From: Bart Van Assche <bvanassche@acm.org>
Date: Sun, 21 Jun 2020 21:31:08 +0000 (-0700)
Subject: Optimize fio_gettime_offload()
X-Git-Tag: fio-3.21~28^2
X-Git-Url: https://git.kernel.dk/?p=fio.git;a=commitdiff_plain;h=817a1d8301824effbf9ea3352e6e722a3515473c

Optimize fio_gettime_offload()

This patch not only an optimization but also a bug fix because it guarantees
that fio_gettime_offload() never returns an inconsistent timestamp, something
that could happen before. In this context inconsistent means returning the
.tv_sec value from one gettimeofday() call and the .tv_usec value from another
gettimeofday() call.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---

diff --git a/gettime-thread.c b/gettime-thread.c
index 0a2cc6c4..953e4e67 100644
--- a/gettime-thread.c
+++ b/gettime-thread.c
@@ -2,9 +2,10 @@
 #include <time.h>
 
 #include "fio.h"
+#include "lib/seqlock.h"
 #include "smalloc.h"
 
-struct timespec *fio_ts = NULL;
+struct fio_ts *fio_ts;
 int fio_gtod_offload = 0;
 static pthread_t gtod_thread;
 static os_cpu_mask_t fio_gtod_cpumask;
@@ -19,15 +20,17 @@ void fio_gtod_init(void)
 
 static void fio_gtod_update(void)
 {
-	if (fio_ts) {
-		struct timeval __tv;
-
-		gettimeofday(&__tv, NULL);
-		fio_ts->tv_sec = __tv.tv_sec;
-		write_barrier();
-		fio_ts->tv_nsec = __tv.tv_usec * 1000;
-		write_barrier();
-	}
+	struct timeval __tv;
+
+	if (!fio_ts)
+		return;
+
+	gettimeofday(&__tv, NULL);
+
+	write_seqlock_begin(&fio_ts->seqlock);
+	fio_ts->ts.tv_sec = __tv.tv_sec;
+	fio_ts->ts.tv_nsec = __tv.tv_usec * 1000;
+	write_seqlock_end(&fio_ts->seqlock);
 }
 
 struct gtod_cpu_data {
diff --git a/gettime.h b/gettime.h
index f92ee8c4..c55f5cba 100644
--- a/gettime.h
+++ b/gettime.h
@@ -4,6 +4,7 @@
 #include <sys/time.h>
 
 #include "arch/arch.h"
+#include "lib/seqlock.h"
 
 /*
  * Clock sources
@@ -22,20 +23,22 @@ extern int fio_start_gtod_thread(void);
 extern int fio_monotonic_clocktest(int debug);
 extern void fio_local_clock_init(void);
 
-extern struct timespec *fio_ts;
+extern struct fio_ts {
+	struct seqlock seqlock;
+	struct timespec ts;
+} *fio_ts;
 
 static inline int fio_gettime_offload(struct timespec *ts)
 {
-	time_t last_sec;
+	unsigned int seq;
 
 	if (!fio_ts)
 		return 0;
 
 	do {
-		read_barrier();
-		last_sec = ts->tv_sec = fio_ts->tv_sec;
-		ts->tv_nsec = fio_ts->tv_nsec;
-	} while (fio_ts->tv_sec != last_sec);
+		seq = read_seqlock_begin(&fio_ts->seqlock);
+		*ts = fio_ts->ts;
+	} while (read_seqlock_retry(&fio_ts->seqlock, seq));
 
 	return 1;
 }
diff --git a/t/debug.c b/t/debug.c
index 8965cfbc..0c913368 100644
--- a/t/debug.c
+++ b/t/debug.c
@@ -1,7 +1,7 @@
 #include <stdio.h>
 
 FILE *f_err;
-struct timespec *fio_ts = NULL;
+void *fio_ts;
 unsigned long fio_debug = 0;
 
 void __dprint(int type, const char *str, ...)