[PATCH v2 3/3] powerpc/mm/hash: Avoid multiple HPT resize-downs on memory hotunplug

[Leonardo Bras]

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 <leobras.c@xxxxxxxxx>
---
 arch/powerpc/include/asm/book3s/64/hash.h     |  2 +
 arch/powerpc/mm/book3s64/hash_utils.c         | 45 +++++++++++++++++--
 .../platforms/pseries/hotplug-memory.c        | 26 +++++++++++
 3 files changed, 70 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/include/asm/book3s/64/hash.h b/arch/powerpc/include/asm/book3s/64/hash.h
index fad4af8b8543..6cd66e7e98c9 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_batch_expand_prepare(unsigned long newsize);
+void hash_batch_shrink_begin(void);
+void hash_batch_shrink_end(void);
 
 #endif /* !__ASSEMBLY__ */
 #endif /* __KERNEL__ */
diff --git a/arch/powerpc/mm/book3s64/hash_utils.c b/arch/powerpc/mm/book3s64/hash_utils.c
index 3fa395b3fe57..73ecd0f61acd 100644
--- a/arch/powerpc/mm/book3s64/hash_utils.c
+++ b/arch/powerpc/mm/book3s64/hash_utils.c
@@ -795,6 +795,9 @@ static unsigned long __init htab_get_table_size(void)
 }
 
 #ifdef CONFIG_MEMORY_HOTPLUG
+
+static DEFINE_MUTEX(hpt_resize_down_lock);
+
 static int resize_hpt_for_hotplug(unsigned long new_mem_size, bool shrinking)
 {
 	unsigned target_hpt_shift;
@@ -805,7 +808,7 @@ static int resize_hpt_for_hotplug(unsigned long new_mem_size, bool shrinking)
 	target_hpt_shift = htab_shift_for_mem_size(new_mem_size);
 
 	if (shrinking) {
-
+		int ret;
 		/*
 		 * To avoid lots of HPT resizes if memory size is fluctuating
 		 * across a boundary, we deliberately have some hysterisis
@@ -818,10 +821,20 @@ static int resize_hpt_for_hotplug(unsigned long new_mem_size, bool shrinking)
 		if (target_hpt_shift >= ppc64_pft_size - 1)
 			return 0;
 
-	} else if (target_hpt_shift <= ppc64_pft_size) {
-		return 0;
+		/* When batch removing entries, only resizes HPT at the end. */
+
+		if (!mutex_trylock(&hpt_resize_down_lock))
+			return 0;
+
+		ret = mmu_hash_ops.resize_hpt(target_hpt_shift);
+
+		mutex_unlock(&hpt_resize_down_lock);
+		return ret;
 	}
 
+	if (target_hpt_shift <= ppc64_pft_size)
+		return 0;
+
 	return mmu_hash_ops.resize_hpt(target_hpt_shift);
 }
 
@@ -879,6 +892,32 @@ void hash_batch_expand_prepare(unsigned long newsize)
 			break;
 	}
 }
+
+void hash_batch_shrink_begin(void)
+{
+	/* Disable HPT resize-down during hot-unplug */
+	mutex_lock(&hpt_resize_down_lock);
+}
+
+void hash_batch_shrink_end(void)
+{
+	const u64 starting_size = ppc64_pft_size;
+	unsigned long newsize;
+
+	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");
+
+		/* Do not try to resize to the starting size, or bigger value */
+		if (htab_shift_for_mem_size(newsize) >= starting_size)
+			break;
+	}
+
+	/* Re-enables HPT resize-down after hot-unplug */
+	mutex_unlock(&hpt_resize_down_lock);
+}
 #endif /* CONFIG_MEMORY_HOTPLUG */
 
 static void __init hash_init_partition_table(phys_addr_t hash_table,
diff --git a/arch/powerpc/platforms/pseries/hotplug-memory.c b/arch/powerpc/platforms/pseries/hotplug-memory.c
index 48b2cfe4ce69..44bc50d72353 100644
--- a/arch/powerpc/platforms/pseries/hotplug-memory.c
+++ b/arch/powerpc/platforms/pseries/hotplug-memory.c
@@ -426,6 +426,9 @@ static int dlpar_memory_remove_by_count(u32 lmbs_to_remove)
 		return -EINVAL;
 	}
 
+	if (!radix_enabled())
+		hash_batch_shrink_begin();
+
 	for_each_drmem_lmb(lmb) {
 		rc = dlpar_remove_lmb(lmb);
 		if (rc)
@@ -471,6 +474,9 @@ static int dlpar_memory_remove_by_count(u32 lmbs_to_remove)
 		rc = 0;
 	}
 
+	if (!radix_enabled())
+		hash_batch_shrink_end();
+
 	return rc;
 }
 
@@ -533,6 +539,9 @@ static int dlpar_memory_remove_by_ic(u32 lmbs_to_remove, u32 drc_index)
 	if (lmbs_available < lmbs_to_remove)
 		return -EINVAL;
 
+	if (!radix_enabled())
+		hash_batch_shrink_begin();
+
 	for_each_drmem_lmb_in_range(lmb, start_lmb, end_lmb) {
 		if (!(lmb->flags & DRCONF_MEM_ASSIGNED))
 			continue;
@@ -573,6 +582,9 @@ static int dlpar_memory_remove_by_ic(u32 lmbs_to_remove, u32 drc_index)
 		}
 	}
 
+	if (!radix_enabled())
+		hash_batch_shrink_end();
+
 	return rc;
 }
 
@@ -703,6 +715,9 @@ 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");
 
+		if (!radix_enabled())
+			hash_batch_shrink_begin();
+
 		for_each_drmem_lmb(lmb) {
 			if (!drmem_lmb_reserved(lmb))
 				continue;
@@ -716,6 +731,10 @@ static int dlpar_memory_add_by_count(u32 lmbs_to_add)
 
 			drmem_remove_lmb_reservation(lmb);
 		}
+
+		if (!radix_enabled())
+			hash_batch_shrink_end();
+
 		rc = -EINVAL;
 	} else {
 		for_each_drmem_lmb(lmb) {
@@ -817,6 +836,9 @@ 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");
 
+		if (!radix_enabled())
+			hash_batch_shrink_begin();
+
 		for_each_drmem_lmb_in_range(lmb, start_lmb, end_lmb) {
 			if (!drmem_lmb_reserved(lmb))
 				continue;
@@ -830,6 +852,10 @@ static int dlpar_memory_add_by_ic(u32 lmbs_to_add, u32 drc_index)
 
 			drmem_remove_lmb_reservation(lmb);
 		}
+
+		if (!radix_enabled())
+			hash_batch_shrink_end();
+
 		rc = -EINVAL;
 	} else {
 		for_each_drmem_lmb_in_range(lmb, start_lmb, end_lmb) {
-- 
2.30.2