docs: ftrace: always use canonical ftrace path

The canonical location for the tracefs filesystem is at /sys/kernel/tracing.

But, from Documentation/trace/ftrace.rst:

Before 4.1, all ftrace tracing control files were within the debugfs
file system, which is typically located at /sys/kernel/debug/tracing.
For backward compatibility, when mounting the debugfs file system,
the tracefs file system will be automatically mounted at:

/sys/kernel/debug/tracing

Many parts of Documentation still reference this older debugfs path, so
let's update them to avoid confusion.

Signed-off-by: Ross Zwisler <zwisler@google.com>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Link: https://lore.kernel.org/r/20230125213251.2013791-1-zwisler@google.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation: trace/histogram-design: drop doubled words

Drop the doubled words "to" and "the".

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ingo Molnar <mingo@redhat.com>
Link: https://lore.kernel.org/r/20200703212453.30553-3-rdunlap@infradead.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

tracing/doc: Fix ascii-art in histogram-design.rst

This fixes the Sphinx parallel build error when building htmldocs:

docutils.utils.SystemMessage: /home/sfr/next/next/Documentation/trace/histogram-design.rst:219: (SEVERE/4) Unexpected section title.

It also fixes a bunch of other warnings I noticed when fixing the
above, caused by mixing ascii-art and text.

Link: https://lkml.kernel.org/r/69c291c76964642a417e5dd170d183ba6b552010.camel@kernel.org

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Tested-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

tracing/doc: Fix typos in histogram-design.rst

There's a few typos in the histogram-design.rst document that need need to
be fixed.

Cc: Tom Zanussi <zanussi@kernel.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

tracing: Add histogram-design document

Add a new Documentation/trace/histogram-design.rst file describing the
ftrace histogram low-level design, meant to help developers trying to
understand the internals when extending or making use of the hist
triggers for higher-level tools.

This documentation refers to the hist_debug files implemented by
'tracing: Add hist_debug trace event files for histogram debugging' so
users wishing to try out the test examples here should make sure
CONFIG_HIST_TRIGGERS_DEBUG is enabled.

Link: http://lkml.kernel.org/r/256b29c3274bb89a10157c4a8d1a8bce7e74849e.1585941485.git.zanussi@kernel.org

Signed-off-by: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>