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SubjectRe: [PATCH 1/4] tracing: keep ring buffer to minimum size till used
On Wed, 11 Mar 2009 22:37:21 -0400 Steven Rostedt <rostedt@goodmis.org> wrote:

> From: Steven Rostedt <srostedt@redhat.com>
>
> Impact: less memory impact on systems not using tracer
>
> When the kernel boots up that has tracing configured, it allocates
> the default size of the ring buffer. This currently happens to be
> 1.4Megs per possible CPU. This is quite a bit of wasted memory if
> the system is never using the tracer.
>
> The current solution is to keep the ring buffers to a minimum size
> until the user uses them. Once a tracer is piped into the current_tracer
> the ring buffer will be expanded to the default size. If the user
> changes the size of the ring buffer, it will take the size given
> by the user immediately.
>
> If the user adds a "ftrace=" to the kernel command line, then the ring
> buffers will be set to the default size on initialization.
>
> Signed-off-by: Steven Rostedt <srostedt@redhat.com>
> ---
> kernel/trace/trace.c | 79 +++++++++++++++++++++++++++++++++++++------------
> 1 files changed, 59 insertions(+), 20 deletions(-)
>
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 4c97947..0c1dc18 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -45,6 +45,12 @@ unsigned long __read_mostly tracing_max_latency;
> unsigned long __read_mostly tracing_thresh;
>
> /*
> + * On boot up, the ring buffer is set to the minimum size, so that
> + * we do not waste memory on systems that are not using tracing.
> + */
> +static int ring_buffer_expanded;
> +
> +/*
> * We need to change this state when a selftest is running.
> * A selftest will lurk into the ring-buffer to count the
> * entries inserted during the selftest although some concurrent
> @@ -128,6 +134,8 @@ static int __init set_ftrace(char *str)
> {
> strncpy(bootup_tracer_buf, str, BOOTUP_TRACER_SIZE);
> default_bootup_tracer = bootup_tracer_buf;
> + /* We are using ftrace early, expand it */
> + ring_buffer_expanded = 1;
> return 1;
> }
> __setup("ftrace=", set_ftrace);
> @@ -2315,6 +2323,40 @@ int tracer_init(struct tracer *t, struct trace_array *tr)
> return t->init(tr);
> }
>
> +static int tracing_resize_ring_buffer(unsigned long size)
> +{
> + int ret;
> +
> + /*
> + * If kernel or user changes the size of the ring buffer
> + * it get completed.
> + */

That comment needs help.

> + ring_buffer_expanded = 1;
> +
> + ret = ring_buffer_resize(global_trace.buffer, size);
> + if (ret < 0)
> + return ret;
> +
> + ret = ring_buffer_resize(max_tr.buffer, size);
> + if (ret < 0) {
> + int r;
> +
> + r = ring_buffer_resize(global_trace.buffer,
> + global_trace.entries);
> + if (r < 0) {
> + /* AARGH! We are left with different
> + * size max buffer!!!! */

So does that one, but at least it's understandable ;)

> + WARN_ON(1);
> + tracing_disabled = 1;
> + }
> + return ret;
> + }
> +
> + global_trace.entries = size;
> +
> + return ret;
> +}
> +
> struct trace_option_dentry;
>
> static struct trace_option_dentry *
> @@ -2330,6 +2372,13 @@ static int tracing_set_tracer(const char *buf)
> struct tracer *t;
> int ret = 0;
>
> + if (!ring_buffer_expanded) {
> + ret = tracing_resize_ring_buffer(trace_buf_size);
> + if (ret < 0)
> + return ret;
> + ret = 0;
> + }

This would look a bit less racy were it to happen inside the lock.

> mutex_lock(&trace_types_lock);
> for (t = trace_types; t; t = t->next) {
> if (strcmp(t->name, buf) == 0)



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