Message-ID: <4BAF1450.1090307@cs.helsinki.fi>
Date:	Sun, 28 Mar 2010 11:33:20 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro>
CC:	mingo@...e.hu, randy.dunlap@...cle.com, lizf@...fujitsu.com,
	fweisbec@...il.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Remove Documentation/trace/kmemtrace.txt

Eduard - Gabriel Munteanu wrote:
> The current implementation of kmemtrace has been based on ftrace for a
> while. Documentation/trace/kmemtrace.txt concerns the old, relay-based
> implementation, so it's outdated. This removes it to avoid confusion.
> 
> Signed-off-by: Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro>

Acked-by: Pekka Enberg <penberg@...helsinki.fi>

> ---
>  Documentation/trace/kmemtrace.txt |  126 -------------------------------------
>  1 files changed, 0 insertions(+), 126 deletions(-)
>  delete mode 100644 Documentation/trace/kmemtrace.txt
> 
> diff --git a/Documentation/trace/kmemtrace.txt b/Documentation/trace/kmemtrace.txt
> deleted file mode 100644
> index 6308735..0000000
> --- a/Documentation/trace/kmemtrace.txt
> +++ /dev/null
> @@ -1,126 +0,0 @@
> -			kmemtrace - Kernel Memory Tracer
> -
> -			  by Eduard - Gabriel Munteanu
> -			     <eduard.munteanu@...ux360.ro>
> -
> -I. Introduction
> -===============
> -
> -kmemtrace helps kernel developers figure out two things:
> -1) how different allocators (SLAB, SLUB etc.) perform
> -2) how kernel code allocates memory and how much
> -
> -To do this, we trace every allocation and export information to the userspace
> -through the relay interface. We export things such as the number of requested
> -bytes, the number of bytes actually allocated (i.e. including internal
> -fragmentation), whether this is a slab allocation or a plain kmalloc() and so
> -on.
> -
> -The actual analysis is performed by a userspace tool (see section III for
> -details on where to get it from). It logs the data exported by the kernel,
> -processes it and (as of writing this) can provide the following information:
> -- the total amount of memory allocated and fragmentation per call-site
> -- the amount of memory allocated and fragmentation per allocation
> -- total memory allocated and fragmentation in the collected dataset
> -- number of cross-CPU allocation and frees (makes sense in NUMA environments)
> -
> -Moreover, it can potentially find inconsistent and erroneous behavior in
> -kernel code, such as using slab free functions on kmalloc'ed memory or
> -allocating less memory than requested (but not truly failed allocations).
> -
> -kmemtrace also makes provisions for tracing on some arch and analysing the
> -data on another.
> -
> -II. Design and goals
> -====================
> -
> -kmemtrace was designed to handle rather large amounts of data. Thus, it uses
> -the relay interface to export whatever is logged to userspace, which then
> -stores it. Analysis and reporting is done asynchronously, that is, after the
> -data is collected and stored. By design, it allows one to log and analyse
> -on different machines and different arches.
> -
> -As of writing this, the ABI is not considered stable, though it might not
> -change much. However, no guarantees are made about compatibility yet. When
> -deemed stable, the ABI should still allow easy extension while maintaining
> -backward compatibility. This is described further in Documentation/ABI.
> -
> -Summary of design goals:
> -	- allow logging and analysis to be done across different machines
> -	- be fast and anticipate usage in high-load environments (*)
> -	- be reasonably extensible
> -	- make it possible for GNU/Linux distributions to have kmemtrace
> -	included in their repositories
> -
> -(*) - one of the reasons Pekka Enberg's original userspace data analysis
> -    tool's code was rewritten from Perl to C (although this is more than a
> -    simple conversion)
> -
> -
> -III. Quick usage guide
> -======================
> -
> -1) Get a kernel that supports kmemtrace and build it accordingly (i.e. enable
> -CONFIG_KMEMTRACE).
> -
> -2) Get the userspace tool and build it:
> -$ git clone git://repo.or.cz/kmemtrace-user.git		# current repository
> -$ cd kmemtrace-user/
> -$ ./autogen.sh
> -$ ./configure
> -$ make
> -
> -3) Boot the kmemtrace-enabled kernel if you haven't, preferably in the
> -'single' runlevel (so that relay buffers don't fill up easily), and run
> -kmemtrace:
> -# '$' does not mean user, but root here.
> -$ mount -t debugfs none /sys/kernel/debug
> -$ mount -t proc none /proc
> -$ cd path/to/kmemtrace-user/
> -$ ./kmemtraced
> -Wait a bit, then stop it with CTRL+C.
> -$ cat /sys/kernel/debug/kmemtrace/total_overruns	# Check if we didn't
> -							# overrun, should
> -							# be zero.
> -$ (Optionally) [Run kmemtrace_check separately on each cpu[0-9]*.out file to
> -		check its correctness]
> -$ ./kmemtrace-report
> -
> -Now you should have a nice and short summary of how the allocator performs.
> -
> -IV. FAQ and known issues
> -========================
> -
> -Q: 'cat /sys/kernel/debug/kmemtrace/total_overruns' is non-zero, how do I fix
> -this? Should I worry?
> -A: If it's non-zero, this affects kmemtrace's accuracy, depending on how
> -large the number is. You can fix it by supplying a higher
> -'kmemtrace.subbufs=N' kernel parameter.
> ----
> -
> -Q: kmemtrace_check reports errors, how do I fix this? Should I worry?
> -A: This is a bug and should be reported. It can occur for a variety of
> -reasons:
> -	- possible bugs in relay code
> -	- possible misuse of relay by kmemtrace
> -	- timestamps being collected unorderly
> -Or you may fix it yourself and send us a patch.
> ----
> -
> -Q: kmemtrace_report shows many errors, how do I fix this? Should I worry?
> -A: This is a known issue and I'm working on it. These might be true errors
> -in kernel code, which may have inconsistent behavior (e.g. allocating memory
> -with kmem_cache_alloc() and freeing it with kfree()). Pekka Enberg pointed
> -out this behavior may work with SLAB, but may fail with other allocators.
> -
> -It may also be due to lack of tracing in some unusual allocator functions.
> -
> -We don't want bug reports regarding this issue yet.
> ----
> -
> -V. See also
> -===========
> -
> -Documentation/kernel-parameters.txt
> -Documentation/ABI/testing/debugfs-kmemtrace
> -

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