From: Larry Woodman <lwoodman@redhat.com>
Date: Tue, 28 Apr 2009 15:11:38 -0400
Subject: [mm] allow tuning of MAX_WRITEBACK_PAGES
Message-id: 1240945898.16032.19.camel@dhcp-100-19-198.bos.redhat.com
O-Subject: [RHEL5-U4 patch] Allow tuning of MAX_WRITEBACK_PAGES.
Bugzilla: 479079
RH-Acked-by: Josef Bacik <josef@redhat.com>
RH-Acked-by: Rik van Riel <riel@redhat.com>
RH-Acked-by: Dave Anderson <anderson@redhat.com>

On April 15(tax day!!!) I posted a patch to mpage_writepages() and
wb_kupdate() to prevent kupdate from not writing all modified pagecache
pages if you continuously extend a file fast enough.

In addition to that patch the same customers are asking to be able to
tune MAX_WRITEBACK_PAGES.  It is #defined to 1024 and controls the
maximum number of pages that pdflush writes out during each iteration of
background_writeout() and wb_kupdate().  The upstream status is
MAX_WRITEBACK_PAGES is still #defined to 1024 but that kernel is much
smarter about write-backs on a per-device basis.   On large systems with
lots of RAM and fast IO devices you can clearly see that increasing it
will allow more pages to be written during each iteration before
blocking due to IO WRITE congestion and therefore more work gets done
faster.

I left the default at 1024 and just made it tunable
vi /proc/sys/vm/max_writeback_pages.

This patch is needed in conjunction with the kupdate patch to fix BZ
479079.

diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index d4b00d4..65bb74a 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -206,6 +206,7 @@ enum
 	VM_PAGECACHE=37,        /* favor reclaiming unmapped pagecache pages */
 	VM_MMAP_MIN_ADDR=38, 	/* amount of memory to protect from mmap */
 	VM_FLUSH_MMAP=39,       /* flush mmap()d pagecache pages */
+	VM_MAX_WRITEBACK_PAGES=40, /*maximum pages written per writeback loop */
 };
 
 
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index e181616..c70404f 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -78,6 +78,7 @@ extern int sysctl_drop_caches;
 extern int percpu_pagelist_fraction;
 extern int compat_log;
 extern int flush_mmap_pages;
+extern int max_writeback_pages;
 
 #if defined(CONFIG_X86_LOCAL_APIC) && defined(CONFIG_X86)
 extern int proc_unknown_nmi_panic(ctl_table *, int, struct file *,
@@ -1130,6 +1131,16 @@ static ctl_table vm_table[] = {
 		.strategy	= &sysctl_intvec,
 		.extra1		= &zero,
 	},
+	{
+		.ctl_name	= VM_MAX_WRITEBACK_PAGES,
+		.procname	= "max_writeback_pages",
+		.data		= &max_writeback_pages,
+		.maxlen		= sizeof(max_writeback_pages),
+		.mode		= 0644,
+		.proc_handler	= &proc_dointvec,
+		.strategy	= &sysctl_intvec,
+		.extra1		= &zero,
+	},
 	{ .ctl_name = 0 }
 };
 
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 4edf8e9..6b3a818 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -40,6 +40,7 @@
  * the dirty each time it has written this many pages.
  */
 #define MAX_WRITEBACK_PAGES	1024
+int max_writeback_pages = MAX_WRITEBACK_PAGES;
 
 /*
  * After a CPU has dirtied this many pages, balance_dirty_pages_ratelimited
@@ -347,10 +348,10 @@ static void background_writeout(unsigned long _min_pages)
 				&& min_pages <= 0)
 			break;
 		wbc.encountered_congestion = 0;
-		wbc.nr_to_write = MAX_WRITEBACK_PAGES;
+		wbc.nr_to_write = max_writeback_pages;
 		wbc.pages_skipped = 0;
 		writeback_inodes(&wbc);
-		min_pages -= MAX_WRITEBACK_PAGES - wbc.nr_to_write;
+		min_pages -= max_writeback_pages - wbc.nr_to_write;
 		if (wbc.nr_to_write > 0 || wbc.pages_skipped > 0) {
 			/* Wrote less than expected */
 			blk_congestion_wait(WRITE, HZ/10);
@@ -422,7 +423,7 @@ static void wb_kupdate(unsigned long arg)
 			(inodes_stat.nr_inodes - inodes_stat.nr_unused);
 	while (nr_to_write > 0) {
 		wbc.encountered_congestion = 0;
-		wbc.nr_to_write = MAX_WRITEBACK_PAGES;
+		wbc.nr_to_write = max_writeback_pages;
 		writeback_inodes(&wbc);
 		if (wbc.nr_to_write > 0) {
 			if (wbc.encountered_congestion)
@@ -430,7 +431,7 @@ static void wb_kupdate(unsigned long arg)
 			else
 				break;	/* All the old data is written */
 		}
-		nr_to_write -= MAX_WRITEBACK_PAGES - wbc.nr_to_write;
+		nr_to_write -= max_writeback_pages - wbc.nr_to_write;
 	}
 	if (time_before(next_jif, jiffies + HZ))
 		next_jif = jiffies + HZ;