[Qemu-devel] [PATCH] simpletrace: Thread-safe tracing

[Stefan Hajnoczi]

Trace events outside the global mutex cannot be used with the simple
trace backend since it is not thread-safe.  There is no check to prevent
them being enabled so people sometimes learn this the hard way.

This patch restructures the simple trace backend with a ring buffer
suitable for multiple concurrent writers.  A writeout thread empties the
trace buffer when threshold fill levels are reached.  Should the
writeout thread be unable to keep up with trace generation, records will
simply be dropped.

Each time events are dropped a special record is written to the trace
file indicating how many events were dropped.  The event ID is
0xfffffffffffffffe and its signature is dropped(uint32_t count).

Signed-off-by: Stefan Hajnoczi <address@hidden>
---
v2:
 * Add 'dropped' event so we know when events were lost.

 docs/tracing.txt       |    5 -
 scripts/simpletrace.py |    3 +-
 simpletrace.c          |  309 ++++++++++++++++++++++++++++++++----------------
 simpletrace.h          |    8 ++
 vl.c                   |   16 +--
 5 files changed, 219 insertions(+), 122 deletions(-)

diff --git a/docs/tracing.txt b/docs/tracing.txt
index a6cc56f..f15069c 100644
--- a/docs/tracing.txt
+++ b/docs/tracing.txt
@@ -141,11 +141,6 @@ source tree.  It may not be as powerful as 
platform-specific or third-party
 trace backends but it is portable.  This is the recommended trace backend
 unless you have specific needs for more advanced backends.
 
-Warning: the "simple" backend is not thread-safe so only enable trace events
-that are executed while the global mutex is held.  Much of QEMU meets this
-requirement but some utility functions like qemu_malloc() or thread-related
-code cannot be safely traced using the "simple" backend.
-
 ==== Monitor commands ====
 
 * info trace
diff --git a/scripts/simpletrace.py b/scripts/simpletrace.py
index 9fe3dda..2ad5699 100755
--- a/scripts/simpletrace.py
+++ b/scripts/simpletrace.py
@@ -16,6 +16,7 @@ import inspect
 header_event_id = 0xffffffffffffffff
 header_magic    = 0xf2b177cb0aa429b4
 header_version  = 0
+dropped_event_id = 0xfffffffffffffffe
 
 trace_fmt = '=QQQQQQQQ'
 trace_len = struct.calcsize(trace_fmt)
@@ -28,7 +29,7 @@ def parse_events(fobj):
         """Extract argument names from a parameter list."""
         return tuple(arg.split()[-1].lstrip('*') for arg in args.split(','))
 
-    events = {}
+    events = {dropped_event_id: ('dropped', 'count')}
     event_num = 0
     for line in fobj:
         m = event_re.match(line.strip())
diff --git a/simpletrace.c b/simpletrace.c
index 9ea0d1f..9926ab3 100644
--- a/simpletrace.c
+++ b/simpletrace.c
@@ -12,6 +12,9 @@
 #include <stdint.h>
 #include <stdio.h>
 #include <time.h>
+#include <signal.h>
+#include <pthread.h>
+#include "qerror.h"
 #include "qemu-timer.h"
 #include "trace.h"
 
@@ -24,6 +27,12 @@
 /** Trace file version number, bump if format changes */
 #define HEADER_VERSION 0
 
+/** Records were dropped event ID */
+#define DROPPED_EVENT_ID (~(uint64_t)0 - 1)
+
+/** Trace record is valid */
+#define TRACE_RECORD_VALID ((uint64_t)1 << 63)
+
 /** Trace buffer entry */
 typedef struct {
     uint64_t event;
@@ -37,126 +46,135 @@ typedef struct {
 } TraceRecord;
 
 enum {
-    TRACE_BUF_LEN = 64 * 1024 / sizeof(TraceRecord),
+    TRACE_BUF_LEN = 4096,
+    TRACE_BUF_FLUSH_THRESHOLD = TRACE_BUF_LEN / 4,
 };
 
+/*
+ * Trace records are written out by a dedicated thread.  The thread waits for
+ * records to become available, writes them out, and then waits again.
+ */
+static pthread_mutex_t trace_lock = PTHREAD_MUTEX_INITIALIZER;
+static pthread_cond_t trace_available_cond = PTHREAD_COND_INITIALIZER;
+static pthread_cond_t trace_empty_cond = PTHREAD_COND_INITIALIZER;
+static bool trace_available;
+static bool trace_writeout_enabled;
+
 static TraceRecord trace_buf[TRACE_BUF_LEN];
 static unsigned int trace_idx;
 static FILE *trace_fp;
 static char *trace_file_name = NULL;
-static bool trace_file_enabled = false;
-
-void st_print_trace_file_status(FILE *stream, int (*stream_printf)(FILE 
*stream, const char *fmt, ...))
-{
-    stream_printf(stream, "Trace file \"%s\" %s.\n",
-                  trace_file_name, trace_file_enabled ? "on" : "off");
-}
-
-static bool write_header(FILE *fp)
-{
-    static const TraceRecord header = {
-        .event = HEADER_EVENT_ID,
-        .timestamp_ns = HEADER_MAGIC,
-        .x1 = HEADER_VERSION,
-    };
-
-    return fwrite(&header, sizeof header, 1, fp) == 1;
-}
 
 /**
- * set_trace_file : To set the name of a trace file.
- * @file : pointer to the name to be set.
- *         If NULL, set to the default name-<pid> set at config time.
+ * Read a trace record from the trace buffer
+ *
+ * @idx         Trace buffer index
+ * @record      Trace record to fill
+ *
+ * Returns false if the record is not valid.
  */
-bool st_set_trace_file(const char *file)
+static bool get_trace_record(unsigned int idx, TraceRecord *record)
 {
-    st_set_trace_file_enabled(false);
-
-    free(trace_file_name);
-
-    if (!file) {
-        if (asprintf(&trace_file_name, CONFIG_TRACE_FILE, getpid()) < 0) {
-            trace_file_name = NULL;
-            return false;
-        }
-    } else {
-        if (asprintf(&trace_file_name, "%s", file) < 0) {
-            trace_file_name = NULL;
-            return false;
-        }
+    if (!(trace_buf[idx].event & TRACE_RECORD_VALID)) {
+        return false;
     }
 
-    st_set_trace_file_enabled(true);
+    __sync_synchronize(); /* read memory barrier before accessing record */
+
+    *record = trace_buf[idx];
+    record->event &= ~TRACE_RECORD_VALID;
     return true;
 }
 
-static void flush_trace_file(void)
+/**
+ * Kick writeout thread
+ *
+ * @wait        Whether to wait for writeout thread to complete
+ */
+static void flush_trace_file(bool wait)
 {
-    /* If the trace file is not open yet, open it now */
-    if (!trace_fp) {
-        trace_fp = fopen(trace_file_name, "w");
-        if (!trace_fp) {
-            /* Avoid repeatedly trying to open file on failure */
-            trace_file_enabled = false;
-            return;
-        }
-        write_header(trace_fp);
-    }
+    pthread_mutex_lock(&trace_lock);
+    trace_available = true;
+    pthread_cond_signal(&trace_available_cond);
 
-    if (trace_fp) {
-        size_t unused; /* for when fwrite(3) is declared warn_unused_result */
-        unused = fwrite(trace_buf, trace_idx * sizeof(trace_buf[0]), 1, 
trace_fp);
+    if (wait) {
+        pthread_cond_wait(&trace_empty_cond, &trace_lock);
     }
+
+    pthread_mutex_unlock(&trace_lock);
 }
 
-void st_flush_trace_buffer(void)
+static void wait_for_trace_records_available(void)
 {
-    if (trace_file_enabled) {
-        flush_trace_file();
+    pthread_mutex_lock(&trace_lock);
+    while (!(trace_available && trace_writeout_enabled)) {
+        pthread_cond_signal(&trace_empty_cond);
+        pthread_cond_wait(&trace_available_cond, &trace_lock);
     }
-
-    /* Discard written trace records */
-    trace_idx = 0;
+    trace_available = false;
+    pthread_mutex_unlock(&trace_lock);
 }
 
-void st_set_trace_file_enabled(bool enable)
+static void *writeout_thread(void *opaque)
 {
-    if (enable == trace_file_enabled) {
-        return; /* no change */
-    }
+    TraceRecord record;
+    unsigned int writeout_idx = 0;
+    unsigned int num_available, idx;
+    size_t unused;
+
+    for (;;) {
+        wait_for_trace_records_available();
+
+        num_available = trace_idx - writeout_idx;
+        if (num_available > TRACE_BUF_LEN) {
+            record = (TraceRecord){
+                .event = DROPPED_EVENT_ID,
+                .x1 = num_available,
+            };
+            unused = fwrite(&record, sizeof(record), 1, trace_fp);
+            writeout_idx += num_available;
+        }
 
-    /* Flush/discard trace buffer */
-    st_flush_trace_buffer();
+        idx = writeout_idx % TRACE_BUF_LEN;
+        while (get_trace_record(idx, &record)) {
+            trace_buf[idx].event = 0; /* clear valid bit */
+            unused = fwrite(&record, sizeof(record), 1, trace_fp);
+            idx = ++writeout_idx % TRACE_BUF_LEN;
+        }
 
-    /* To disable, close trace file */
-    if (!enable) {
-        fclose(trace_fp);
-        trace_fp = NULL;
+        fflush(trace_fp);
     }
-
-    trace_file_enabled = enable;
+    return NULL;
 }
 
 static void trace(TraceEventID event, uint64_t x1, uint64_t x2, uint64_t x3,
                   uint64_t x4, uint64_t x5, uint64_t x6)
 {
-    TraceRecord *rec = &trace_buf[trace_idx];
+    unsigned int idx;
+    uint64_t timestamp;
 
     if (!trace_list[event].state) {
         return;
     }
 
-    rec->event = event;
-    rec->timestamp_ns = get_clock();
-    rec->x1 = x1;
-    rec->x2 = x2;
-    rec->x3 = x3;
-    rec->x4 = x4;
-    rec->x5 = x5;
-    rec->x6 = x6;
-
-    if (++trace_idx == TRACE_BUF_LEN) {
-        st_flush_trace_buffer();
+    timestamp = get_clock();
+
+    idx = __sync_fetch_and_add(&trace_idx, 1) % TRACE_BUF_LEN;
+    trace_buf[idx] = (TraceRecord){
+        .event = event,
+        .timestamp_ns = timestamp,
+        .x1 = x1,
+        .x2 = x2,
+        .x3 = x3,
+        .x4 = x4,
+        .x5 = x5,
+        .x6 = x6,
+    };
+    __sync_synchronize(); /* write barrier before marking as valid */
+    trace_buf[idx].event |= TRACE_RECORD_VALID;
+
+    if ((idx + 1) % TRACE_BUF_FLUSH_THRESHOLD == 0) {
+        flush_trace_file(false);
     }
 }
 
@@ -195,24 +213,93 @@ void trace6(TraceEventID event, uint64_t x1, uint64_t x2, 
uint64_t x3, uint64_t
     trace(event, x1, x2, x3, x4, x5, x6);
 }
 
+void st_set_trace_file_enabled(bool enable)
+{
+    if (enable == !!trace_fp) {
+        return; /* no change */
+    }
+
+    /* Halt trace writeout */
+    flush_trace_file(true);
+    trace_writeout_enabled = false;
+    flush_trace_file(true);
+
+    if (enable) {
+        static const TraceRecord header = {
+            .event = HEADER_EVENT_ID,
+            .timestamp_ns = HEADER_MAGIC,
+            .x1 = HEADER_VERSION,
+        };
+
+        trace_fp = fopen(trace_file_name, "w");
+        if (!trace_fp) {
+            return;
+        }
+
+        if (fwrite(&header, sizeof header, 1, trace_fp) != 1) {
+            fclose(trace_fp);
+            trace_fp = NULL;
+            return;
+        }
+
+        /* Resume trace writeout */
+        trace_writeout_enabled = true;
+        flush_trace_file(false);
+    } else {
+        fclose(trace_fp);
+        trace_fp = NULL;
+    }
+}
+
 /**
- * Flush the trace buffer on exit
+ * Set the name of a trace file
+ *
+ * @file        The trace file name or NULL for the default name-<pid> set at
+ *              config time
  */
-static void __attribute__((constructor)) st_init(void)
+bool st_set_trace_file(const char *file)
 {
-    atexit(st_flush_trace_buffer);
+    st_set_trace_file_enabled(false);
+
+    free(trace_file_name);
+
+    if (!file) {
+        if (asprintf(&trace_file_name, CONFIG_TRACE_FILE, getpid()) < 0) {
+            trace_file_name = NULL;
+            return false;
+        }
+    } else {
+        if (asprintf(&trace_file_name, "%s", file) < 0) {
+            trace_file_name = NULL;
+            return false;
+        }
+    }
+
+    st_set_trace_file_enabled(true);
+    return true;
+}
+
+void st_print_trace_file_status(FILE *stream, int (*stream_printf)(FILE 
*stream, const char *fmt, ...))
+{
+    stream_printf(stream, "Trace file \"%s\" %s.\n",
+                  trace_file_name, trace_fp ? "on" : "off");
 }
 
 void st_print_trace(FILE *stream, int (*stream_printf)(FILE *stream, const 
char *fmt, ...))
 {
     unsigned int i;
 
-    for (i = 0; i < trace_idx; i++) {
+    for (i = 0; i < TRACE_BUF_LEN; i++) {
+        TraceRecord record;
+
+        if (!get_trace_record(i, &record)) {
+            continue;
+        }
         stream_printf(stream, "Event %" PRIu64 " : %" PRIx64 " %" PRIx64
                       " %" PRIx64 " %" PRIx64 " %" PRIx64 " %" PRIx64 "\n",
-                      trace_buf[i].event, trace_buf[i].x1, trace_buf[i].x2,
-                      trace_buf[i].x3, trace_buf[i].x4, trace_buf[i].x5,
-                      trace_buf[i].x6);
+                      record.event, record.x1, record.x2,
+                      record.x3, record.x4, record.x5,
+                      record.x6);
     }
 }
 
@@ -226,30 +313,44 @@ void st_print_trace_events(FILE *stream, int 
(*stream_printf)(FILE *stream, cons
     }
 }
 
-static TraceEvent* find_trace_event_by_name(const char *tname)
+bool st_change_trace_event_state(const char *name, bool enabled)
 {
     unsigned int i;
 
-    if (!tname) {
-        return NULL;
-    }
-
     for (i = 0; i < NR_TRACE_EVENTS; i++) {
-        if (!strcmp(trace_list[i].tp_name, tname)) {
-            return &trace_list[i];
+        if (!strcmp(trace_list[i].tp_name, name)) {
+            trace_list[i].state = enabled;
+            return true;
         }
     }
-    return NULL; /* indicates end of list reached without a match */
+    return false;
+}
+
+void st_flush_trace_buffer(void)
+{
+    flush_trace_file(true);
 }
 
-bool st_change_trace_event_state(const char *tname, bool tstate)
+void st_init(const char *file)
 {
-    TraceEvent *tp;
+    pthread_t thread;
+    pthread_attr_t attr;
+    sigset_t set, oldset;
+    int ret;
+
+    pthread_attr_init(&attr);
+    pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
 
-    tp = find_trace_event_by_name(tname);
-    if (tp) {
-        tp->state = tstate;
-        return true;
+    sigfillset(&set);
+    pthread_sigmask(SIG_SETMASK, &set, &oldset);
+    ret = pthread_create(&thread, &attr, writeout_thread, NULL);
+    pthread_sigmask(SIG_SETMASK, &oldset, NULL);
+
+    if (ret != 0) {
+        error_report("warning: unable to create trace file thread\n");
+        return;
     }
-    return false;
+
+    atexit(st_flush_trace_buffer);
+    st_set_trace_file(file);
 }
diff --git a/simpletrace.h b/simpletrace.h
index 2f44ed3..3a5bd9f 100644
--- a/simpletrace.h
+++ b/simpletrace.h
@@ -15,6 +15,7 @@
 #include <stdbool.h>
 #include <stdio.h>
 
+#ifdef CONFIG_SIMPLE_TRACE
 typedef uint64_t TraceEventID;
 
 typedef struct {
@@ -36,5 +37,12 @@ void st_print_trace_file_status(FILE *stream, 
fprintf_function stream_printf);
 void st_set_trace_file_enabled(bool enable);
 bool st_set_trace_file(const char *file);
 void st_flush_trace_buffer(void);
+void st_init(const char *file);
+#else
+static inline void st_init(const char *file)
+{
+    /* Do nothing */
+}
+#endif /* !CONFIG_SIMPLE_TRACE */
 
 #endif /* SIMPLETRACE_H */
diff --git a/vl.c b/vl.c
index b436952..5e007a7 100644
--- a/vl.c
+++ b/vl.c
@@ -47,9 +47,6 @@
 #include <dirent.h>
 #include <netdb.h>
 #include <sys/select.h>
-#ifdef CONFIG_SIMPLE_TRACE
-#include "trace.h"
-#endif
 
 #ifdef CONFIG_BSD
 #include <sys/stat.h>
@@ -159,6 +156,7 @@ int main(int argc, char **argv)
 #include "slirp/libslirp.h"
 
 #include "trace.h"
+#include "simpletrace.h"
 #include "qemu-queue.h"
 #include "cpus.h"
 #include "arch_init.h"
@@ -1941,10 +1939,8 @@ int main(int argc, char **argv, char **envp)
     const char *incoming = NULL;
     int show_vnc_port = 0;
     int defconfig = 1;
-
-#ifdef CONFIG_SIMPLE_TRACE
     const char *trace_file = NULL;
-#endif
+
     atexit(qemu_run_exit_notifiers);
     error_set_progname(argv[0]);
 
@@ -2770,6 +2766,8 @@ int main(int argc, char **argv, char **envp)
     }
     loc_set_none();
 
+    st_init(trace_file);
+
     /* If no data_dir is specified then try to find it relative to the
        executable path.  */
     if (!data_dir) {
@@ -2780,12 +2778,6 @@ int main(int argc, char **argv, char **envp)
         data_dir = CONFIG_QEMU_DATADIR;
     }
 
-#ifdef CONFIG_SIMPLE_TRACE
-    /*
-     * Set the trace file name, if specified.
-     */
-    st_set_trace_file(trace_file);
-#endif
     /*
      * Default to max_cpus = smp_cpus, in case the user doesn't
      * specify a max_cpus value.
-- 
1.7.2.3