1. Build with the 'simple' trace backend:
- ./configure --trace-backend=simple
+ ./configure --enable-trace-backend=simple
make
-2. Enable trace events you are interested in:
+2. Create a file with the events you want to trace:
- $EDITOR trace-events # remove "disable" from events you want
+ echo bdrv_aio_readv > /tmp/events
+ echo bdrv_aio_writev >> /tmp/events
3. Run the virtual machine to produce a trace file:
- qemu ... # your normal QEMU invocation
+ qemu -trace events=/tmp/events ... # your normal QEMU invocation
4. Pretty-print the binary trace file:
- ./simpletrace.py trace-events trace-*
+ ./scripts/simpletrace.py trace-events trace-*
== Trace events ==
file. Each trace event declaration names the event, its arguments, and the
format string which can be used for pretty-printing:
- qemu_malloc(size_t size, void *ptr) "size %zu ptr %p"
- qemu_free(void *ptr) "ptr %p"
+ qemu_vmalloc(size_t size, void *ptr) "size %zu ptr %p"
+ qemu_vfree(void *ptr) "ptr %p"
The "trace-events" file is processed by the "tracetool" script during build to
generate code for the trace events. Trace events are invoked directly from
source code like this:
#include "trace.h" /* needed for trace event prototype */
-
- void *qemu_malloc(size_t size)
+
+ void *qemu_vmalloc(size_t size)
{
void *ptr;
- if (!size && !allow_zero_malloc()) {
- abort();
+ size_t align = QEMU_VMALLOC_ALIGN;
+
+ if (size < align) {
+ align = getpagesize();
}
- ptr = oom_check(malloc(size ? size : 1));
- trace_qemu_malloc(size, ptr); /* <-- trace event */
+ ptr = qemu_memalign(align, size);
+ trace_qemu_vmalloc(size, ptr);
return ptr;
}
cannot include all user-defined struct declarations and it is therefore
necessary to use void * for pointers to structs.
- Pointers (including char *) cannot be dereferenced easily (or at all) in
- some trace backends. If pointers are used, ensure they are meaningful by
- themselves and do not assume the data they point to will be traced. Do
- not pass in string arguments.
-
* For everything else, use primitive scalar types (char, int, long) with the
appropriate signedness.
Format strings should reflect the types defined in the trace event. Take
special care to use PRId64 and PRIu64 for int64_t and uint64_t types,
-respectively. This ensures portability between 32- and 64-bit platforms. Note
-that format strings must begin and end with double quotes. When using
-portability macros, ensure they are preceded and followed by double quotes:
-"value %"PRIx64"".
+respectively. This ensures portability between 32- and 64-bit platforms.
=== Hints for adding new trace events ===
4. Name trace events after their function. If there are multiple trace events
in one function, append a unique distinguisher at the end of the name.
-5. Declare trace events with the "disable" keyword. Some trace events can
- produce a lot of output and users are typically only interested in a subset
- of trace events. Marking trace events disabled by default saves the user
- from having to manually disable noisy trace events.
+== Generic interface and monitor commands ==
+
+You can programmatically query and control the state of trace events through a
+backend-agnostic interface provided by the header "trace/control.h".
+
+Note that some of the backends do not provide an implementation for some parts
+of this interface, in which case QEMU will just print a warning (please refer to
+header "trace/control.h" to see which routines are backend-dependent).
+
+The state of events can also be queried and modified through monitor commands:
+
+* info trace-events
+ View available trace events and their state. State 1 means enabled, state 0
+ means disabled.
+
+* trace-event NAME on|off
+ Enable/disable a given trace event or a group of events (using wildcards).
+
+The "-trace events=<file>" command line argument can be used to enable the
+events listed in <file> from the very beginning of the program. This file must
+contain one event name per line.
+
+If a line in the "-trace events=<file>" file begins with a '-', the trace event
+will be disabled instead of enabled. This is useful when a wildcard was used
+to enable an entire family of events but one noisy event needs to be disabled.
+
+Wildcard matching is supported in both the monitor command "trace-event" and the
+events list file. That means you can enable/disable the events having a common
+prefix in a batch. For example, virtio-blk trace events could be enabled using
+the following monitor command:
+
+ trace-event virtio_blk_* on
== Trace backends ==
can optimize out trace events completely. This is the default and imposes no
performance penalty.
+Note that regardless of the selected trace backend, events with the "disable"
+property will be generated with the "nop" backend.
+
=== Stderr ===
The "stderr" backend sends trace events directly to standard error. This
trace backends but it is portable. This is the recommended trace backend
unless you have specific needs for more advanced backends.
-==== Monitor commands ====
+The "simple" backend currently does not capture string arguments, it simply
+records the char* pointer value instead of the string that is pointed to.
-* info trace
- Display the contents of trace buffer. This command dumps the trace buffer
- with simple formatting. For full pretty-printing, use the simpletrace.py
- script on a binary trace file.
+=== Ftrace ===
- The trace buffer is written into until full. The full trace buffer is
- flushed and emptied. This means the 'info trace' will display few or no
- entries if the buffer has just been flushed.
+The "ftrace" backend writes trace data to ftrace marker. This effectively
+sends trace events to ftrace ring buffer, and you can compare qemu trace
+data and kernel(especially kvm.ko when using KVM) trace data.
-* info trace-events
- View available trace events and their state. State 1 means enabled, state 0
- means disabled.
+if you use KVM, enable kvm events in ftrace:
-* trace-event NAME on|off
- Enable/disable a given trace event.
+ # echo 1 > /sys/kernel/debug/tracing/events/kvm/enable
-* trace-file on|off|flush|set <path>
- Enable/disable/flush the trace file or set the trace file name.
+After running qemu by root user, you can get the trace:
-==== Enabling/disabling trace events programmatically ====
+ # cat /sys/kernel/debug/tracing/trace
-The st_change_trace_event_state() function can be used to enable or disable trace
-events at runtime inside QEMU:
+Restriction: "ftrace" backend is restricted to Linux only.
- #include "trace.h"
-
- st_change_trace_event_state("virtio_irq", true); /* enable */
- [...]
- st_change_trace_event_state("virtio_irq", false); /* disable */
+==== Monitor commands ====
+
+* trace-file on|off|flush|set <path>
+ Enable/disable/flush the trace file or set the trace file name.
==== Analyzing trace files ====
simpletrace.py script. The script takes the "trace-events" file and the binary
trace:
- ./simpletrace.py trace-events trace-12345
+ ./scripts/simpletrace.py trace-events trace-12345
You must ensure that the same "trace-events" file was used to build QEMU,
otherwise trace event declarations may have changed and output will not be
--target-type system \
--target-arch x86_64 \
<trace-events >qemu.stp
+
+== Trace event properties ==
+
+Each event in the "trace-events" file can be prefixed with a space-separated
+list of zero or more of the following event properties.
+
+=== "disable" ===
+
+If a specific trace event is going to be invoked a huge number of times, this
+might have a noticeable performance impact even when the event is
+programmatically disabled.
+
+In this case you should declare such event with the "disable" property. This
+will effectively disable the event at compile time (by using the "nop" backend),
+thus having no performance impact at all on regular builds (i.e., unless you
+edit the "trace-events" file).
+
+In addition, there might be cases where relatively complex computations must be
+performed to generate values that are only used as arguments for a trace
+function. In these cases you can use the macro 'TRACE_${EVENT_NAME}_ENABLED' to
+guard such computations and avoid its compilation when the event is disabled:
+
+ #include "trace.h" /* needed for trace event prototype */
+
+ void *qemu_vmalloc(size_t size)
+ {
+ void *ptr;
+ size_t align = QEMU_VMALLOC_ALIGN;
+
+ if (size < align) {
+ align = getpagesize();
+ }
+ ptr = qemu_memalign(align, size);
+ if (TRACE_QEMU_VMALLOC_ENABLED) { /* preprocessor macro */
+ void *complex;
+ /* some complex computations to produce the 'complex' value */
+ trace_qemu_vmalloc(size, ptr, complex);
+ }
+ return ptr;
+ }
+
+You can check both if the event has been disabled and is dynamically enabled at
+the same time using the 'trace_event_get_state' routine (see header
+"trace/control.h" for more information).
projects / qemu.git / blobdiff