Re: [PATCH 4/9] agent doc

Yao Qi
2012-02-20T02:41:28Z

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On 02/18/2012 08:40 PM, Eli Zaretskii wrote:
> Then I suggest the following modified wording of that paragraph:
>
> Although called @dfn{agent expression}, because they originally
> referred to the target-side debugging agent (@pxref{Agent}), these
> expressions can be used in conjunction with many unrelated
> @value{GDBN} features, such as expressions used in tracepoints for
> data collection, expressions used in breakpoint condition evaluation,
> etc. Note that the expressions may also denote registers and objects
> in memory---structures or arrays, for example---whose values
> @value{GDBN} should record.

I replaced 'target-side debugging agent (@pxref{Agent})' with
'in-process agent (@pxref{In-Process Agent})', to explictly name `agent'
as `In-Process Agent', to avoid of confusion. Here is the new patch on doc.

--
Yao (é½å°§)

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gdb/doc/
2012-02-20 Yao Qi <yao@codesourcery.com>

* agentexpr.texi (Agent Expressions): Update some parts for agent expression
can not only be used in agent, but also in remote stubs.
Move some contents to ...
* gdb.texinfo (In-Process Agent): ... to here. New node.
Document new commands.
---
gdb/doc/agentexpr.texi | 37 ++++++++++-------------------
gdb/doc/gdb.texinfo | 58 ++++++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 71 insertions(+), 24 deletions(-)

diff --git a/gdb/doc/agentexpr.texi b/gdb/doc/agentexpr.texi
index d0f6f15..65e4423 100644
--- a/gdb/doc/agentexpr.texi
+++ b/gdb/doc/agentexpr.texi
@@ -13,32 +13,21 @@

@node Agent Expressions
@appendix The GDB Agent Expression Mechanism
-
-In some applications, it is not feasible for the debugger to interrupt
-the program's execution long enough for the developer to learn anything
-helpful about its behavior. If the program's correctness depends on its
-real-time behavior, delays introduced by a debugger might cause the
-program to fail, even when the code itself is correct. It is useful to
-be able to observe the program's behavior without interrupting it.
-
-Using GDB's @code{trace} and @code{collect} commands, the user can
-specify locations in the program, and arbitrary expressions to evaluate
-when those locations are reached. Later, using the @code{tfind}
-command, she can examine the values those expressions had when the
-program hit the trace points. The expressions may also denote objects
-in memory --- structures or arrays, for example --- whose values GDB
-should record; while visiting a particular tracepoint, the user may
-inspect those objects as if they were in memory at that moment.
-However, because GDB records these values without interacting with the
-user, it can do so quickly and unobtrusively, hopefully not disturbing
-the program's behavior.
-
-When GDB is debugging a remote target, the GDB @dfn{agent} code running
+Although called @dfn{agent expression}, because they originally
+referred to the in-process agent (@pxref{In-Process Agent}), these
+expressions can be used in conjunction with many unrelated @value{GDBN}
+features, such as expressions used in tracepoints for data collection,
+expressions used in breakpoint condition evaluation, etc. Note that the
+expressions may also denote registers and objects in memory---structures
+or arrays, for example---whose values @value{GDBN} should record.
+
+When @value{GDBN} is debugging, the @value{GDBN} agent code running
on the target computes the values of the expressions itself. To avoid
-having a full symbolic expression evaluator on the agent, GDB translates
-expressions in the source language into a simpler bytecode language, and
-then sends the bytecode to the agent; the agent then executes the
-bytecode, and records the values for GDB to retrieve later.
+having a full symbolic expression evaluator on the agent or remote stub,
+@value{GDBN} translates expressions in the source language into a simpler
+bytecode language, and then sends the bytecode to the agent; the agent
+then executes the bytecode, and records the values for @value{GDBN} to
+retrieve later.

The bytecode language is simple; there are forty-odd opcodes, the bulk
of which are the usual vocabulary of C operands (addition, subtraction,
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index 34bf77e..cd2482f 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -154,6 +154,7 @@ software in general. We will miss him.
* GDB/MI:: @value{GDBN}'s Machine Interface.
* Annotations:: @value{GDBN}'s annotation interface.
* JIT Interface:: Using the JIT debugging interface.
+* In-Process Agent:: In-Process Agent

* GDB Bugs:: Reporting bugs in @value{GDBN}

@@ -32191,6 +32192,63 @@ frame and to write out the values of the registers in the previous
frame. Both have a callback (@code{target_read}) to read bytes off the
target's address space.

+@node In-Process Agent
+@chapter In-Process Agent
+@cindex debugging agent
+The traditional debugging model is conceptually low-speed, but works fine,
+because most bugs can be reproduced in debugging-mode execution. However,
+as multi-core or many-core processors are becoming mainstream, and
+multi-threaded programs become more and more popular, there should be more
+and more bugs that only manifest themselves at normal-mode execution, for
+example, thread races, because debugger's interference with the program's
+timing may conceal the bugs. On the other hand, in some applications,
+it is not feasible for the debugger to interrupt the program's execution
+long enough for the developer to learn anything helpful about its behavior.
+If the program's correctness depends on its real-time behavior, delays
+introduced by a debugger might cause the program to fail, even when the
+code itself is correct. It is useful to be able to observe the program's
+behavior without interrupting it.
+
+Therefore, traditional debugging model is too intrusive to reproduce
+some bugs. In order to reduce the interference with the program, we can
+reduce the number of operations performed by debugger. @dfn{Agent},
+a shared library, is running within the same process with inferior, and is
+able to perform some debugging operations itself. As a result, debugger
+is only involved when necessary, and performance of debugging can be
+improved accordingly. Note that interference with program can be
+reduced but can't be removed completely, because the agent will still stop
+or slow down the program.
+
+The agent can interpret and execute Agent Expressions
+(@pxref{Agent Expressions}) during performing debugging operations. The
+agent expressions can be used for different purposes, such as collecting
+data in tracepoints, and condition evaluation in breakpoints.
+
+@anchor{Control Agent}
+You can control whether the agent is used as an aid for debugging
+with the following commands:
+
+@table @code
+@kindex set agent on
+@item set agent on
+Causes the agent to perform some operations on behalf of the
+debugger. Just which operations requested by the user will be done
+by the agent depends on the agent's capabilities. For example, if
+you request to evaluate breakpoint conditions in the agent, and the
+agent has such capability as well, then breakpoint conditions will be
+evaluated in the agent.
+
+@kindex set agent off
+@item set agent off
+Disables execution of debugging operations by the agent. All of the
+operations will be performed by @value{GDBN}.
+
+@kindex show agent
+@item show agent
+Display the current setting of execution of debugging operations by
+the agent.
+@end table
+
@node GDB Bugs
@chapter Reporting Bugs in @value{GDBN}
@cindex bugs in @value{GDBN}
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1.7.0.4