Subject: [PATCH 3/3] powerpc/mm/hash: Avoid multiple HPT resize-downs on memory hotunplug

From: Leonardo Bras <[email-masked]>
Date: Fri, 12 Mar 2021 04:29:41 -0300

During memory hotunplug, after each LMB is removed, the HPT may be
resized-down if it would map a max of 4 times the current amount of memory.
(2 shifts, due to introduced histeresis)

It usually is not an issue, but it can take a lot of time if HPT
resizing-down fails. This happens  because resize-down failures
usually repeat at each LMB removal, until there are no more bolted entries
conflict, which can take a while to happen.

This can be solved by doing a single HPT resize at the end of memory
hotunplug, after all requested entries are removed.

To make this happen, it's necessary to temporarily disable all HPT
resize-downs before hotunplug, re-enable them after hotunplug ends,
and then resize-down HPT to the current memory size.

As an example, hotunplugging 256GB from a 385GB guest took 621s without
this patch, and 100s after applied.

Signed-off-by: Leonardo Bras <[email-masked]>
---
 arch/powerpc/include/asm/book3s/64/hash.h     |  2 ++
 arch/powerpc/include/asm/sparsemem.h          |  2 ++
 arch/powerpc/mm/book3s64/hash_utils.c         | 28 +++++++++++++++++++
 arch/powerpc/mm/book3s64/pgtable.c            | 12 ++++++++
 .../platforms/pseries/hotplug-memory.c        | 16 +++++++++++
 5 files changed, 60 insertions(+)

diff --git a/arch/powerpc/include/asm/book3s/64/hash.h b/arch/powerpc/include/asm/book3s/64/hash.h
index 843b0a178590..f92697c107f7 100644
--- a/arch/powerpc/include/asm/book3s/64/hash.h
+++ b/arch/powerpc/include/asm/book3s/64/hash.h
@@ -256,6 +256,8 @@ int hash__create_section_mapping(unsigned long start, unsigned long end,
 int hash__remove_section_mapping(unsigned long start, unsigned long end);
 
 void hash_memory_batch_expand_prepare(unsigned long newsize);
+void hash_memory_batch_shrink_begin(void);
+void hash_memory_batch_shrink_end(void);
 
 #endif /* !__ASSEMBLY__ */
 #endif /* __KERNEL__ */
diff --git a/arch/powerpc/include/asm/sparsemem.h b/arch/powerpc/include/asm/sparsemem.h
index 16b5f5300c84..a7a8a0d070fc 100644
--- a/arch/powerpc/include/asm/sparsemem.h
+++ b/arch/powerpc/include/asm/sparsemem.h
@@ -18,6 +18,8 @@ extern int memory_add_physaddr_to_nid(u64 start);
 #define memory_add_physaddr_to_nid memory_add_physaddr_to_nid
 
 void memory_batch_expand_prepare(unsigned long newsize);
+void memory_batch_shrink_begin(void);
+void memory_batch_shrink_end(void);
 
 #ifdef CONFIG_NUMA
 extern int hot_add_scn_to_nid(unsigned long scn_addr);
diff --git a/arch/powerpc/mm/book3s64/hash_utils.c b/arch/powerpc/mm/book3s64/hash_utils.c
index 1f6aa0bf27e7..e16f207de8e4 100644
--- a/arch/powerpc/mm/book3s64/hash_utils.c
+++ b/arch/powerpc/mm/book3s64/hash_utils.c
@@ -794,6 +794,9 @@ static unsigned long __init htab_get_table_size(void)
 }
 
 #ifdef CONFIG_MEMORY_HOTPLUG
+
+atomic_t hpt_resize_disable = ATOMIC_INIT(0);
+
 static int resize_hpt_for_hotplug(unsigned long new_mem_size, bool shrinking)
 {
 	unsigned target_hpt_shift;
@@ -805,6 +808,10 @@ static int resize_hpt_for_hotplug(unsigned long new_mem_size, bool shrinking)
 
 	if (shrinking) {
 
+		/* When batch removing entries, only resizes HPT at the end. */
+		if (atomic_read_acquire(&hpt_resize_disable))
+			return 0;
+
 		/*
 		 * To avoid lots of HPT resizes if memory size is fluctuating
 		 * across a boundary, we deliberately have some hysterisis
@@ -872,6 +879,27 @@ void hash_memory_batch_expand_prepare(unsigned long newsize)
 		pr_warn("Hash collision while resizing HPT\n");
 	}
 }
+
+void hash_memory_batch_shrink_begin(void)
+{
+	/* Disable HPT resize-down during hot-unplug */
+	atomic_set_release(&hpt_resize_disable, 1);
+}
+
+void hash_memory_batch_shrink_end(void)
+{
+	unsigned long newsize;
+
+	/* Re-enables HPT resize-down after hot-unplug */
+	atomic_set_release(&hpt_resize_disable, 0);
+
+	newsize = memblock_phys_mem_size();
+	/* Resize to smallest SHIFT possible */
+	while (resize_hpt_for_hotplug(newsize, true) == -ENOSPC) {
+		newsize *= 2;
+		pr_warn("Hash collision while resizing HPT\n");
+	}
+}
 #endif /* CONFIG_MEMORY_HOTPLUG */
 
 static void __init hash_init_partition_table(phys_addr_t hash_table,
diff --git a/arch/powerpc/mm/book3s64/pgtable.c b/arch/powerpc/mm/book3s64/pgtable.c
index f1cd8af0f67f..e01681e22e00 100644
--- a/arch/powerpc/mm/book3s64/pgtable.c
+++ b/arch/powerpc/mm/book3s64/pgtable.c
@@ -199,6 +199,18 @@ void memory_batch_expand_prepare(unsigned long newsize)
 	if (!radix_enabled())
 		hash_memory_batch_expand_prepare(newsize);
 }
+
+void memory_batch_shrink_begin(void)
+{
+	if (!radix_enabled())
+		hash_memory_batch_shrink_begin();
+}
+
+void memory_batch_shrink_end(void)
+{
+	if (!radix_enabled())
+		hash_memory_batch_shrink_end();
+}
 #endif /* CONFIG_MEMORY_HOTPLUG */
 
 void __init mmu_partition_table_init(void)
diff --git a/arch/powerpc/platforms/pseries/hotplug-memory.c b/arch/powerpc/platforms/pseries/hotplug-memory.c
index 353c71249214..9182fb5b5c01 100644
--- a/arch/powerpc/platforms/pseries/hotplug-memory.c
+++ b/arch/powerpc/platforms/pseries/hotplug-memory.c
@@ -425,6 +425,8 @@ static int dlpar_memory_remove_by_count(u32 lmbs_to_remove)
 		return -EINVAL;
 	}
 
+	memory_batch_shrink_begin();
+
 	for_each_drmem_lmb(lmb) {
 		rc = dlpar_remove_lmb(lmb);
 		if (rc)
@@ -470,6 +472,8 @@ static int dlpar_memory_remove_by_count(u32 lmbs_to_remove)
 		rc = 0;
 	}
 
+	memory_batch_shrink_end();
+
 	return rc;
 }
 
@@ -481,6 +485,8 @@ static int dlpar_memory_remove_by_index(u32 drc_index)
 
 	pr_debug("Attempting to hot-remove LMB, drc index %x\n", drc_index);
 
+	memory_batch_shrink_begin();
+
 	lmb_found = 0;
 	for_each_drmem_lmb(lmb) {
 		if (lmb->drc_index == drc_index) {
@@ -502,6 +508,8 @@ static int dlpar_memory_remove_by_index(u32 drc_index)
 	else
 		pr_debug("Memory at %llx was hot-removed\n", lmb->base_addr);
 
+	memory_batch_shrink_end();
+
 	return rc;
 }
 
@@ -532,6 +540,8 @@ static int dlpar_memory_remove_by_ic(u32 lmbs_to_remove, u32 drc_index)
 	if (lmbs_available < lmbs_to_remove)
 		return -EINVAL;
 
+	memory_batch_shrink_begin();
+
 	for_each_drmem_lmb_in_range(lmb, start_lmb, end_lmb) {
 		if (!(lmb->flags & DRCONF_MEM_ASSIGNED))
 			continue;
@@ -572,6 +582,8 @@ static int dlpar_memory_remove_by_ic(u32 lmbs_to_remove, u32 drc_index)
 		}
 	}
 
+	memory_batch_shrink_end();
+
 	return rc;
 }
 
@@ -700,6 +712,7 @@ static int dlpar_memory_add_by_count(u32 lmbs_to_add)
 	if (lmbs_added != lmbs_to_add) {
 		pr_err("Memory hot-add failed, removing any added LMBs\n");
 
+		memory_batch_shrink_begin();
 		for_each_drmem_lmb(lmb) {
 			if (!drmem_lmb_reserved(lmb))
 				continue;
@@ -713,6 +726,7 @@ static int dlpar_memory_add_by_count(u32 lmbs_to_add)
 
 			drmem_remove_lmb_reservation(lmb);
 		}
+		memory_batch_shrink_end();
 		rc = -EINVAL;
 	} else {
 		for_each_drmem_lmb(lmb) {
@@ -814,6 +828,7 @@ static int dlpar_memory_add_by_ic(u32 lmbs_to_add, u32 drc_index)
 	if (rc) {
 		pr_err("Memory indexed-count-add failed, removing any added LMBs\n");
 
+		memory_batch_shrink_begin();
 		for_each_drmem_lmb_in_range(lmb, start_lmb, end_lmb) {
 			if (!drmem_lmb_reserved(lmb))
 				continue;
@@ -827,6 +842,7 @@ static int dlpar_memory_add_by_ic(u32 lmbs_to_add, u32 drc_index)
 
 			drmem_remove_lmb_reservation(lmb);
 		}
+		memory_batch_shrink_end();
 		rc = -EINVAL;
 	} else {
 		for_each_drmem_lmb_in_range(lmb, start_lmb, end_lmb) {
-- 
2.29.2