[PATCH 12/18] writeback: separate out domain_dirty_limits()

Tejun Heo
2015-03-23T05:11:03Z

global_dirty_limits() calculates thresh and bg_thresh (confusingly
called *pdirty and *pbackground in the function) assuming
global_wb_domain; however, cgroup writeback support requires
considering per-memcg wb_domain too.

This patch separates out domain_dirty_limits() which takes
dirty_throttle_control out of global_dirty_limits(). As thresh and
bg_thresh calculation needs the amount of dirtyable memory in the
domain, dirty_throttle_control->avail is added. The new function
calculates the two thresholds and store them directly in the
dirty_throttle_control.

Also, as memcg domains can't follow vm_dirty_bytes and
dirty_background_bytes settings directly. If those are set and
domain_dirty_limits() is invoked for a !global domain, the settings
are translated to ratios by scaling them against globally available
memory. dirty_throttle_control->gdtc is added to enable this when
CONFIG_CGROUP_WRITEBACK.

global_dirty_limits() is now a thin wrapper around
domain_dirty_limits() and balance_dirty_pages() is updated to use the
new function too.

This patch doesn't introduce any behavioral changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Jan Kara <jack@suse.cz>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Greg Thelen <gthelen@google.com>
---
mm/page-writeback.c | 111 ++++++++++++++++++++++++++++++++++++++++------------
1 file changed, 86 insertions(+), 25 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index e6c7572..7e9922f 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -128,10 +128,12 @@ struct wb_domain global_wb_domain;
struct dirty_throttle_control {
#ifdef CONFIG_CGROUP_WRITEBACK
struct wb_domain *dom;
+ struct dirty_throttle_control *gdtc; /* only set in memcg dtc's */
#endif
struct bdi_writeback *wb;
struct fprop_local_percpu *wb_completions;

+ unsigned long avail; /* dirtyable */
unsigned long dirty; /* file_dirty + write + nfs */
unsigned long thresh; /* dirty threshold */
unsigned long bg_thresh; /* dirty background threshold */
@@ -157,12 +159,18 @@ struct dirty_throttle_control {

#define GDTC_INIT(__wb) .dom = &global_wb_domain, \
DTC_INIT_COMMON(__wb)
+#define GDTC_INIT_NO_WB .dom = &global_wb_domain

static struct wb_domain *dtc_dom(struct dirty_throttle_control *dtc)
{
return dtc->dom;
}

+static struct dirty_throttle_control *mdtc_gdtc(struct dirty_throttle_control *mdtc)
+{
+ return mdtc->gdtc;
+}
+
static void wb_min_max_ratio(struct bdi_writeback *wb,
unsigned long *minp, unsigned long *maxp)
{
@@ -193,12 +201,18 @@ static void wb_min_max_ratio(struct bdi_writeback *wb,
#else /* CONFIG_CGROUP_WRITEBACK */

#define GDTC_INIT(__wb) DTC_INIT_COMMON(__wb)
+#define GDTC_INIT_NO_WB

static struct wb_domain *dtc_dom(struct dirty_throttle_control *dtc)
{
return &global_wb_domain;
}

+static struct dirty_throttle_control *mdtc_gdtc(struct dirty_throttle_control *mdtc)
+{
+ return NULL;
+}
+
static void wb_min_max_ratio(struct bdi_writeback *wb,
unsigned long *minp, unsigned long *maxp)
{
@@ -303,42 +317,88 @@ static unsigned long global_dirtyable_memory(void)
return x + 1; /* Ensure that we never return 0 */
}

-/*
- * global_dirty_limits - background-writeback and dirty-throttling thresholds
+/**
+ * domain_dirty_limits - calculate thresh and bg_thresh for a wb_domain
+ * @dtc: dirty_throttle_control of interest
*
- * Calculate the dirty thresholds based on sysctl parameters
- * - vm.dirty_background_ratio or vm.dirty_background_bytes
- * - vm.dirty_ratio or vm.dirty_bytes
- * The dirty limits will be lifted by 1/4 for PF_LESS_THROTTLE (ie. nfsd) and
+ * Calculate @dtc->thresh and ->bg_thresh considering
+ * vm_dirty_{bytes|ratio} and dirty_background_{bytes|ratio}. The caller
+ * must ensure that @dtc->avail is set before calling this function. The
+ * dirty limits will be lifted by 1/4 for PF_LESS_THROTTLE (ie. nfsd) and
* real-time tasks.
*/
-void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
-{
- const unsigned long available_memory = global_dirtyable_memory();
- unsigned long background;
- unsigned long dirty;
+static void domain_dirty_limits(struct dirty_throttle_control *dtc)
+{
+ const unsigned long available_memory = dtc->avail;
+ 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;
+ unsigned long thresh;
+ unsigned long bg_thresh;
struct task_struct *tsk;

- if (vm_dirty_bytes)
- dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE);
+ /* gdtc is !NULL iff @dtc is for memcg domain */
+ if (gdtc) {
+ unsigned long global_avail = gdtc->avail;
+
+ /*
+ * The byte settings can't be applied directly to memcg
+ * domains. Convert them to ratios by scaling against
+ * globally available memory.
+ */
+ if (bytes)
+ ratio = min(DIV_ROUND_UP(bytes, PAGE_SIZE) * 100 /
+ global_avail, 100UL);
+ if (bg_bytes)
+ bg_ratio = min(DIV_ROUND_UP(bg_bytes, PAGE_SIZE) * 100 /
+ global_avail, 100UL);
+ bytes = bg_bytes = 0;
+ }
+
+ if (bytes)
+ thresh = DIV_ROUND_UP(bytes, PAGE_SIZE);
else
- dirty = (vm_dirty_ratio * available_memory) / 100;
+ thresh = (ratio * available_memory) / 100;

- if (dirty_background_bytes)
- background = DIV_ROUND_UP(dirty_background_bytes, PAGE_SIZE);
+ if (bg_bytes)
+ bg_thresh = DIV_ROUND_UP(bg_bytes, PAGE_SIZE);
else
- background = (dirty_background_ratio * available_memory) / 100;
+ bg_thresh = (bg_ratio * available_memory) / 100;

- if (background >= dirty)
- background = dirty / 2;
+ if (bg_thresh >= thresh)
+ bg_thresh = thresh / 2;
tsk = current;
if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
- background += background / 4;
- dirty += dirty / 4;
+ bg_thresh += bg_thresh / 4;
+ thresh += thresh / 4;
}
- *pbackground = background;
- *pdirty = dirty;
- trace_global_dirty_state(background, dirty);
+ dtc->thresh = thresh;
+ dtc->bg_thresh = bg_thresh;
+
+ /* we should eventually report the domain in the TP */
+ if (!gdtc)
+ trace_global_dirty_state(bg_thresh, thresh);
+}
+
+/**
+ * global_dirty_limits - background-writeback and dirty-throttling thresholds
+ * @pbackground: out parameter for bg_thresh
+ * @pdirty: out parameter for thresh
+ *
+ * Calculate bg_thresh and thresh for global_wb_domain. See
+ * domain_dirty_limits() for details.
+ */
+void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
+{
+ struct dirty_throttle_control gdtc = { GDTC_INIT_NO_WB };
+
+ gdtc.avail = global_dirtyable_memory();
+ domain_dirty_limits(&gdtc);
+
+ *pbackground = gdtc.bg_thresh;
+ *pdirty = gdtc.thresh;
}

/**
@@ -1421,9 +1481,10 @@ static void balance_dirty_pages(struct address_space *mapping,
*/
nr_reclaimable = global_page_state(NR_FILE_DIRTY) +
global_page_state(NR_UNSTABLE_NFS);
+ gdtc->avail = global_dirtyable_memory();
gdtc->dirty = nr_reclaimable + global_page_state(NR_WRITEBACK);

- global_dirty_limits(&gdtc->bg_thresh, &gdtc->thresh);
+ domain_dirty_limits(gdtc);

if (unlikely(strictlimit)) {
wb_dirty_limits(gdtc);
--
2.1.0