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percpu: optimize locking in pcpu_balance_workfn()

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pcpu_balance_workfn() unconditionally calls pcpu_balance_free(),
pcpu_reclaim_populated(), pcpu_balance_populated() and
pcpu_balance_free() again.

Each call to pcpu_balance_free() and pcpu_reclaim_populated() will
cause at least one acquisition of the pcpu_lock. So even if the
balancing was scheduled because of a failed atomic allocation,
pcpu_lock will be acquired at least 4 times. This obviously
increases the contention on the pcpu_lock.

To optimize the scheme let's grab the pcpu_lock on the upper level
(in pcpu_balance_workfn()) and keep it generally locked for the whole
duration of the scheduled work, but release conditionally to perform
any slow operations like chunk (de)population and creation of new
chunks.

Signed-off-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>
This commit is contained in:
Roman Gushchin 2021-06-17 12:03:22 -07:00 committed by Dennis Zhou
parent 4829c791b2
commit e4d777003a
1 changed files with 29 additions and 12 deletions
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41
mm/percpu.c
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@ -1980,6 +1980,9 @@ void __percpu *__alloc_reserved_percpu(size_t size, size_t align)
* If empty_only is %false, reclaim all fully free chunks regardless of the
* number of populated pages. Otherwise, only reclaim chunks that have no
* populated pages.
*
* CONTEXT:
* pcpu_lock (can be dropped temporarily)
*/
static void pcpu_balance_free(bool empty_only)
{
@ -1987,12 +1990,12 @@ static void pcpu_balance_free(bool empty_only)
struct list_head *free_head = &pcpu_chunk_lists[pcpu_free_slot];
struct pcpu_chunk *chunk, *next;
lockdep_assert_held(&pcpu_lock);
/*
* There's no reason to keep around multiple unused chunks and VM
* areas can be scarce. Destroy all free chunks except for one.
*/
spin_lock_irq(&pcpu_lock);
list_for_each_entry_safe(chunk, next, free_head, list) {
WARN_ON(chunk->immutable);
@ -2004,8 +2007,10 @@ static void pcpu_balance_free(bool empty_only)
list_move(&chunk->list, &to_free);
}
spin_unlock_irq(&pcpu_lock);
if (list_empty(&to_free))
return;
spin_unlock_irq(&pcpu_lock);
list_for_each_entry_safe(chunk, next, &to_free, list) {
unsigned int rs, re;
@ -2019,6 +2024,7 @@ static void pcpu_balance_free(bool empty_only)
pcpu_destroy_chunk(chunk);
cond_resched();
}
spin_lock_irq(&pcpu_lock);
}
/**
@ -2029,6 +2035,9 @@ static void pcpu_balance_free(bool empty_only)
* OOM killer to be triggered. We should avoid doing so until an actual
* allocation causes the failure as it is possible that requests can be
* serviced from already backed regions.
*
* CONTEXT:
* pcpu_lock (can be dropped temporarily)
*/
static void pcpu_balance_populated(void)
{
@ -2037,6 +2046,8 @@ static void pcpu_balance_populated(void)
struct pcpu_chunk *chunk;
int slot, nr_to_pop, ret;
lockdep_assert_held(&pcpu_lock);
/*
* Ensure there are certain number of free populated pages for
* atomic allocs. Fill up from the most packed so that atomic
@ -2064,13 +2075,11 @@ retry_pop:
if (!nr_to_pop)
break;
spin_lock_irq(&pcpu_lock);
list_for_each_entry(chunk, &pcpu_chunk_lists[slot], list) {
nr_unpop = chunk->nr_pages - chunk->nr_populated;
if (nr_unpop)
break;
}
spin_unlock_irq(&pcpu_lock);
if (!nr_unpop)
continue;
@ -2080,12 +2089,13 @@ retry_pop:
chunk->nr_pages) {
int nr = min_t(int, re - rs, nr_to_pop);
spin_unlock_irq(&pcpu_lock);
ret = pcpu_populate_chunk(chunk, rs, rs + nr, gfp);
cond_resched();
spin_lock_irq(&pcpu_lock);
if (!ret) {
nr_to_pop -= nr;
spin_lock_irq(&pcpu_lock);
pcpu_chunk_populated(chunk, rs, rs + nr);
spin_unlock_irq(&pcpu_lock);
} else {
nr_to_pop = 0;
}
@ -2097,11 +2107,12 @@ retry_pop:
if (nr_to_pop) {
/* ran out of chunks to populate, create a new one and retry */
spin_unlock_irq(&pcpu_lock);
chunk = pcpu_create_chunk(gfp);
cond_resched();
spin_lock_irq(&pcpu_lock);
if (chunk) {
spin_lock_irq(&pcpu_lock);
pcpu_chunk_relocate(chunk, -1);
spin_unlock_irq(&pcpu_lock);
goto retry_pop;
}
}
@ -2117,6 +2128,10 @@ retry_pop:
* populated pages threshold, reintegrate the chunk if it has empty free pages.
* Each chunk is scanned in the reverse order to keep populated pages close to
* the beginning of the chunk.
*
* CONTEXT:
* pcpu_lock (can be dropped temporarily)
*
*/
static void pcpu_reclaim_populated(void)
{
@ -2124,7 +2139,7 @@ static void pcpu_reclaim_populated(void)
struct pcpu_block_md *block;
int i, end;
spin_lock_irq(&pcpu_lock);
lockdep_assert_held(&pcpu_lock);
restart:
/*
@ -2190,8 +2205,6 @@ restart:
list_move(&chunk->list,
&pcpu_chunk_lists[pcpu_sidelined_slot]);
}
spin_unlock_irq(&pcpu_lock);
}
/**
@ -2212,10 +2225,14 @@ static void pcpu_balance_workfn(struct work_struct *work)
* appropriate.
*/
mutex_lock(&pcpu_alloc_mutex);
spin_lock_irq(&pcpu_lock);
pcpu_balance_free(false);
pcpu_reclaim_populated();
pcpu_balance_populated();
pcpu_balance_free(true);
spin_unlock_irq(&pcpu_lock);
mutex_unlock(&pcpu_alloc_mutex);
}