Re: [PATCH 18/45] writeback: introduce wait queue forbalance_dirty_pages()

[Wu Fengguang]

On Thu, Oct 08, 2009 at 10:40:37AM +0800, KAMEZAWA Hiroyuki wrote:
> On Thu, 8 Oct 2009 09:58:22 +0800
> Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:
> > On Thu, Oct 08, 2009 at 09:01:59AM +0800, KAMEZAWA Hiroyuki wrote:
> > > IIUC, "iowait" cpustat data was calculated by runqueue->nr_iowait as
> > > == kernel/schec.c
> > > void account_idle_time(cputime_t cputime)
> > > {
> > >         struct cpu_usage_stat *cpustat = &kstat_this_cpu.cpustat;
> > >         cputime64_t cputime64 = cputime_to_cputime64(cputime);
> > >         struct rq *rq = this_rq();
> > > 
> > >         if (atomic_read(&rq->nr_iowait) > 0)
> > >                 cpustat->iowait = cputime64_add(cpustat->iowait, cputime64);
> > >         else
> > >                 cpustat->idle = cputime64_add(cpustat->idle, cputime64);
> > > }
> > > ==
> > > Then, for showing "cpu is in iowait", runqueue->nr_iowait should be modified
> > > at some places. In old kernel, congestion_wait() at el did that by calling
> > > io_schedule_timeout().
> > > 
> > > How this runqueue->nr_iowait is handled now ?
> > 
> > Good question. io_schedule() has an old comment for throttling IO wait:
> > 
> >          * But don't do that if it is a deliberate, throttling IO wait (this task
> >          * has set its backing_dev_info: the queue against which it should throttle)
> >          */
> >         void __sched io_schedule(void)
> > 
> > So it looks both Jens' and this patch behaves right in ignoring the
> > iowait accounting for balance_dirty_pages() :)
> > 
> Thank you for clarification.
> Then, hmm, %iotwait (which 'top' shows) didn't work as desgined and we need
> to update throttle_vm_writeout() and some in vmscan.c. Thanks for input.

Thanks, you also reminds me to do io_schedule() in the nfs writeback
wait queue :)

> BTW, I'm glad if I can know "how many threads/ios are throttoled now" per bdi.

Good suggestion. How about this patch?

---
writeback: show per-bdi throttled tasks

All currently throttled tasks will be listed, showing the pages to
writeback for them, and total wait time since blocked.

	# cat /debug/bdi/8:0/throttle_list
	goal=6144kb	waited=32ms
	goal=6144kb     waited=48ms

CC: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
 fs/fs-writeback.c           |    1 +
 include/linux/backing-dev.h |    2 ++
 mm/backing-dev.c            |   33 +++++++++++++++++++++++++++++++++
 3 files changed, 36 insertions(+)

--- linux.orig/include/linux/backing-dev.h	2009-10-08 11:46:28.000000000 +0800
+++ linux/include/linux/backing-dev.h	2009-10-08 11:47:39.000000000 +0800
@@ -101,6 +101,7 @@ struct backing_dev_info {
 #ifdef CONFIG_DEBUG_FS
 	struct dentry *debug_dir;
 	struct dentry *debug_stats;
+	struct dentry *debug_throttle;
 #endif
 };
 
@@ -116,6 +117,7 @@ struct backing_dev_info {
 #define DIRTY_THROTTLE_PAGES_STOP	(1 << 22)
 
 struct dirty_throttle_task {
+	unsigned long		start_time;
 	long			nr_pages;
 	struct list_head	list;
 	struct completion	complete;
--- linux.orig/mm/backing-dev.c	2009-10-08 11:47:37.000000000 +0800
+++ linux/mm/backing-dev.c	2009-10-08 11:59:06.000000000 +0800
@@ -115,6 +115,23 @@ static int bdi_debug_stats_show(struct s
 	return 0;
 }
 
+static int bdi_debug_throttle_show(struct seq_file *m, void *v)
+{
+	struct backing_dev_info *bdi = m->private;
+	struct dirty_throttle_task *tt;
+	unsigned long flags;
+
+	spin_lock_irqsave(&bdi->throttle_lock, flags);
+	list_for_each_entry(tt, &bdi->throttle_list, list) {
+		seq_printf(m, "goal=%lukb\twaited=%lums\n",
+			   tt->nr_pages << (PAGE_SHIFT - 10),
+			   (jiffies - tt->start_time) * 1000 / HZ);
+	}
+	spin_unlock_irqrestore(&bdi->throttle_lock, flags);
+
+	return 0;
+}
+
 static int bdi_debug_stats_open(struct inode *inode, struct file *file)
 {
 	return single_open(file, bdi_debug_stats_show, inode->i_private);
@@ -127,15 +144,31 @@ static const struct file_operations bdi_
 	.release	= single_release,
 };
 
+static int bdi_debug_throttle_open(struct inode *inode, struct file *file)
+{
+	return single_open(file, bdi_debug_throttle_show, inode->i_private);
+}
+
+static const struct file_operations bdi_debug_throttle_fops = {
+	.open		= bdi_debug_throttle_open,
+	.read		= seq_read,
+	.llseek		= seq_lseek,
+	.release	= single_release,
+};
+
 static void bdi_debug_register(struct backing_dev_info *bdi, const char *name)
 {
 	bdi->debug_dir = debugfs_create_dir(name, bdi_debug_root);
 	bdi->debug_stats = debugfs_create_file("stats", 0444, bdi->debug_dir,
 					       bdi, &bdi_debug_stats_fops);
+	bdi->debug_throttle = debugfs_create_file("throttle_list", 0444,
+						  bdi->debug_dir, bdi,
+						  &bdi_debug_throttle_fops);
 }
 
 static void bdi_debug_unregister(struct backing_dev_info *bdi)
 {
+	debugfs_remove(bdi->debug_throttle);
 	debugfs_remove(bdi->debug_stats);
 	debugfs_remove(bdi->debug_dir);
 }
--- linux.orig/fs/fs-writeback.c	2009-10-08 11:44:27.000000000 +0800
+++ linux/fs/fs-writeback.c	2009-10-08 11:47:39.000000000 +0800
@@ -284,6 +284,7 @@ static void bdi_calc_write_bandwidth(str
 void bdi_writeback_wait(struct backing_dev_info *bdi, long nr_pages)
 {
 	struct dirty_throttle_task tt = {
+		.start_time = jiffies,
 		.nr_pages = nr_pages,
 		.complete = COMPLETION_INITIALIZER_ONSTACK(tt.complete),
 	};
--
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