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SubjectRe: [PATCH 1/4] tracing: keep ring buffer to minimum size till used

On Wed, 11 Mar 2009, Andrew Morton wrote:

> 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.

Hmm, I wonder what kind of drugs I was on when I wrote that ;-)
Seriously, I think that was a cut and paste error. Looks like I some
how killed a line :-/

>
> > + 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.

Agreed.

Thanks, I'll go make the updates.

-- Steve

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


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