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Message-ID: <20090312150100.GB5270@nowhere>
Date: Thu, 12 Mar 2009 16:01:01 +0100
From: Frederic Weisbecker <fweisbec@...il.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
Andrew Morton <akpm@...ux-foundation.org>,
Peter Zijlstra <peterz@...radead.org>,
Wu Fengguang <fengguang.wu@...el.com>,
Pierre Ossman <drzeus@...eus.cx>, Pekka Paalanen <pq@....fi>,
Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH 1/4] tracing: keep ring buffer to minimum size till used
On Wed, Mar 11, 2009 at 10:37:21PM -0400, Steven Rostedt wrote:
> From: Steven Rostedt <srostedt@...hat.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@...hat.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.
> + */
Indeed :-)
I just wonder about one thing.
Sometimes I use trace_printk to trace some events that come on bootup.
The only solution I see that could avoid me this minimum size would be
to use ftrace=nop that I sometimes use to avoid ftrace selftests.
I guess that few developers will be aware of this trick so may be
it could be worth it to explain it on the ftrace documentation and
on kernel.h (though a simple ftrace_printk as a parameter would be
more obvious, or ftrace=printk).
Anyway this patch is a very good thing, especially on embeeded systems.
Frederic.
> + 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!!!! */
> + 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;
> + }
> +
> mutex_lock(&trace_types_lock);
> for (t = trace_types; t; t = t->next) {
> if (strcmp(t->name, buf) == 0)
> @@ -2903,28 +2952,11 @@ tracing_entries_write(struct file *filp, const char __user *ubuf,
> val <<= 10;
>
> if (val != global_trace.entries) {
> - ret = ring_buffer_resize(global_trace.buffer, val);
> + ret = tracing_resize_ring_buffer(val);
> if (ret < 0) {
> cnt = ret;
> goto out;
> }
> -
> - ret = ring_buffer_resize(max_tr.buffer, val);
> - if (ret < 0) {
> - int r;
> - cnt = ret;
> - r = ring_buffer_resize(global_trace.buffer,
> - global_trace.entries);
> - if (r < 0) {
> - /* AARGH! We are left with different
> - * size max buffer!!!! */
> - WARN_ON(1);
> - tracing_disabled = 1;
> - }
> - goto out;
> - }
> -
> - global_trace.entries = val;
> }
>
> filp->f_pos += cnt;
> @@ -3916,6 +3948,7 @@ void ftrace_dump(void)
> __init static int tracer_alloc_buffers(void)
> {
> struct trace_array_cpu *data;
> + int ring_buf_size;
> int i;
> int ret = -ENOMEM;
>
> @@ -3928,12 +3961,18 @@ __init static int tracer_alloc_buffers(void)
> if (!alloc_cpumask_var(&tracing_reader_cpumask, GFP_KERNEL))
> goto out_free_tracing_cpumask;
>
> + /* To save memory, keep the ring buffer size to its minimum */
> + if (ring_buffer_expanded)
> + ring_buf_size = trace_buf_size;
> + else
> + ring_buf_size = 1;
> +
> cpumask_copy(tracing_buffer_mask, cpu_possible_mask);
> cpumask_copy(tracing_cpumask, cpu_all_mask);
> cpumask_clear(tracing_reader_cpumask);
>
> /* TODO: make the number of buffers hot pluggable with CPUS */
> - global_trace.buffer = ring_buffer_alloc(trace_buf_size,
> + global_trace.buffer = ring_buffer_alloc(ring_buf_size,
> TRACE_BUFFER_FLAGS);
> if (!global_trace.buffer) {
> printk(KERN_ERR "tracer: failed to allocate ring buffer!\n");
> @@ -3944,7 +3983,7 @@ __init static int tracer_alloc_buffers(void)
>
>
> #ifdef CONFIG_TRACER_MAX_TRACE
> - max_tr.buffer = ring_buffer_alloc(trace_buf_size,
> + max_tr.buffer = ring_buffer_alloc(ring_buf_size,
> TRACE_BUFFER_FLAGS);
> if (!max_tr.buffer) {
> printk(KERN_ERR "tracer: failed to allocate max ring buffer!\n");
> --
> 1.6.1.3
>
> --
--
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