On Thu, Jan 11, 2024 at 8:08 PM Marco Elver <elver@xxxxxxxxxx> wrote:
>
> On Thu, Jan 11, 2024 at 04:36AM -0800, Andi Kleen wrote:
> > > stackdepot is severely limited in what kernel facilities it may use
> > > due to being used by such low level facilities as the allocator
> > > itself.
> >
> > RCU can be done quite low level too (e.g. there is NMI safe RCU)
>
> How about the below? This should get us back the performance of the old
> lock-less version. Although it's using rculist, we don't actually need
> to synchronize via RCU.
>
> Thanks,
> -- Marco
>
> ------ >8 ------
>
> From: Marco Elver <elver@xxxxxxxxxx>
> Date: Tue, 9 Jan 2024 10:21:56 +0100
> Subject: [PATCH] stackdepot: make fast paths lock-less again
>
> stack_depot_put() unconditionally takes the pool_rwlock as a writer.
> This is unnecessary if the stack record is not going to be freed.
> Furthermore, reader-writer locks have inherent cache contention, which
> does not scale well on machines with large CPU counts.
>
> Instead, rework the synchronization story of stack depot to again avoid
> taking any locks in the fast paths. This is done by relying on RCU
> primitives to give us lock-less list traversal. See code comments for
> more details.
>
> Fixes: 108be8def46e ("lib/stackdepot: allow users to evict stack traces")
> Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
> ---
> lib/stackdepot.c | 222 ++++++++++++++++++++++++++++-------------------
> 1 file changed, 133 insertions(+), 89 deletions(-)
>
> diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> index a0be5d05c7f0..9eaf46f8abc4 100644
> --- a/lib/stackdepot.c
> +++ b/lib/stackdepot.c
> @@ -19,10 +19,13 @@
> #include <linux/kernel.h>
> #include <linux/kmsan.h>
> #include <linux/list.h>
> +#include <linux/llist.h>
> #include <linux/mm.h>
> #include <linux/mutex.h>
> #include <linux/percpu.h>
> #include <linux/printk.h>
> +#include <linux/rculist.h>
> +#include <linux/rcupdate.h>
> #include <linux/refcount.h>
> #include <linux/slab.h>
> #include <linux/spinlock.h>
> @@ -67,7 +70,8 @@ union handle_parts {
> };
>
> struct stack_record {
> - struct list_head list; /* Links in hash table or freelist */
> + struct list_head hash_list; /* Links in the hash table */
> + struct llist_node free_list; /* Links in the freelist */
> u32 hash; /* Hash in hash table */
> u32 size; /* Number of stored frames */
> union handle_parts handle;
> @@ -104,7 +108,7 @@ static void *new_pool;
> /* Number of pools in stack_pools. */
> static int pools_num;
> /* Freelist of stack records within stack_pools. */
> -static LIST_HEAD(free_stacks);
> +static LLIST_HEAD(free_stacks);
> /*
> * Stack depot tries to keep an extra pool allocated even before it runs out
> * of space in the currently used pool. This flag marks whether this extra pool
> @@ -112,8 +116,8 @@ static LIST_HEAD(free_stacks);
> * yet allocated or if the limit on the number of pools is reached.
> */
> static bool new_pool_required = true;
> -/* Lock that protects the variables above. */
> -static DEFINE_RWLOCK(pool_rwlock);
> +/* The lock must be held when performing pool or free list modifications. */
> +static DEFINE_RAW_SPINLOCK(pool_lock);
>
> static int __init disable_stack_depot(char *str)
> {
> @@ -263,9 +267,7 @@ static void depot_init_pool(void *pool)
> {
> int offset;
>
> - lockdep_assert_held_write(&pool_rwlock);
> -
> - WARN_ON(!list_empty(&free_stacks));
> + lockdep_assert_held(&pool_lock);
>
> /* Initialize handles and link stack records into the freelist. */
> for (offset = 0; offset <= DEPOT_POOL_SIZE - DEPOT_STACK_RECORD_SIZE;
> @@ -276,18 +278,25 @@ static void depot_init_pool(void *pool)
> stack->handle.offset = offset >> DEPOT_STACK_ALIGN;
> stack->handle.extra = 0;
>
> - list_add(&stack->list, &free_stacks);
> + llist_add(&stack->free_list, &free_stacks);
> + INIT_LIST_HEAD(&stack->hash_list);
> }
>
> /* Save reference to the pool to be used by depot_fetch_stack(). */
> stack_pools[pools_num] = pool;
> - pools_num++;
> +
> + /*
> + * Release of pool pointer assignment above. Pairs with the
> + * smp_load_acquire() in depot_fetch_stack().
> + */
> + smp_store_release(&pools_num, pools_num + 1);
> + ASSERT_EXCLUSIVE_WRITER(pools_num);
> }
>
> /* Keeps the preallocated memory to be used for a new stack depot pool. */
> static void depot_keep_new_pool(void **prealloc)
> {
> - lockdep_assert_held_write(&pool_rwlock);
> + lockdep_assert_held(&pool_lock);
>
> /*
> * If a new pool is already saved or the maximum number of
> @@ -310,16 +319,16 @@ static void depot_keep_new_pool(void **prealloc)
> * number of pools is reached. In either case, take note that
> * keeping another pool is not required.
> */
> - new_pool_required = false;
> + WRITE_ONCE(new_pool_required, false);
> }
>
> /* Updates references to the current and the next stack depot pools. */
> static bool depot_update_pools(void **prealloc)
> {
> - lockdep_assert_held_write(&pool_rwlock);
> + lockdep_assert_held(&pool_lock);
>
> /* Check if we still have objects in the freelist. */
> - if (!list_empty(&free_stacks))
> + if (!llist_empty(&free_stacks))
> goto out_keep_prealloc;
>
> /* Check if we have a new pool saved and use it. */
> @@ -329,7 +338,7 @@ static bool depot_update_pools(void **prealloc)
>
> /* Take note that we might need a new new_pool. */
> if (pools_num < DEPOT_MAX_POOLS)
> - new_pool_required = true;
> + WRITE_ONCE(new_pool_required, true);
>
> /* Try keeping the preallocated memory for new_pool. */
> goto out_keep_prealloc;
> @@ -362,20 +371,19 @@ static struct stack_record *
> depot_alloc_stack(unsigned long *entries, int size, u32 hash, void **prealloc)
> {
> struct stack_record *stack;
> + struct llist_node *free;
>
> - lockdep_assert_held_write(&pool_rwlock);
> + lockdep_assert_held(&pool_lock);
>
> /* Update current and new pools if required and possible. */
> if (!depot_update_pools(prealloc))
> return NULL;
>
> /* Check if we have a stack record to save the stack trace. */
> - if (list_empty(&free_stacks))
> + free = llist_del_first(&free_stacks);
> + if (!free)
> return NULL;
> -
> - /* Get and unlink the first entry from the freelist. */
> - stack = list_first_entry(&free_stacks, struct stack_record, list);
> - list_del(&stack->list);
> + stack = llist_entry(free, struct stack_record, free_list);
>
> /* Limit number of saved frames to CONFIG_STACKDEPOT_MAX_FRAMES. */
> if (size > CONFIG_STACKDEPOT_MAX_FRAMES)
> @@ -385,7 +393,6 @@ depot_alloc_stack(unsigned long *entries, int size, u32 hash, void **prealloc)
> stack->hash = hash;
> stack->size = size;
> /* stack->handle is already filled in by depot_init_pool(). */
> - refcount_set(&stack->count, 1);
> memcpy(stack->entries, entries, flex_array_size(stack, entries, size));
>
> /*
> @@ -394,21 +401,30 @@ depot_alloc_stack(unsigned long *entries, int size, u32 hash, void **prealloc)
> */
> kmsan_unpoison_memory(stack, DEPOT_STACK_RECORD_SIZE);
>
> + /*
> + * Release saving of the stack trace. Pairs with smp_mb() in
> + * depot_fetch_stack().
> + */
> + smp_mb__before_atomic();
> + refcount_set(&stack->count, 1);
> +
> return stack;
> }
>
> static struct stack_record *depot_fetch_stack(depot_stack_handle_t handle)
> {
> + /* Acquire the pool pointer written in depot_init_pool(). */
> + const int pools_num_cached = smp_load_acquire(&pools_num);
> union handle_parts parts = { .handle = handle };
> void *pool;
> size_t offset = parts.offset << DEPOT_STACK_ALIGN;
> struct stack_record *stack;
>
> - lockdep_assert_held(&pool_rwlock);
> + lockdep_assert_not_held(&pool_lock);
>
> - if (parts.pool_index > pools_num) {
> + if (parts.pool_index > pools_num_cached) {
> WARN(1, "pool index %d out of bounds (%d) for stack id %08x\n",
> - parts.pool_index, pools_num, handle);
> + parts.pool_index, pools_num_cached, handle);
> return NULL;
> }
>
> @@ -417,15 +433,35 @@ static struct stack_record *depot_fetch_stack(depot_stack_handle_t handle)
> return NULL;
>
> stack = pool + offset;
> +
> + /*
> + * Acquire the stack trace. Pairs with smp_mb() in depot_alloc_stack().
> + *
> + * This does not protect against a stack_depot_put() freeing the record
> + * and having it subsequently being reused. Callers are responsible to
> + * avoid using stack depot handles after passing to stack_depot_put().
> + */
> + if (!refcount_read(&stack->count))
> + return NULL;
Can this happen? It seems that depot_fetch_stack should only be called
for handles that were returned from stack_depot_save_flags before all
puts and thus the the refcount should > 0. Or is this a safeguard
against improper API usage?
> + smp_mb__after_atomic();
> +
> return stack;
> }
>
> /* Links stack into the freelist. */
> static void depot_free_stack(struct stack_record *stack)
> {
> - lockdep_assert_held_write(&pool_rwlock);
> + unsigned long flags;
> +
> + lockdep_assert_not_held(&pool_lock);
> +
> + raw_spin_lock_irqsave(&pool_lock, flags);
> + printk_deferred_enter();
> + list_del_rcu(&stack->hash_list);
> + printk_deferred_exit();
> + raw_spin_unlock_irqrestore(&pool_lock, flags);
>
> - list_add(&stack->list, &free_stacks);
> + llist_add(&stack->free_list, &free_stacks);
This llist_add is outside of the lock just because we can (i.e.
llist_add can run concurrently with the other free_stacks operations,
which are all under the lock), right? This slightly contradicts the
comment above the free_stacks definition.
If we put this under the lock and use normal list instead of llist, I
think we can then combine the hash_list with the free_list like before
to save up on some space for stack_record. Would that make sense?
> }
>
> /* Calculates the hash for a stack. */
> @@ -453,22 +489,55 @@ int stackdepot_memcmp(const unsigned long *u1, const unsigned long *u2,
>
> /* Finds a stack in a bucket of the hash table. */
> static inline struct stack_record *find_stack(struct list_head *bucket,
> - unsigned long *entries, int size,
> - u32 hash)
> + unsigned long *entries, int size,
> + u32 hash, depot_flags_t flags)
> {
> - struct list_head *pos;
> - struct stack_record *found;
> + struct stack_record *stack, *ret = NULL;
>
> - lockdep_assert_held(&pool_rwlock);
> + /*
> + * Due to being used from low-level code paths such as the allocators,
> + * NMI, or even RCU itself, stackdepot cannot rely on primitives that
> + * would sleep (such as synchronize_rcu()) or end up recursively call
> + * into stack depot again (such as call_rcu()).
> + *
> + * Instead, lock-less readers only rely on RCU primitives for correct
> + * memory ordering, but do not use RCU-based synchronization otherwise.
> + * Instead, we perform 3-pass validation below to ensure that the stack
> + * record we accessed is actually valid. If we fail to obtain a valid
> + * stack record here, the slow-path in stack_depot_save_flags() will
> + * retry to avoid inserting duplicates.
> + *
> + * If STACK_DEPOT_FLAG_GET is not used, it is undefined behaviour to
> + * call stack_depot_put() later - i.e. in the non-refcounted case, we do
> + * not have to worry that the entry will be recycled.
> + */
> +
> + list_for_each_entry_rcu(stack, bucket, hash_list) {
So we don't need rcu_read_lock here, because we don't rely on call_rcu
etc., right?
> + /* 1. Check if this entry could potentially match. */
> + if (data_race(stack->hash != hash || stack->size != size))
> + continue;
> +
> + /*
> + * 2. Increase refcount if not zero. If this is successful, we
> + * know that this stack record is valid and will not be freed by
> + * stack_depot_put().
> + */
> + if ((flags & STACK_DEPOT_FLAG_GET) && unlikely(!refcount_inc_not_zero(&stack->count)))
> + continue;
> +
> + /* 3. Do full validation of the record. */
> + if (likely(stack->hash == hash && stack->size == size &&
> + !stackdepot_memcmp(entries, stack->entries, size))) {
> + ret = stack;
> + break;
> + }
>
> - list_for_each(pos, bucket) {
> - found = list_entry(pos, struct stack_record, list);
> - if (found->hash == hash &&
> - found->size == size &&
> - !stackdepot_memcmp(entries, found->entries, size))
> - return found;
> + /* Undo refcount - could have raced with stack_depot_put(). */
> + if ((flags & STACK_DEPOT_FLAG_GET) && unlikely(refcount_dec_and_test(&stack->count)))
> + depot_free_stack(stack);
> }
> - return NULL;
> +
> + return ret;
> }
>
> depot_stack_handle_t stack_depot_save_flags(unsigned long *entries,
> @@ -482,7 +551,6 @@ depot_stack_handle_t stack_depot_save_flags(unsigned long *entries,
> struct page *page = NULL;
> void *prealloc = NULL;
> bool can_alloc = depot_flags & STACK_DEPOT_FLAG_CAN_ALLOC;
> - bool need_alloc = false;
> unsigned long flags;
> u32 hash;
>
> @@ -505,31 +573,16 @@ depot_stack_handle_t stack_depot_save_flags(unsigned long *entries,
> hash = hash_stack(entries, nr_entries);
> bucket = &stack_table[hash & stack_hash_mask];
>
> - read_lock_irqsave(&pool_rwlock, flags);
> - printk_deferred_enter();
> -
> - /* Fast path: look the stack trace up without full locking. */
> - found = find_stack(bucket, entries, nr_entries, hash);
> - if (found) {
> - if (depot_flags & STACK_DEPOT_FLAG_GET)
> - refcount_inc(&found->count);
> - printk_deferred_exit();
> - read_unlock_irqrestore(&pool_rwlock, flags);
> + /* Fast path: look the stack trace up without locking. */
> + found = find_stack(bucket, entries, nr_entries, hash, depot_flags);
> + if (found)
> goto exit;
> - }
> -
> - /* Take note if another stack pool needs to be allocated. */
> - if (new_pool_required)
> - need_alloc = true;
> -
> - printk_deferred_exit();
> - read_unlock_irqrestore(&pool_rwlock, flags);
>
> /*
> * Allocate memory for a new pool if required now:
> * we won't be able to do that under the lock.
> */
> - if (unlikely(can_alloc && need_alloc)) {
> + if (unlikely(can_alloc && READ_ONCE(new_pool_required))) {
> /*
> * Zero out zone modifiers, as we don't have specific zone
> * requirements. Keep the flags related to allocation in atomic
> @@ -543,31 +596,33 @@ depot_stack_handle_t stack_depot_save_flags(unsigned long *entries,
> prealloc = page_address(page);
> }
>
> - write_lock_irqsave(&pool_rwlock, flags);
> + raw_spin_lock_irqsave(&pool_lock, flags);
> printk_deferred_enter();
>
> - found = find_stack(bucket, entries, nr_entries, hash);
> + /* Try to find again, to avoid concurrently inserting duplicates. */
> + found = find_stack(bucket, entries, nr_entries, hash, depot_flags);
> if (!found) {
> struct stack_record *new =
> depot_alloc_stack(entries, nr_entries, hash, &prealloc);
>
> if (new) {
> - list_add(&new->list, bucket);
> + /*
> + * This releases the stack record into the bucket and
> + * makes it visible to readers in find_stack().
> + */
> + list_add_rcu(&new->hash_list, bucket);
> found = new;
> }
> - } else {
> - if (depot_flags & STACK_DEPOT_FLAG_GET)
> - refcount_inc(&found->count);
> + } else if (prealloc) {
> /*
> * Stack depot already contains this stack trace, but let's
> * keep the preallocated memory for future.
> */
> - if (prealloc)
> - depot_keep_new_pool(&prealloc);
> + depot_keep_new_pool(&prealloc);
> }
>
> printk_deferred_exit();
> - write_unlock_irqrestore(&pool_rwlock, flags);
> + raw_spin_unlock_irqrestore(&pool_lock, flags);
> exit:
> if (prealloc) {
> /* Stack depot didn't use this memory, free it. */
> @@ -592,7 +647,6 @@ unsigned int stack_depot_fetch(depot_stack_handle_t handle,
> unsigned long **entries)
> {
> struct stack_record *stack;
> - unsigned long flags;
>
> *entries = NULL;
> /*
> @@ -604,13 +658,12 @@ unsigned int stack_depot_fetch(depot_stack_handle_t handle,
> if (!handle || stack_depot_disabled)
> return 0;
>
> - read_lock_irqsave(&pool_rwlock, flags);
> - printk_deferred_enter();
> -
> stack = depot_fetch_stack(handle);
> -
> - printk_deferred_exit();
> - read_unlock_irqrestore(&pool_rwlock, flags);
> + /*
> + * Should never be NULL, otherwise this is a use-after-put.
> + */
> + if (WARN_ON(!stack))
> + return 0;
>
> *entries = stack->entries;
> return stack->size;
> @@ -620,29 +673,20 @@ EXPORT_SYMBOL_GPL(stack_depot_fetch);
> void stack_depot_put(depot_stack_handle_t handle)
> {
> struct stack_record *stack;
> - unsigned long flags;
>
> if (!handle || stack_depot_disabled)
> return;
>
> - write_lock_irqsave(&pool_rwlock, flags);
> - printk_deferred_enter();
> -
> stack = depot_fetch_stack(handle);
> + /*
> + * Should always be able to find the stack record, otherwise this is an
> + * unbalanced put attempt.
> + */
> if (WARN_ON(!stack))
> - goto out;
> -
> - if (refcount_dec_and_test(&stack->count)) {
> - /* Unlink stack from the hash table. */
> - list_del(&stack->list);
> + return;
>
> - /* Free stack. */
> + if (refcount_dec_and_test(&stack->count))
> depot_free_stack(stack);
> - }
> -
> -out:
> - printk_deferred_exit();
> - write_unlock_irqrestore(&pool_rwlock, flags);
> }
> EXPORT_SYMBOL_GPL(stack_depot_put);
>
> --
> 2.43.0.275.g3460e3d667-goog
>
Looks good to me from the functional perspective (modulo the
clarification comments I left above), but it would be great to get a
review from someone with a better understanding of the low-level
synchronization primitives.
Thank you!
