demand_paging: add NRU algorithm

Simple textbook Not Recently Used eviction algorithm.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>

subsys/demand_paging/eviction/CMakeLists.txt

@@ -1,2 +1,14 @@
 # Copyright (c) 2020 Intel Corporation
 # SPDX-License-Identifier: Apache-2.0
+
+add_definitions(-D__ZEPHYR_SUPERVISOR__)
+
+include_directories(
+  ${ZEPHYR_BASE}/kernel/include
+  ${ARCH_DIR}/${ARCH}/include
+  )
+
+if(NOT DEFINED CONFIG_EVICTION_CUSTOM)
+  zephyr_library()
+  zephyr_library_sources_ifdef(CONFIG_EVICTION_NRU            nru.c)
+endif()

subsys/demand_paging/eviction/Kconfig

@@ -5,7 +5,7 @@
 
 choice EVICTION_CHOICE
 	prompt "Page frame eviction algorithms"
-	default EVICTION_CUSTOM
+	default EVICTION_NRU
 	depends on DEMAND_PAGING
 
 config EVICTION_CUSTOM
@@ -14,4 +14,27 @@ config EVICTION_CUSTOM
 	  This option is chosen when the eviction algorithm will be implemented
 	  by the application, instead of using one included in Zephyr.
 
+config EVICTION_NRU
+	bool "Not Recently Used (NRU) page eviction algorithm"
+	help
+	  This implements a Not Recently Used page eviction algorithm.
+	  A periodic timer will clear the accessed state of all virtual pages.
+	  When a page frame needs to be evicted, the algorithm will prefer to
+	  evict page frames using an ascending order of priority:
+
+	   - recently accessed, dirty
+	   - recently accessed, clean
+	   - not recently accessed, dirty
+	   - not recently accessed, clean
+
 endchoice
+
+if EVICTION_NRU
+config EVICTION_NRU_PERIOD
+	int "Recently accessed period, in milliseconds"
+	default 100
+	help
+	  A periodic timer will fire that clears the accessed state of all virtual
+	  pages that are capable of being paged out. At eviction time, if a page
+	  still has the accessed property, it will be considered as recently used.
+endif # EVICTION_NRU

subsys/demand_paging/eviction/nru.c

@@ -0,0 +1,96 @@
+/*
+ * Copyright (c) 2020 Intel Corporation
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Not Recently Used (NRU) eviction algorithm for demand paging
+ */
+#include <kernel.h>
+#include <mmu.h>
+#include <kernel_arch_interface.h>
+#include <init.h>
+
+/* The accessed and dirty states of each page frame are used to create
+ * a hierarchy with a numerical value. When evicting a page, try to evict
+ * page with the highest value (we prefer clean, not accessed pages).
+ *
+ * In this ontology, "accessed" means "recently accessed" and gets cleared
+ * during the periodic update.
+ *
+ * 0 not accessed, clean
+ * 1 not accessed, dirty
+ * 2 accessed, clean
+ * 3 accessed, dirty
+ */
+static void nru_periodic_update(struct k_timer *timer)
+{
+	uintptr_t phys;
+	struct z_page_frame *pf;
+	int key = irq_lock();
+
+	Z_PAGE_FRAME_FOREACH(phys, pf) {
+		if (!z_page_frame_is_evictable(pf)) {
+			continue;
+		}
+
+		/* Clear accessed bit in page tables */
+		(void)arch_page_info_get(pf->addr, NULL, true);
+	}
+
+	irq_unlock(key);
+}
+
+struct z_page_frame *z_eviction_select(bool *dirty_ptr)
+{
+	unsigned int last_prec = 4U;
+	struct z_page_frame *last_pf = NULL, *pf;
+	bool accessed;
+	bool dirty = false;
+	uintptr_t flags, phys;
+
+	Z_PAGE_FRAME_FOREACH(phys, pf) {
+		unsigned int prec;
+
+		if (!z_page_frame_is_evictable(pf)) {
+			continue;
+		}
+
+		flags = arch_page_info_get(pf->addr, NULL, false);
+		accessed = (flags & ARCH_DATA_PAGE_ACCESSED) != 0UL;
+		dirty = (flags & ARCH_DATA_PAGE_DIRTY) != 0UL;
+
+		/* Implies a mismatch with page frame ontology and page
+		 * tables
+		 */
+		__ASSERT((flags & ARCH_DATA_PAGE_LOADED) != 0U,
+			 "non-present page, %s",
+			 ((flags & ARCH_DATA_PAGE_NOT_MAPPED) != 0U) ?
+			 "un-mapped" : "paged out");
+
+		prec = (dirty ? 1U : 0U) + (accessed ? 2U : 0U);
+		if (prec == 0) {
+			/* If we find a not accessed, clean page we're done */
+			last_pf = pf;
+			break;
+		}
+
+		if (prec < last_prec) {
+			last_prec = prec;
+			last_pf = pf;
+		}
+	}
+	/* Shouldn't ever happen unless every page is pinned */
+	__ASSERT(last_pf != NULL, "no page to evict");
+
+	*dirty_ptr = dirty;
+
+	return last_pf;
+}
+
+static K_TIMER_DEFINE(nru_timer, nru_periodic_update, NULL);
+
+void z_eviction_init(void)
+{
+	k_timer_start(&nru_timer, K_NO_WAIT,
+		      K_MSEC(CONFIG_EVICTION_NRU_PERIOD));
+}