demand_paging: eviction/lru: add simulated LRU in software

This adds bits to simulate LRU in software for architectures
that do not provide a native mean to figure out least recently
used pages. Enabling this changes the eviction selection to
clear the access flag if it is set and requeues the page frame
for later consideration. Without this, the LRU would simply
devolve into picking the earliest paged in frame for
eviction.

Signed-off-by: Daniel Leung <[email protected]>

Author: dcpleung

subsys/demand_paging/eviction/Kconfig

@@ -48,3 +48,16 @@ config EVICTION_NRU_PERIOD
 	  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
+
+if EVICTION_LRU
+
+config EVICTION_LRU_SW_SIMULATED
+	bool
+	default y if X86
+	help
+	  This is for architectures where hardware does not provide a native
+	  mean to figure out least recently used pages. Enabling this changes
+	  the eviction selection to clear the access flag if it is set and
+	  requeues the page frame for later consideration.
+
+endif # EVICTION_LRU

subsys/demand_paging/eviction/lru.c

@@ -160,14 +160,48 @@ void k_mem_paging_eviction_accessed(uintptr_t phys)
 
 struct k_mem_page_frame *k_mem_paging_eviction_select(bool *dirty_ptr)
 {
-	uint32_t head_pf_idx = LRU_PF_HEAD;
+	uint32_t pf_idx = LRU_PF_HEAD;
+	struct k_mem_page_frame *pf;
+	uintptr_t flags;
 
-	if (head_pf_idx == 0) {
+	if (pf_idx == 0) {
 		return NULL;
 	}
 
-	struct k_mem_page_frame *pf = idx_to_pf(head_pf_idx);
-	uintptr_t flags = arch_page_info_get(k_mem_page_frame_to_virt(pf), NULL, false);
+#ifndef CONFIG_EVICTION_LRU_SW_SIMULATED
+	pf = idx_to_pf(pf_idx);
+	flags = arch_page_info_get(k_mem_page_frame_to_virt(pf), NULL, false);
+#else
+	do {
+		/*
+		 * During page selection for eviction, if a page has access bit set,
+		 * clear the access bit and put it back at the end of queue.
+		 *
+		 * A subtle fail safe to avoid infinite loop is that the head of queue
+		 * when this function is entered will have its access bit cleared.
+		 * So when we have exhausted every page and looped back to that page,
+		 * it will be selected for eviction.
+		 */
+		pf = idx_to_pf(pf_idx);
+		flags = arch_page_info_get(k_mem_page_frame_to_virt(pf), NULL, true);
+
+		if ((flags & ARCH_DATA_PAGE_ACCESSED) != ARCH_DATA_PAGE_ACCESSED) {
+			/* Page has not been accessed recently, so select this. */
+			break;
+		}
+
+		/*
+		 * Page has been accessed recently.
+		 * Put the page frame back to the end of queue.
+		 * Use unlink() here since we do not need the extra logic inside remove().
+		 */
+		lru_pf_unlink(pf_idx);
+		lru_pf_append(pf_idx);
+
+		/* Grab the new head of queue. */
+		pf_idx = LRU_PF_HEAD;
+	} while (pf_idx != 0);
+#endif /* !CONFIG_EVICTION_LRU_SW_SIMULATED */
 
 	__ASSERT(k_mem_page_frame_is_evictable(pf), "");
 	*dirty_ptr = ((flags & ARCH_DATA_PAGE_DIRTY) != 0);