[PATCH rfc 3/4] percpu: on demand chunk depopulation

Roman Gushchin
2021-03-24T19:07:34Z

To return unused memory to the system schedule an async
depopulation of percpu chunks.

To balance between scanning too much and creating an overhead because
of the pcpu_lock contention and scanning not enough, let's track an
amount of chunks to scan and mark chunks which are potentially a good
target for the depopulation with a new boolean flag. The async
depopulation work will clear the flag after trying to depopulate a
chunk (successfully or not).

This commit suggest the following logic: if a chunk
1) has more than 1/4 of total pages free and populated
2) isn't a reserved chunk
3) isn't entirely free
4) isn't alone in the corresponding slot
it's a good target for depopulation.

If there are 2 or more of such chunks, an async depopulation
is scheduled.

Because chunk population and depopulation are opposite processes
which make a little sense together, split out the shrinking part of
pcpu_balance_populated() into pcpu_grow_populated() and make
pcpu_balance_populated() calling into pcpu_grow_populated() or
pcpu_shrink_populated() conditionally.

Signed-off-by: Roman Gushchin <guro@fb.com>
---
mm/percpu-internal.h | 1 +
mm/percpu.c | 111 ++++++++++++++++++++++++++++++++-----------
2 files changed, 85 insertions(+), 27 deletions(-)

diff --git a/mm/percpu-internal.h b/mm/percpu-internal.h
index 18b768ac7dca..1c5b92af02eb 100644
--- a/mm/percpu-internal.h
+++ b/mm/percpu-internal.h
@@ -67,6 +67,7 @@ struct pcpu_chunk {

void *data; /* chunk data */
bool immutable; /* no [de]population allowed */
+ bool depopulate; /* depopulation hint */
int start_offset; /* the overlap with the previous
region to have a page aligned
base_addr */
diff --git a/mm/percpu.c b/mm/percpu.c
index 015d076893f5..148137f0fc0b 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -178,6 +178,12 @@ static LIST_HEAD(pcpu_map_extend_chunks);
*/
int pcpu_nr_empty_pop_pages;

+/*
+ * Track the number of chunks with a lot of free memory.
+ * It's used to release unused pages to the system.
+ */
+static int pcpu_nr_chunks_to_depopulate;
+
/*
* The number of populated pages in use by the allocator, protected by
* pcpu_lock. This number is kept per a unit per chunk (i.e. when a page gets
@@ -1955,6 +1961,11 @@ static void pcpu_balance_free(enum pcpu_chunk_type type)
if (chunk == list_first_entry(free_head, struct pcpu_chunk, list))
continue;

+ if (chunk->depopulate) {
+ chunk->depopulate = false;
+ pcpu_nr_chunks_to_depopulate--;
+ }
+
list_move(&chunk->list, &to_free);
}

@@ -1976,7 +1987,7 @@ static void pcpu_balance_free(enum pcpu_chunk_type type)
}

/**
- * pcpu_balance_populated - manage the amount of populated pages
+ * pcpu_grow_populated - populate chunk(s) to satisfy atomic allocations
* @type: chunk type
*
* Maintain a certain amount of populated pages to satisfy atomic allocations.
@@ -1985,35 +1996,15 @@ static void pcpu_balance_free(enum pcpu_chunk_type type)
* allocation causes the failure as it is possible that requests can be
* serviced from already backed regions.
*/
-static void pcpu_balance_populated(enum pcpu_chunk_type type)
+static void pcpu_grow_populated(enum pcpu_chunk_type type, int nr_to_pop)
{
/* gfp flags passed to underlying allocators */
const gfp_t gfp = GFP_KERNEL | __GFP_NORETRY | __GFP_NOWARN;
struct list_head *pcpu_slot = pcpu_chunk_list(type);
struct pcpu_chunk *chunk;
- int slot, nr_to_pop, ret;
+ int slot, ret;

- /*
- * Ensure there are certain number of free populated pages for
- * atomic allocs. Fill up from the most packed so that atomic
- * allocs don't increase fragmentation. If atomic allocation
- * failed previously, always populate the maximum amount. This
- * should prevent atomic allocs larger than PAGE_SIZE from keeping
- * failing indefinitely; however, large atomic allocs are not
- * something we support properly and can be highly unreliable and
- * inefficient.
- */
retry_pop:
- if (pcpu_atomic_alloc_failed) {
- nr_to_pop = PCPU_EMPTY_POP_PAGES_HIGH;
- /* best effort anyway, don't worry about synchronization */
- pcpu_atomic_alloc_failed = false;
- } else {
- nr_to_pop = clamp(PCPU_EMPTY_POP_PAGES_HIGH -
- pcpu_nr_empty_pop_pages,
- 0, PCPU_EMPTY_POP_PAGES_HIGH);
- }
-
for (slot = pcpu_size_to_slot(PAGE_SIZE); slot < pcpu_nr_slots; slot++) {
unsigned int nr_unpop = 0, rs, re;

@@ -2084,9 +2075,18 @@ static void pcpu_shrink_populated(enum pcpu_chunk_type type)
list_for_each_entry(chunk, &pcpu_slot[slot], list) {
bool isolated = false;

- if (pcpu_nr_empty_pop_pages < PCPU_EMPTY_POP_PAGES_HIGH)
+ if (pcpu_nr_empty_pop_pages < PCPU_EMPTY_POP_PAGES_HIGH ||
+ pcpu_nr_chunks_to_depopulate < 1)
break;

+ /*
+ * Don't try to depopulate a chunk again and again.
+ */
+ if (!chunk->depopulate)
+ continue;
+ chunk->depopulate = false;
+ pcpu_nr_chunks_to_depopulate--;
+
for (i = 0, start = -1; i < chunk->nr_pages; i++) {
if (!chunk->nr_empty_pop_pages)
break;
@@ -2153,6 +2153,41 @@ static void pcpu_shrink_populated(enum pcpu_chunk_type type)
spin_unlock_irq(&pcpu_lock);
}

+/**
+ * pcpu_balance_populated - manage the amount of populated pages
+ * @type: chunk type
+ *
+ * Populate or depopulate chunks to maintain a certain amount
+ * of free pages to satisfy atomic allocations, but not waste
+ * large amounts of memory.
+ */
+static void pcpu_balance_populated(enum pcpu_chunk_type type)
+{
+ int nr_to_pop;
+
+ /*
+ * Ensure there are certain number of free populated pages for
+ * atomic allocs. Fill up from the most packed so that atomic
+ * allocs don't increase fragmentation. If atomic allocation
+ * failed previously, always populate the maximum amount. This
+ * should prevent atomic allocs larger than PAGE_SIZE from keeping
+ * failing indefinitely; however, large atomic allocs are not
+ * something we support properly and can be highly unreliable and
+ * inefficient.
+ */
+ if (pcpu_atomic_alloc_failed) {
+ nr_to_pop = PCPU_EMPTY_POP_PAGES_HIGH;
+ /* best effort anyway, don't worry about synchronization */
+ pcpu_atomic_alloc_failed = false;
+ pcpu_grow_populated(type, nr_to_pop);
+ } else if (pcpu_nr_empty_pop_pages < PCPU_EMPTY_POP_PAGES_HIGH) {
+ nr_to_pop = PCPU_EMPTY_POP_PAGES_HIGH - pcpu_nr_empty_pop_pages;
+ pcpu_grow_populated(type, nr_to_pop);
+ } else if (pcpu_nr_chunks_to_depopulate > 0) {
+ pcpu_shrink_populated(type);
+ }
+}
+
/**
* pcpu_balance_workfn - manage the amount of free chunks and populated pages
* @work: unused
@@ -2188,6 +2223,7 @@ void free_percpu(void __percpu *ptr)
int size, off;
bool need_balance = false;
struct list_head *pcpu_slot;
+ struct pcpu_chunk *pos;

if (!ptr)
return;
@@ -2207,15 +2243,36 @@ void free_percpu(void __percpu *ptr)

pcpu_memcg_free_hook(chunk, off, size);

- /* if there are more than one fully free chunks, wake up grim reaper */
if (chunk->free_bytes == pcpu_unit_size) {
- struct pcpu_chunk *pos;
-
+ /*
+ * If there are more than one fully free chunks,
+ * wake up grim reaper.
+ */
list_for_each_entry(pos, &pcpu_slot[pcpu_nr_slots - 1], list)
if (pos != chunk) {
need_balance = true;
break;
}
+
+ } else if (chunk->nr_empty_pop_pages > chunk->nr_pages / 4) {
+ /*
+ * If there is more than one chunk in the slot and
+ * at least 1/4 of its pages are empty, mark the chunk
+ * as a target for the depopulation. If there is more
+ * than one chunk like this, schedule an async balancing.
+ */
+ int nslot = pcpu_chunk_slot(chunk);
+
+ list_for_each_entry(pos, &pcpu_slot[nslot], list)
+ if (pos != chunk && !chunk->depopulate &&
+ !chunk->immutable) {
+ chunk->depopulate = true;
+ pcpu_nr_chunks_to_depopulate++;
+ break;
+ }
+
+ if (pcpu_nr_chunks_to_depopulate > 1)
+ need_balance = true;
}

trace_percpu_free_percpu(chunk->base_addr, off, ptr);
--
2.30.2