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    SubjectRe: [PATCH 2/3] tracing: export stats of ring buffers to userspace
    On Thu, Apr 30, 2009 at 10:22:12PM -0400, Steven Rostedt wrote:
    > From: Steven Rostedt <srostedt@redhat.com>
    >
    > This patch adds stats to the ftrace ring buffers:
    >
    > # cat /debugfs/tracing/per_cpu/cpu0/stats
    > entries: 42360
    > overrun: 30509326
    > commit overrun: 0
    > nmi dropped: 0
    >
    > Where entries are the total number of data entries in the buffer.
    >
    > overrun is the number of entries not consumed and were overwritten by
    > the writer.
    >
    > commit overrun is the number of entries dropped due to nested writers
    > wrapping the buffer before the initial writer finished the commit.


    I feel a bit confused with this one.
    How such a thing can happen? The write page and the commit page
    are not the same. So is that because we can have (ring-buffer inspires
    all of us to try ascii-art):


    Write page Commit page (which becomes new write page)
    ------------------------------------ -----------------
    | | | | |
    Writer 1 | Writer 2 | Writer n | | Writer n + 1 | .....
    reserved | reserved | reserved | | reserved |
    ----------------------------------- ----------------
    | ^
    | |
    ---------------- Was supposed to commit here--|


    I know this is silly, my picture seem to show a data copy whereas
    the ring buffer deals with page pointers.
    But the commit page on the ring buffer is a mistery for me.
    Just because you haven't drawn in in ascii in your comments :)

    Thanks.
    Frederic.

    > nmi dropped is the number of entries dropped due to the ring buffer
    > lock being held when an nmi was going to write to the ring buffer.
    > Note, this field will be meaningless and will go away when the ring
    > buffer becomes lockless.
    >
    > [ Impact: let userspace know what is happening in the ring buffers ]
    >
    > Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
    > ---
    > kernel/trace/trace.c | 42 ++++++++++++++++++++++++++++++++++++++++++
    > 1 files changed, 42 insertions(+), 0 deletions(-)
    >
    > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
    > index f5427e0..74df029 100644
    > --- a/kernel/trace/trace.c
    > +++ b/kernel/trace/trace.c
    > @@ -3595,6 +3595,45 @@ static const struct file_operations tracing_buffers_fops = {
    > .llseek = no_llseek,
    > };
    >
    > +static ssize_t
    > +tracing_stats_read(struct file *filp, char __user *ubuf,
    > + size_t count, loff_t *ppos)
    > +{
    > + unsigned long cpu = (unsigned long)filp->private_data;
    > + struct trace_array *tr = &global_trace;
    > + struct trace_seq *s;
    > + unsigned long cnt;
    > +
    > + s = kmalloc(sizeof(*s), GFP_ATOMIC);
    > + if (!s)
    > + return ENOMEM;
    > +
    > + trace_seq_init(s);
    > +
    > + cnt = ring_buffer_entries_cpu(tr->buffer, cpu);
    > + trace_seq_printf(s, "entries: %ld\n", cnt);
    > +
    > + cnt = ring_buffer_overrun_cpu(tr->buffer, cpu);
    > + trace_seq_printf(s, "overrun: %ld\n", cnt);
    > +
    > + cnt = ring_buffer_commit_overrun_cpu(tr->buffer, cpu);
    > + trace_seq_printf(s, "commit overrun: %ld\n", cnt);
    > +
    > + cnt = ring_buffer_nmi_dropped_cpu(tr->buffer, cpu);
    > + trace_seq_printf(s, "nmi dropped: %ld\n", cnt);
    > +
    > + count = simple_read_from_buffer(ubuf, count, ppos, s->buffer, s->len);
    > +
    > + kfree(s);
    > +
    > + return count;
    > +}
    > +
    > +static const struct file_operations tracing_stats_fops = {
    > + .open = tracing_open_generic,
    > + .read = tracing_stats_read,
    > +};
    > +
    > #ifdef CONFIG_DYNAMIC_FTRACE
    >
    > int __weak ftrace_arch_read_dyn_info(char *buf, int size)
    > @@ -3708,6 +3747,9 @@ static void tracing_init_debugfs_percpu(long cpu)
    >
    > trace_create_file("trace_pipe_raw", 0444, d_cpu,
    > (void *) cpu, &tracing_buffers_fops);
    > +
    > + trace_create_file("stats", 0444, d_cpu,
    > + (void *) cpu, &tracing_stats_fops);
    > }
    >
    > #ifdef CONFIG_FTRACE_SELFTEST
    > --
    > 1.6.2.1
    >
    > --



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