->tr->data[cpu];
> __raw_spin_lock(&data->lock);
> +
> + if (data->overrun > iter->last_overrun[cpu])
> + iter->overrun[cpu] +=
> + data->overrun - iter->last_overrun[cpu];
> + iter->last_overrun[cpu] = data->overrun;

Option 1: move this code earlier.
(Could also remove the `if' and make it unconditional.)

> }
>
> while (find_next_entry_inc(iter) != NULL) {
> Index: linux-sched-devel.git/kernel/trace/trace.h
> ===================================================================
> --- linux-sched-devel.git.orig/kernel/trace/trace.h 2008-04-21 13:47:30.000000000 -0400
> +++ linux-sched-devel.git/kernel/trace/trace.h 2008-04-21 14:38:09.000000000 -0400
> @@ -102,6 +102,7 @@ struct trace_array_cpu {
> void *trace_head; /* producer */
> void *trace_tail; /* consumer */
> unsigned long trace_idx;
> + unsigned long overrun;

Option 2: Change this into atomic_t.

> unsigned long saved_latency;
> unsigned long critical_start;
> unsigned long critical_end;
> @@ -162,10 +163,13 @@ struct trace_seq {
> * results to users and which routines might sleep, etc:
> */
> struct trace_iterator {
> - struct trace_seq seq;
> struct trace_array *tr;
> struct tracer *trace;
> + long last_overrun[NR_CPUS];
> + long overrun[NR_CPUS];

The problem with these fields is that they are updated only after the read
hook has been called in tracing_read_pipe(), which means all "N events
lost!" entries will be one read call behind. This can be any number of
events.

If trace_array_cpu::overrun was atomic_t, I could process that in
the read callback. But, trace_iterator::overrun would be good, because
I could just reset it to zero every time I encounter a non-zero value.

I can think of two options, the ones noted above: moving the update
earlier, or using atomic_t. Or maybe do both, so that we don't have to
lock the trace_array_cpu structs early and also avoid duplicating the
overrun/last_overrun logic in a tracer.

What shall we do?


Thanks.

--
Pekka Paalanen
http://www.iki.fi/pq/


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