[PATCH 2/2] mm/vmalloc: rework the drain logic

From: Uladzislau Rezki (Sony)
Date: Mon Nov 16 2020 - 17:01:09 EST

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A current "lazy drain" model suffers from at least two issues.

First one is related to the unsorted list of vmap areas, thus
in order to identify the [min:max] range of areas to be drained,
it requires a full list scan. What is a time consuming if the
list is too long.

Second one and as a next step is about merging all fragments
with a free space. What is also a time consuming because it
has to iterate over entire list which holds outstanding lazy
areas.

See below the "preemptirqsoff" tracer that illustrates a high
latency. It is ~24 676us. Our workloads like audio and video
are effected by such long latency:

<snip>
tracer: preemptirqsoff

preemptirqsoff latency trace v1.1.5 on 4.9.186-perf+
--------------------------------------------------------------------
latency: 24676 us, #4/4, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 P:8)
-----------------
| task: crtc_commit:112-261 (uid:0 nice:0 policy:1 rt_prio:16)
-----------------
=> started at: __purge_vmap_area_lazy
=> ended at: __purge_vmap_area_lazy

_------=> CPU#
/ _-----=> irqs-off
| / _----=> need-resched
|| / _---=> hardirq/softirq
||| / _--=> preempt-depth
|||| / delay
cmd pid ||||| time | caller
\ / ||||| \ | /
crtc_com-261 1...1 1us*: _raw_spin_lock <-__purge_vmap_area_lazy
[...]
crtc_com-261 1...1 24675us : _raw_spin_unlock <-__purge_vmap_area_lazy
crtc_com-261 1...1 24677us : trace_preempt_on <-__purge_vmap_area_lazy
crtc_com-261 1...1 24683us : <stack trace>
=> free_vmap_area_noflush
=> remove_vm_area
=> __vunmap
=> vfree
=> drm_property_free_blob
=> drm_mode_object_unreference
=> drm_property_unreference_blob
=> __drm_atomic_helper_crtc_destroy_state
=> sde_crtc_destroy_state
=> drm_atomic_state_default_clear
=> drm_atomic_state_clear
=> drm_atomic_state_free
=> complete_commit
=> _msm_drm_commit_work_cb
=> kthread_worker_fn
=> kthread
=> ret_from_fork
<snip>

To address those two issues we can redesign a purging of the
outstanding lazy areas. Instead of queuing vmap areas to the
list, we replace it by the separate rb-tree. In hat case an
area is located in the tree/list in ascending order. It will
give us below advantages:

a) Outstanding vmap areas are merged creating bigger coalesced
blocks, thus it becomes less fragmented.

b) It is possible to calculate a flush range [min:max] without
scanning all elements. It is O(1) access time or complexity;

c) The final merge of areas with the rb-tree that represents a
free space is faster because of (a). As a result the lock
contention is also reduced.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
---
include/linux/vmalloc.h | 8 ++--
mm/vmalloc.c | 90 +++++++++++++++++++++++------------------
2 files changed, 53 insertions(+), 45 deletions(-)

diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
index 938eaf9517e2..80c0181c411d 100644
--- a/include/linux/vmalloc.h
+++ b/include/linux/vmalloc.h
@@ -72,16 +72,14 @@ struct vmap_area {
struct list_head list; /* address sorted list */

/*
- * The following three variables can be packed, because
- * a vmap_area object is always one of the three states:
+ * The following two variables can be packed, because
+ * a vmap_area object can be either:
* 1) in "free" tree (root is vmap_area_root)
- * 2) in "busy" tree (root is free_vmap_area_root)
- * 3) in purge list (head is vmap_purge_list)
+ * 2) or "busy" tree (root is free_vmap_area_root)
*/
union {
unsigned long subtree_max_size; /* in "free" tree */
struct vm_struct *vm; /* in "busy" tree */
- struct llist_node purge_list; /* in purge list */
};
};

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index b08b06a8cc2a..f16a71fb0624 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -413,10 +413,13 @@ static DEFINE_SPINLOCK(vmap_area_lock);
static DEFINE_SPINLOCK(free_vmap_area_lock);
/* Export for kexec only */
LIST_HEAD(vmap_area_list);
-static LLIST_HEAD(vmap_purge_list);
static struct rb_root vmap_area_root = RB_ROOT;
static bool vmap_initialized __read_mostly;

+static struct rb_root purge_vmap_area_root = RB_ROOT;
+static LIST_HEAD(purge_vmap_area_list);
+static DEFINE_SPINLOCK(purge_vmap_area_lock);
+
/*
* This kmem_cache is used for vmap_area objects. Instead of
* allocating from slab we reuse an object from this cache to
@@ -820,10 +823,17 @@ merge_or_add_vmap_area(struct vmap_area *va,
if (!merged)
link_va(va, root, parent, link, head);

- /*
- * Last step is to check and update the tree.
- */
- augment_tree_propagate_from(va);
+ return va;
+}
+
+static __always_inline struct vmap_area *
+merge_or_add_vmap_area_augment(struct vmap_area *va,
+ struct rb_root *root, struct list_head *head)
+{
+ va = merge_or_add_vmap_area(va, root, head);
+ if (va)
+ augment_tree_propagate_from(va);
+
return va;
}

@@ -1138,7 +1148,7 @@ static void free_vmap_area(struct vmap_area *va)
* Insert/Merge it back to the free tree/list.
*/
spin_lock(&free_vmap_area_lock);
- merge_or_add_vmap_area(va, &free_vmap_area_root, &free_vmap_area_list);
+ merge_or_add_vmap_area_augment(va, &free_vmap_area_root, &free_vmap_area_list);
spin_unlock(&free_vmap_area_lock);
}

@@ -1326,32 +1336,32 @@ void set_iounmap_nonlazy(void)
static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end)
{
unsigned long resched_threshold;
- struct llist_node *valist;
- struct vmap_area *va;
- struct vmap_area *n_va;
+ struct list_head local_pure_list;
+ struct vmap_area *va, *n_va;

lockdep_assert_held(&vmap_purge_lock);

- valist = llist_del_all(&vmap_purge_list);
- if (unlikely(valist == NULL))
+ spin_lock(&purge_vmap_area_lock);
+ purge_vmap_area_root = RB_ROOT;
+ list_replace_init(&purge_vmap_area_list, &local_pure_list);
+ spin_unlock(&purge_vmap_area_lock);
+
+ if (unlikely(list_empty(&local_pure_list)))
return false;

- /*
- * TODO: to calculate a flush range without looping.
- * The list can be up to lazy_max_pages() elements.
- */
- llist_for_each_entry(va, valist, purge_list) {
- if (va->va_start < start)
- start = va->va_start;
- if (va->va_end > end)
- end = va->va_end;
- }
+ start = min(start,
+ list_first_entry(&local_pure_list,
+ struct vmap_area, list)->va_start);
+
+ end = max(end,
+ list_last_entry(&local_pure_list,
+ struct vmap_area, list)->va_end);

flush_tlb_kernel_range(start, end);
resched_threshold = lazy_max_pages() << 1;

spin_lock(&free_vmap_area_lock);
- llist_for_each_entry_safe(va, n_va, valist, purge_list) {
+ list_for_each_entry_safe(va, n_va, &local_pure_list, list) {
unsigned long nr = (va->va_end - va->va_start) >> PAGE_SHIFT;
unsigned long orig_start = va->va_start;
unsigned long orig_end = va->va_end;
@@ -1361,8 +1371,8 @@ static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end)
* detached and there is no need to "unlink" it from
* anything.
*/
- va = merge_or_add_vmap_area(va, &free_vmap_area_root,
- &free_vmap_area_list);
+ va = merge_or_add_vmap_area_augment(va, &free_vmap_area_root,
+ &free_vmap_area_list);

if (!va)
continue;
@@ -1419,9 +1429,15 @@ static void free_vmap_area_noflush(struct vmap_area *va)
nr_lazy = atomic_long_add_return((va->va_end - va->va_start) >>
PAGE_SHIFT, &vmap_lazy_nr);

- /* After this point, we may free va at any time */
- llist_add(&va->purge_list, &vmap_purge_list);
+ /*
+ * Merge or place it to the purge tree/list.
+ */
+ spin_lock(&purge_vmap_area_lock);
+ merge_or_add_vmap_area(va,
+ &purge_vmap_area_root, &purge_vmap_area_list);
+ spin_unlock(&purge_vmap_area_lock);

+ /* After this point, we may free va at any time */
if (unlikely(nr_lazy > lazy_max_pages()))
try_purge_vmap_area_lazy();
}
@@ -3351,8 +3367,8 @@ struct vm_struct **pcpu_get_vm_areas(const unsigned long *offsets,
while (area--) {
orig_start = vas[area]->va_start;
orig_end = vas[area]->va_end;
- va = merge_or_add_vmap_area(vas[area], &free_vmap_area_root,
- &free_vmap_area_list);
+ va = merge_or_add_vmap_area_augment(vas[area], &free_vmap_area_root,
+ &free_vmap_area_list);
if (va)
kasan_release_vmalloc(orig_start, orig_end,
va->va_start, va->va_end);
@@ -3401,8 +3417,8 @@ struct vm_struct **pcpu_get_vm_areas(const unsigned long *offsets,
for (area = 0; area < nr_vms; area++) {
orig_start = vas[area]->va_start;
orig_end = vas[area]->va_end;
- va = merge_or_add_vmap_area(vas[area], &free_vmap_area_root,
- &free_vmap_area_list);
+ va = merge_or_add_vmap_area_augment(vas[area], &free_vmap_area_root,
+ &free_vmap_area_list);
if (va)
kasan_release_vmalloc(orig_start, orig_end,
va->va_start, va->va_end);
@@ -3482,18 +3498,15 @@ static void show_numa_info(struct seq_file *m, struct vm_struct *v)

static void show_purge_info(struct seq_file *m)
{
- struct llist_node *head;
struct vmap_area *va;

- head = READ_ONCE(vmap_purge_list.first);
- if (head == NULL)
- return;
-
- llist_for_each_entry(va, head, purge_list) {
+ spin_lock(&purge_vmap_area_lock);
+ list_for_each_entry(va, &purge_vmap_area_list, list) {
seq_printf(m, "0x%pK-0x%pK %7ld unpurged vm_area\n",
(void *)va->va_start, (void *)va->va_end,
va->va_end - va->va_start);
}
+ spin_unlock(&purge_vmap_area_lock);
}

static int s_show(struct seq_file *m, void *p)
@@ -3551,10 +3564,7 @@ static int s_show(struct seq_file *m, void *p)
seq_putc(m, '\n');

/*
- * As a final step, dump "unpurged" areas. Note,
- * that entire "/proc/vmallocinfo" output will not
- * be address sorted, because the purge list is not
- * sorted.
+ * As a final step, dump "unpurged" areas.
*/
if (list_is_last(&va->list, &vmap_area_list))
show_purge_info(m);
--
2.20.1

Next message: Uladzislau Rezki (Sony): "[PATCH 1/2] mm/vmalloc: use free_vm_area() if an allocation fails"
Previous message: Thomas Gleixner: "Re: [PATCH v2] x86/dumpstack: Fix misleading instruction pointer error message"
In reply to: Uladzislau Rezki (Sony): "[PATCH 1/2] mm/vmalloc: use free_vm_area() if an allocation fails"
Next in thread: huang ying: "Re: [PATCH 2/2] mm/vmalloc: rework the drain logic"
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