writeback: avoid unnecessary calculation of bdi dirty thresholds

Split get_dirty_limits() into global_dirty_limits()+bdi_dirty_limit(), so
that the latter can be avoided when under global dirty background
threshold (which is the normal state for most systems).

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 38 insertions, 37 deletions

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 2cf69a5e46e6..1ea13ef350a8 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -267,10 +267,11 @@ static inline void task_dirties_fraction(struct task_struct *tsk,
  *
  *   dirty -= (dirty/8) * p_{t}
  */
-static void task_dirty_limit(struct task_struct *tsk, unsigned long *pdirty)
+static unsigned long task_dirty_limit(struct task_struct *tsk,
+                                       unsigned long bdi_dirty)
 {
         long numerator, denominator;
-        unsigned long dirty = *pdirty;
+        unsigned long dirty = bdi_dirty;
         u64 inv = dirty >> 3;
 
         task_dirties_fraction(tsk, &numerator, &denominator);
@@ -278,10 +279,8 @@ static void task_dirty_limit(struct task_struct *tsk, unsigned long *pdirty)
         do_div(inv, denominator);
 
         dirty -= inv;
-        if (dirty < *pdirty/2)
-                dirty = *pdirty/2;
 
-        *pdirty = dirty;
+        return max(dirty, bdi_dirty/2);
 }
 
 /*
@@ -391,9 +390,7 @@ unsigned long determine_dirtyable_memory(void)
         return x + 1;   /* Ensure that we never return 0 */
 }
 
-void
-get_dirty_limits(unsigned long *pbackground, unsigned long *pdirty,
-                 unsigned long *pbdi_dirty, struct backing_dev_info *bdi)
+void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
 {
         unsigned long background;
         unsigned long dirty;
@@ -425,26 +422,28 @@ get_dirty_limits(unsigned long *pbackground, unsigned long *pdirty,
         }
         *pbackground = background;
         *pdirty = dirty;
+}
 
-        if (bdi) {
-                u64 bdi_dirty;
-                long numerator, denominator;
+unsigned long bdi_dirty_limit(struct backing_dev_info *bdi,
+                               unsigned long dirty)
+{
+        u64 bdi_dirty;
+        long numerator, denominator;
 
-                /*
-                 * Calculate this BDI's share of the dirty ratio.
-                 */
-                bdi_writeout_fraction(bdi, &numerator, &denominator);
+        /*
+         * Calculate this BDI's share of the dirty ratio.
+         */
+        bdi_writeout_fraction(bdi, &numerator, &denominator);
 
-                bdi_dirty = (dirty * (100 - bdi_min_ratio)) / 100;
-                bdi_dirty *= numerator;
-                do_div(bdi_dirty, denominator);
-                bdi_dirty += (dirty * bdi->min_ratio) / 100;
-                if (bdi_dirty > (dirty * bdi->max_ratio) / 100)
-                        bdi_dirty = dirty * bdi->max_ratio / 100;
+        bdi_dirty = (dirty * (100 - bdi_min_ratio)) / 100;
+        bdi_dirty *= numerator;
+        do_div(bdi_dirty, denominator);
 
-                *pbdi_dirty = bdi_dirty;
-                task_dirty_limit(current, pbdi_dirty);
-        }
+        bdi_dirty += (dirty * bdi->min_ratio) / 100;
+        if (bdi_dirty > (dirty * bdi->max_ratio) / 100)
+                bdi_dirty = dirty * bdi->max_ratio / 100;
+
+        return bdi_dirty;
 }
 
 /*
@@ -475,13 +474,24 @@ static void balance_dirty_pages(struct address_space *mapping,
                         .range_cyclic   = 1,
                 };
 
-                get_dirty_limits(&background_thresh, &dirty_thresh,
-                                &bdi_thresh, bdi);
-
                 nr_reclaimable = global_page_state(NR_FILE_DIRTY) +
                                         global_page_state(NR_UNSTABLE_NFS);
                 nr_writeback = global_page_state(NR_WRITEBACK);
 
+                global_dirty_limits(&background_thresh, &dirty_thresh);
+
+                /*
+                 * Throttle it only when the background writeback cannot
+                 * catch-up. This avoids (excessively) small writeouts
+                 * when the bdi limits are ramping up.
+                 */
+                if (nr_reclaimable + nr_writeback <
+                                (background_thresh + dirty_thresh) / 2)
+                        break;
+
+                bdi_thresh = bdi_dirty_limit(bdi, dirty_thresh);
+                bdi_thresh = task_dirty_limit(current, bdi_thresh);
+
                 /*
                  * In order to avoid the stacked BDI deadlock we need
                  * to ensure we accurately count the 'dirty' pages when
@@ -513,15 +523,6 @@ static void balance_dirty_pages(struct address_space *mapping,
                 if (!dirty_exceeded)
                         break;
 
-                /*
-                 * Throttle it only when the background writeback cannot
-                 * catch-up. This avoids (excessively) small writeouts
-                 * when the bdi limits are ramping up.
-                 */
-                if (nr_reclaimable + nr_writeback <
-                                (background_thresh + dirty_thresh) / 2)
-                        break;
-
                 if (!bdi->dirty_exceeded)
                         bdi->dirty_exceeded = 1;
 
@@ -634,7 +635,7 @@ void throttle_vm_writeout(gfp_t gfp_mask)
         unsigned long dirty_thresh;
 
         for ( ; ; ) {
-                get_dirty_limits(&background_thresh, &dirty_thresh, NULL, NULL);
+                global_dirty_limits(&background_thresh, &dirty_thresh);
 
                 /*
                  * Boost the allowable dirty threshold a bit for page