Re: [PATCH 1/2] writeback: Improve busyloop prevention

[Wu Fengguang <fengguang.wu@intel.com>]

Jan,

> > I don't find any misbehaves in the graphs.
> > So in general I think the test results are acceptable.
>   Thanks for running the tests. I looked through the results and given the
> variation I would be happy with them. Will you merge the patch or should I
> resend it?

The patch I tested is this one. It's obviously better than w/ patch.

Looking backing a bit more, the change is related to the old topic
"how can the flush order align better with the LRU order". The general
idea was to make background work behave more like kupdate work which I
still think is a good idea. However the patch I proposed at the time
have some unpleasant/convoluted "goto" and didn't make it eventually.
The code is then leaved in this bad state...

I'll merge this patch.

Thanks,
Fengguang
---
Subject: writeback: Include all dirty inodes in background writeback
Date: Wed, 19 Oct 2011 11:44:41 +0200

From: Jan Kara <jack@suse.cz>

Current livelock avoidance code makes background work to include only inodes
that were dirtied before background writeback has started. However background
writeback can be running for a long time and thus excluding newly dirtied
inodes can eventually exclude significant portion of dirty inodes making
background writeback inefficient. Since background writeback avoids livelocking
the flusher thread by yielding to any other work, there is no real reason why
background work should not include all dirty inodes so change the logic in
wb_writeback().

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 fs/fs-writeback.c |   10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

--- linux-next.orig/fs/fs-writeback.c	2011-10-31 00:14:14.000000000 +0800
+++ linux-next/fs/fs-writeback.c	2011-10-31 21:59:37.000000000 +0800
@@ -780,11 +780,17 @@ static long wb_writeback(struct bdi_writ
 		if (work->for_background && !over_bground_thresh(wb->bdi))
 			break;
 
+		/*
+		 * Kupdate and background works are special and we want to
+		 * include all inodes that need writing. Livelock avoidance is
+		 * handled by these works yielding to any other work so we are
+		 * safe.
+		 */
 		if (work->for_kupdate) {
 			oldest_jif = jiffies -
 				msecs_to_jiffies(dirty_expire_interval * 10);
-			work->older_than_this = &oldest_jif;
-		}
+		} else if (work->for_background)
+			oldest_jif = jiffies;
 
 		trace_writeback_start(wb->bdi, work);
 		if (list_empty(&wb->b_io))