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 <leobra...@gmail.com>
---
 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

Reply via email to