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

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Message-ID: <20090506082814.GB28675@elte.hu>
Date:	Wed, 6 May 2009 10:28:14 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 03/11] tracing: export stats of ring buffers to
	userspace


* Steven Rostedt <rostedt@...dmis.org> wrote:

> From: Steven Rostedt <srostedt@...hat.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
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