Skip to contentSkip to conversation

Patch "writeback: use higher precision calculation in domain_dirty_limits()" has been added to the 4.6-stable tree

PATCH [email protected] 1 participant 1 message Jul 11, 2016

Subsystems (3)
Stable branch Memory management Page cache
lore
Share & tools
lore

[email protected]

author maintainer Applied
#0 lore raw

This is a note to let you know that I've just added the patch titled

writeback: use higher precision calculation in domain_dirty_limits()

to the 4.6-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
writeback-use-higher-precision-calculation-in-domain_dirty_limits.patch
and it can be found in the queue-4.6 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

Quoted 1 line

From 62a584fe05eef1f80ed49a286a29328f1a224fb9 Mon Sep 17 00:00:00 2001

From: Tejun Heo <[email protected]>
Date: Fri, 27 May 2016 14:34:46 -0400
Subject: writeback: use higher precision calculation in domain_dirty_limits()

From: Tejun Heo <[email protected]>

commit 62a584fe05eef1f80ed49a286a29328f1a224fb9 upstream.

As vm.dirty_[background_]bytes can't be applied verbatim to multiple cgroup writeback domains, they get converted to percentages in domain_dirty_limits() and applied the same way as vm.dirty_[background]ratio. However, if the specified bytes is lower than 1% of available memory, the calculated ratios become zero and the writeback domain gets throttled constantly.

Fix it by using per-PAGE_SIZE instead of percentage for ratio calculations. Also, the updated DIV_ROUND_UP() usages now should yield 1/4096 (0.0244%) as the minimum ratio as long as the specified bytes are above zero.

Signed-off-by Tejun Heo <[email protected]>
Reported-by Miao Xie <[email protected]>
Link http://lkml.kernel.org/g/[email protected] open here
Fixes 9fc3a43e1757 ("writeback: separate out domain_dirty_limits()")
Reviewed-by Jan Kara <[email protected]>
Signed-off-by Greg Kroah-Hartman <[email protected]>

Adjusted comment based on Jan's suggestion.

Signed-off-by Jens Axboe <[email protected]>
---
 mm/page-writeback.c |   21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)
mm/page-writeback.c +12 -9
@@ -369,8 +369,9 @@ static void domain_dirty_limits(struct d
 	struct dirty_throttle_control *gdtc = mdtc_gdtc(dtc);
 	unsigned long bytes = vm_dirty_bytes;
 	unsigned long bg_bytes = dirty_background_bytes;
-	unsigned long ratio = vm_dirty_ratio;
-	unsigned long bg_ratio = dirty_background_ratio;
+	/* convert ratios to per-PAGE_SIZE for higher precision */
+	unsigned long ratio = (vm_dirty_ratio * PAGE_SIZE) / 100;
+	unsigned long bg_ratio = (dirty_background_ratio * PAGE_SIZE) / 100;
 	unsigned long thresh;
 	unsigned long bg_thresh;
 	struct task_struct *tsk;

@@ -382,26 +383,28 @@ static void domain_dirty_limits(struct d
 		/*
 		 * The byte settings can't be applied directly to memcg
 		 * domains.  Convert them to ratios by scaling against
-		 * globally available memory.
+		 * globally available memory.  As the ratios are in
+		 * per-PAGE_SIZE, they can be obtained by dividing bytes by
+		 * number of pages.
 		 */
 		if (bytes)
-			ratio = min(DIV_ROUND_UP(bytes, PAGE_SIZE) * 100 /
-				    global_avail, 100UL);
+			ratio = min(DIV_ROUND_UP(bytes, global_avail),
+				    PAGE_SIZE);
 		if (bg_bytes)
-			bg_ratio = min(DIV_ROUND_UP(bg_bytes, PAGE_SIZE) * 100 /
-				       global_avail, 100UL);
+			bg_ratio = min(DIV_ROUND_UP(bg_bytes, global_avail),
+				       PAGE_SIZE);
 		bytes = bg_bytes = 0;
 	}
 
 	if (bytes)
 		thresh = DIV_ROUND_UP(bytes, PAGE_SIZE);
 	else
-		thresh = (ratio * available_memory) / 100;
+		thresh = (ratio * available_memory) / PAGE_SIZE;
 
 	if (bg_bytes)
 		bg_thresh = DIV_ROUND_UP(bg_bytes, PAGE_SIZE);
 	else
-		bg_thresh = (bg_ratio * available_memory) / 100;
+		bg_thresh = (bg_ratio * available_memory) / PAGE_SIZE;
 
 	if (bg_thresh >= thresh)
 		bg_thresh = thresh / 2;

Patches currently in stable-queue which might be from [email protected] are

queue-4.6/writeback-use-higher-precision-calculation-in-domain_dirty_limits.patch