Re: [PATCH 03/11] tracing: export stats of ring buffers to userspace

[Ingo Molnar <mingo@elte.hu>]

* Steven Rostedt <rostedt@goodmis.org> 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.
> 
> 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 ]

nice! I was always wondering about these properties - Frederic 
reported that we drop a lot of trace entries when filtering is 
enabled, due to the rather stupid:

	reserve-buffer
	put-stuff-into-buffer
	drop-buffer-if-filter-false

common sequence we do on heavily filtered input.

	Ingo