[PATCH 21/47] writeback: prevent divide error on tiny HZ

[Wu Fengguang]

As suggested by Andrew and Peter:

I do recall hearing of people who set HZ very low, perhaps because their
huge machines were seeing performance problems when the timer tick went
off.  Probably there's no need to do that any more.

But still, we shouldn't hard-wire the (HZ >= 100) assumption if we don't
absolutely need to, and I don't think it is absolutely needed here.

People who do cpu bring-up on very slow FPGAs also lower HZ as far as
possible.

CC: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
 mm/page-writeback.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

--- linux-next.orig/mm/page-writeback.c	2010-12-08 22:44:28.000000000 +0800
+++ linux-next/mm/page-writeback.c	2010-12-08 22:44:28.000000000 +0800
@@ -527,6 +527,7 @@ void bdi_update_write_bandwidth(struct b
 				unsigned long *bw_time,
 				s64 *bw_written)
 {
+	const unsigned long unit_time = max(HZ/100, 1);
 	unsigned long written;
 	unsigned long elapsed;
 	unsigned long bw;
@@ -536,7 +537,7 @@ void bdi_update_write_bandwidth(struct b
 		goto snapshot;
 
 	elapsed = jiffies - *bw_time;
-	if (elapsed < HZ/100)
+	if (elapsed < unit_time)
 		return;
 
 	/*
@@ -550,7 +551,7 @@ void bdi_update_write_bandwidth(struct b
 
 	written = percpu_counter_read(&bdi->bdi_stat[BDI_WRITTEN]) - *bw_written;
 	bw = (HZ * PAGE_CACHE_SIZE * written + elapsed/2) / elapsed;
-	w = min(elapsed / (HZ/100), 128UL);
+	w = min(elapsed / unit_time, 128UL);
 	bdi->write_bandwidth = (bdi->write_bandwidth * (1024-w) + bw * w) >> 10;
 	bdi->write_bandwidth_update_time = jiffies;
 snapshot:


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