Speed up WB_SYNC_NONE when a WB_SYNC_ALL occurs simultaneously

Page writebacks with WB_SYNC_NONE can take several seconds to complete 
since they wait for the transaction group to close before being 
committed. This is usually not a problem since the caller does not 
need to wait. However, if we're simultaneously doing a writeback 
with WB_SYNC_ALL (e.g via msync), the latter can block for several 
seconds (up to zfs_txg_timeout) due to the active WB_SYNC_NONE 
writeback since it needs to wait for the transaction to complete 
and the PG_writeback bit to be cleared.

This commit deals with 2 cases:

- No page writeback is active. A WB_SYNC_ALL page writeback starts 
  and even completes. But when it's about to check if the PG_writeback 
  bit has been cleared, another writeback with WB_SYNC_NONE starts. 
  The sync page writeback ends up waiting for the non-sync page 
  writeback to complete.

- A page writeback with WB_SYNC_NONE is already active when a 
  WB_SYNC_ALL writeback starts. The WB_SYNC_ALL writeback ends up 
  waiting for the WB_SYNC_NONE writeback.

The fix works by carefully keeping track of active sync/non-sync 
writebacks and committing when beneficial.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Shaan Nobee <sniper111@gmail.com>
Closes #12662
Closes #12790

tests/zfs-tests/cmd/.gitignore

@@ -18,6 +18,7 @@
 /mmap_exec
 /mmap_libaio
 /mmap_seek
+/mmap_sync
 /mmapwrite
 /nvlist_to_lua
 /randfree_file

tests/zfs-tests/cmd/Makefile.am

@@ -80,9 +80,10 @@ mkfiles_SOURCES = mkfiles.c
 mktree_SOURCES = mktree.c
 
 
-pkgexec_PROGRAMS += mmap_exec mmap_seek mmapwrite readmmap
+pkgexec_PROGRAMS += mmap_exec mmap_seek mmap_sync mmapwrite readmmap
 mmap_exec_SOURCES = mmap_exec.c
 mmap_seek_SOURCES = mmap_seek.c
+mmap_sync_SOURCES = mmap_sync.c
 mmapwrite_SOURCES = mmapwrite.c
 mmapwrite_LDADD = -lpthread
 readmmap_SOURCES = readmmap.c

tests/zfs-tests/cmd/mmap_sync.c

@@ -0,0 +1,152 @@
+/*
+ * CDDL HEADER START
+ *
+ * The contents of this file are subject to the terms of the
+ * Common Development and Distribution License (the "License").
+ * You may not use this file except in compliance with the License.
+ *
+ * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+ * or http://opensource.org/licenses/CDDL-1.0.
+ * See the License for the specific language governing permissions
+ * and limitations under the License.
+ *
+ * When distributing Covered Code, include this CDDL HEADER in each
+ * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+ * If applicable, add the following below this CDDL HEADER, with the
+ * fields enclosed by brackets "[]" replaced with your own identifying
+ * information: Portions Copyright [yyyy] [name of copyright owner]
+ *
+ * CDDL HEADER END
+ */
+
+#include <stdlib.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <sys/time.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <time.h>
+
+static void
+cleanup(char *file)
+{
+	remove(file);
+}
+
+int
+main(int argc, char *argv[])
+{
+	char *testdir = getenv("TESTDIR");
+	if (!testdir) {
+		fprintf(stderr, "environment variable TESTDIR not set\n");
+		return (1);
+	}
+
+	struct stat st;
+	umask(0);
+	if (stat(testdir, &st) != 0 &&
+	    mkdir(testdir, 0777) != 0) {
+		perror("mkdir");
+		return (1);
+	}
+
+	if (argc > 3) {
+		fprintf(stderr, "usage: %s "
+		    "[run time in mins] "
+		    "[max msync time in ms]\n", argv[0]);
+		return (1);
+	}
+
+	int run_time_mins = 5;
+	if (argc >= 2) {
+		run_time_mins = atoi(argv[1]);
+	}
+
+	int max_msync_time_ms = 1000;
+	if (argc >= 3) {
+		max_msync_time_ms = atoi(argv[2]);
+	}
+
+	char filepath[512];
+	filepath[0] = '\0';
+	char *file = &filepath[0];
+
+	strcat(file, testdir);
+	strcat(file, "/msync_file");
+
+	const int LEN = 8;
+	cleanup(file);
+
+	int fd = open(file, O_CREAT | O_RDWR, S_IRUSR | S_IWUSR |
+	    S_IRGRP | S_IROTH);
+
+	if (fd == -1) {
+		(void) fprintf(stderr, "%s: %s: ", argv[0], file);
+		perror("open");
+		return (1);
+	}
+
+	if (ftruncate(fd, LEN) != 0) {
+		perror("ftruncate");
+		cleanup(file);
+		return (1);
+	}
+
+	void *ptr = mmap(NULL, LEN, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+
+	if (ptr == MAP_FAILED) {
+		perror("mmap");
+		cleanup(file);
+		return (1);
+	}
+
+	struct timeval tstart;
+	gettimeofday(&tstart, NULL);
+
+	long long x = 0LL;
+
+	for (;;) {
+		*((long long *)ptr) = x;
+		x++;
+
+		struct timeval t1, t2;
+		gettimeofday(&t1, NULL);
+		if (msync(ptr, LEN, MS_SYNC|MS_INVALIDATE) != 0) {
+			perror("msync");
+			cleanup(file);
+			return (1);
+		}
+
+		gettimeofday(&t2, NULL);
+
+		double elapsed = (t2.tv_sec - t1.tv_sec) * 1000.0;
+		elapsed += ((t2.tv_usec - t1.tv_usec) / 1000.0);
+		if (elapsed > max_msync_time_ms) {
+			fprintf(stderr, "slow msync: %f ms\n", elapsed);
+			munmap(ptr, LEN);
+			cleanup(file);
+			return (1);
+		}
+
+		double elapsed_start = (t2.tv_sec - tstart.tv_sec) * 1000.0;
+		elapsed_start += ((t2.tv_usec - tstart.tv_usec) / 1000.0);
+		if (elapsed_start > run_time_mins * 60 * 1000) {
+			break;
+		}
+	}
+
+	if (munmap(ptr, LEN) != 0) {
+		perror("munmap");
+		cleanup(file);
+		return (1);
+	}
+
+	if (close(fd) != 0) {
+		perror("close");
+	}
+
+	cleanup(file);
+	return (0);
+}