ANDROID: add extra free kbytes tunable

Add a userspace visible knob to tell the VM to keep an extra amount
of memory free, by increasing the gap between each zone's min and
low watermarks.

This is useful for realtime applications that call system
calls and have a bound on the number of allocations that happen
in any short time period.  In this application, extra_free_kbytes
would be left at an amount equal to or larger than than the
maximum number of allocations that happen in any burst.

It may also be useful to reduce the memory use of virtual
machines (temporarily?), in a way that does not cause memory
fragmentation like ballooning does.

[ccross]
Revived for use on old kernels where no other solution exists.
The tunable will be removed on kernels that do better at avoiding
direct reclaim.

[surenb]
Will be reverted as soon as Android framework is reworked to
use upstream-supported watermark_scale_factor instead of
extra_free_kbytes.

Bug: 86445363
Bug: 109664768
Bug: 120445732
Change-Id: I765a42be8e964bfd3e2886d1ca85a29d60c3bb3e
Signed-off-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Colin Cross <ccross@android.com>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>

Documentation/sysctl/vm.txt

@@ -31,6 +31,7 @@ Currently, these files are in /proc/sys/vm:
 - dirty_writeback_centisecs
 - drop_caches
 - extfrag_threshold
+- extra_free_kbytes
 - hugetlb_shm_group
 - laptop_mode
 - legacy_va_layout
@@ -274,6 +275,21 @@ any throttling.
 
 ==============================================================
 
+extra_free_kbytes
+
+This parameter tells the VM to keep extra free memory between the threshold
+where background reclaim (kswapd) kicks in, and the threshold where direct
+reclaim (by allocating processes) kicks in.
+
+This is useful for workloads that require low latency memory allocations
+and have a bounded burstiness in memory allocations, for example a
+realtime application that receives and transmits network traffic
+(causing in-kernel memory allocations) with a maximum total message burst
+size of 200MB may need 200MB of extra free memory to avoid direct reclaim
+related latencies.
+
+==============================================================
+
 hugetlb_shm_group
 
 hugetlb_shm_group contains group id that is allowed to create SysV

kernel/sysctl.c

@@ -106,6 +106,7 @@ extern char core_pattern[];
 extern unsigned int core_pipe_limit;
 #endif
 extern int pid_max;
+extern int extra_free_kbytes;
 extern int pid_max_min, pid_max_max;
 extern int percpu_pagelist_fraction;
 extern int latencytop_enabled;
@@ -1459,6 +1460,14 @@ static struct ctl_table vm_table[] = {
 		.extra1		= &one,
 		.extra2		= &one_thousand,
 	},
+	{
+		.procname	= "extra_free_kbytes",
+		.data		= &extra_free_kbytes,
+		.maxlen		= sizeof(extra_free_kbytes),
+		.mode		= 0644,
+		.proc_handler	= min_free_kbytes_sysctl_handler,
+		.extra1		= &zero,
+	},
 	{
 		.procname	= "percpu_pagelist_fraction",
 		.data		= &percpu_pagelist_fraction,

mm/page_alloc.c

@@ -261,10 +261,22 @@ compound_page_dtor * const compound_page_dtors[] = {
 #endif
 };
 
+/*
+ * Try to keep at least this much lowmem free.  Do not allow normal
+ * allocations below this point, only high priority ones. Automatically
+ * tuned according to the amount of memory in the system.
+ */
 int min_free_kbytes = 1024;
 int user_min_free_kbytes = -1;
 int watermark_scale_factor = 10;
 
+/*
+ * Extra memory for the system to try freeing. Used to temporarily
+ * free memory, to make space for new workloads. Anyone can allocate
+ * down to the min watermarks controlled by min_free_kbytes above.
+ */
+int extra_free_kbytes = 0;
+
 static unsigned long nr_kernel_pages __meminitdata;
 static unsigned long nr_all_pages __meminitdata;
 static unsigned long dma_reserve __meminitdata;
@@ -7214,6 +7226,7 @@ static void setup_per_zone_lowmem_reserve(void)
 static void __setup_per_zone_wmarks(void)
 {
 	unsigned long pages_min = min_free_kbytes >> (PAGE_SHIFT - 10);
+	unsigned long pages_low = extra_free_kbytes >> (PAGE_SHIFT - 10);
 	unsigned long lowmem_pages = 0;
 	struct zone *zone;
 	unsigned long flags;
@@ -7225,11 +7238,14 @@ static void __setup_per_zone_wmarks(void)
 	}
 
 	for_each_zone(zone) {
-		u64 tmp;
+		u64 min, low;
 
 		spin_lock_irqsave(&zone->lock, flags);
-		tmp = (u64)pages_min * zone->managed_pages;
-		do_div(tmp, lowmem_pages);
+		min = (u64)pages_min * zone->managed_pages;
+		do_div(min, lowmem_pages);
+		low = (u64)pages_low * zone->managed_pages;
+		do_div(low, vm_total_pages);
+
 		if (is_highmem(zone)) {
 			/*
 			 * __GFP_HIGH and PF_MEMALLOC allocations usually don't
@@ -7250,7 +7266,7 @@ static void __setup_per_zone_wmarks(void)
 			 * If it's a lowmem zone, reserve a number of pages
 			 * proportionate to the zone's size.
 			 */
-			zone->watermark[WMARK_MIN] = tmp;
+			zone->watermark[WMARK_MIN] = min;
 		}
 
 		/*
@@ -7258,12 +7274,14 @@ static void __setup_per_zone_wmarks(void)
 		 * scale factor in proportion to available memory, but
 		 * ensure a minimum size on small systems.
 		 */
-		tmp = max_t(u64, tmp >> 2,
+		min = max_t(u64, min >> 2,
 			    mult_frac(zone->managed_pages,
 				      watermark_scale_factor, 10000));
 
-		zone->watermark[WMARK_LOW]  = min_wmark_pages(zone) + tmp;
-		zone->watermark[WMARK_HIGH] = min_wmark_pages(zone) + tmp * 2;
+		zone->watermark[WMARK_LOW]  = min_wmark_pages(zone) +
+					low + min;
+		zone->watermark[WMARK_HIGH] = min_wmark_pages(zone) +
+					low + min * 2;
 
 		spin_unlock_irqrestore(&zone->lock, flags);
 	}
@@ -7346,7 +7364,7 @@ core_initcall(init_per_zone_wmark_min)
 /*
  * min_free_kbytes_sysctl_handler - just a wrapper around proc_dointvec() so
  *	that we can call two helper functions whenever min_free_kbytes
- *	changes.
+ *	or extra_free_kbytes changes.
  */
 int min_free_kbytes_sysctl_handler(struct ctl_table *table, int write,
 	void __user *buffer, size_t *length, loff_t *ppos)