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percpu_ref: reduce memory footprint of percpu_ref in fast path
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'struct percpu_ref' is often embedded into one user structure, and the
instance is usually referenced in fast path, however actually only
'percpu_count_ptr' is needed in fast path.

So move other fields into one new structure of 'percpu_ref_data', and
allocate it dynamically via kzalloc(), then memory footprint of
'percpu_ref' in fast path is reduced a lot and becomes suitable to put
into hot cacheline of user structure.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Tested-by: Veronika Kabatova <vkabatov@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: Sagi Grimberg <sagi@grimberg.me>
Cc: Tejun Heo <tj@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
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Ming Lei authored and Jens Axboe committed Oct 6, 2020
1 parent cf785af commit 2b0d3d3
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Showing 3 changed files with 123 additions and 62 deletions.
2 changes: 1 addition & 1 deletion drivers/infiniband/sw/rdmavt/mr.c
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Original file line number Diff line number Diff line change
Expand Up @@ -499,7 +499,7 @@ static int rvt_check_refs(struct rvt_mregion *mr, const char *t)
rvt_pr_err(rdi,
"%s timeout mr %p pd %p lkey %x refcount %ld\n",
t, mr, mr->pd, mr->lkey,
atomic_long_read(&mr->refcount.count));
atomic_long_read(&mr->refcount.data->count));
rvt_get_mr(mr);
return -EBUSY;
}
Expand Down
52 changes: 24 additions & 28 deletions include/linux/percpu-refcount.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -92,18 +92,30 @@ enum {
PERCPU_REF_ALLOW_REINIT = 1 << 2,
};

struct percpu_ref {
struct percpu_ref_data {
atomic_long_t count;
/*
* The low bit of the pointer indicates whether the ref is in percpu
* mode; if set, then get/put will manipulate the atomic_t.
*/
unsigned long percpu_count_ptr;
percpu_ref_func_t *release;
percpu_ref_func_t *confirm_switch;
bool force_atomic:1;
bool allow_reinit:1;
struct rcu_head rcu;
struct percpu_ref *ref;
};

struct percpu_ref {
/*
* The low bit of the pointer indicates whether the ref is in percpu
* mode; if set, then get/put will manipulate the atomic_t.
*/
unsigned long percpu_count_ptr;

/*
* 'percpu_ref' is often embedded into user structure, and only
* 'percpu_count_ptr' is required in fast path, move other fields
* into 'percpu_ref_data', so we can reduce memory footprint in
* fast path.
*/
struct percpu_ref_data *data;
};

int __must_check percpu_ref_init(struct percpu_ref *ref,
Expand All @@ -118,6 +130,7 @@ void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
percpu_ref_func_t *confirm_kill);
void percpu_ref_resurrect(struct percpu_ref *ref);
void percpu_ref_reinit(struct percpu_ref *ref);
bool percpu_ref_is_zero(struct percpu_ref *ref);

/**
* percpu_ref_kill - drop the initial ref
Expand Down Expand Up @@ -191,7 +204,7 @@ static inline void percpu_ref_get_many(struct percpu_ref *ref, unsigned long nr)
if (__ref_is_percpu(ref, &percpu_count))
this_cpu_add(*percpu_count, nr);
else
atomic_long_add(nr, &ref->count);
atomic_long_add(nr, &ref->data->count);

rcu_read_unlock();
}
Expand Down Expand Up @@ -231,7 +244,7 @@ static inline bool percpu_ref_tryget_many(struct percpu_ref *ref,
this_cpu_add(*percpu_count, nr);
ret = true;
} else {
ret = atomic_long_add_unless(&ref->count, nr, 0);
ret = atomic_long_add_unless(&ref->data->count, nr, 0);
}

rcu_read_unlock();
Expand Down Expand Up @@ -279,7 +292,7 @@ static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
this_cpu_inc(*percpu_count);
ret = true;
} else if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)) {
ret = atomic_long_inc_not_zero(&ref->count);
ret = atomic_long_inc_not_zero(&ref->data->count);
}

rcu_read_unlock();
Expand All @@ -305,8 +318,8 @@ static inline void percpu_ref_put_many(struct percpu_ref *ref, unsigned long nr)

if (__ref_is_percpu(ref, &percpu_count))
this_cpu_sub(*percpu_count, nr);
else if (unlikely(atomic_long_sub_and_test(nr, &ref->count)))
ref->release(ref);
else if (unlikely(atomic_long_sub_and_test(nr, &ref->data->count)))
ref->data->release(ref);

rcu_read_unlock();
}
Expand Down Expand Up @@ -339,21 +352,4 @@ static inline bool percpu_ref_is_dying(struct percpu_ref *ref)
return ref->percpu_count_ptr & __PERCPU_REF_DEAD;
}

/**
* percpu_ref_is_zero - test whether a percpu refcount reached zero
* @ref: percpu_ref to test
*
* Returns %true if @ref reached zero.
*
* This function is safe to call as long as @ref is between init and exit.
*/
static inline bool percpu_ref_is_zero(struct percpu_ref *ref)
{
unsigned long __percpu *percpu_count;

if (__ref_is_percpu(ref, &percpu_count))
return false;
return !atomic_long_read(&ref->count);
}

#endif
131 changes: 98 additions & 33 deletions lib/percpu-refcount.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -4,6 +4,7 @@
#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/wait.h>
#include <linux/slab.h>
#include <linux/percpu-refcount.h>

/*
Expand Down Expand Up @@ -64,18 +65,25 @@ int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release,
size_t align = max_t(size_t, 1 << __PERCPU_REF_FLAG_BITS,
__alignof__(unsigned long));
unsigned long start_count = 0;
struct percpu_ref_data *data;

ref->percpu_count_ptr = (unsigned long)
__alloc_percpu_gfp(sizeof(unsigned long), align, gfp);
if (!ref->percpu_count_ptr)
return -ENOMEM;

ref->force_atomic = flags & PERCPU_REF_INIT_ATOMIC;
ref->allow_reinit = flags & PERCPU_REF_ALLOW_REINIT;
data = kzalloc(sizeof(*ref->data), gfp);
if (!data) {
free_percpu((void __percpu *)ref->percpu_count_ptr);
return -ENOMEM;
}

data->force_atomic = flags & PERCPU_REF_INIT_ATOMIC;
data->allow_reinit = flags & PERCPU_REF_ALLOW_REINIT;

if (flags & (PERCPU_REF_INIT_ATOMIC | PERCPU_REF_INIT_DEAD)) {
ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC;
ref->allow_reinit = true;
data->allow_reinit = true;
} else {
start_count += PERCPU_COUNT_BIAS;
}
Expand All @@ -85,14 +93,28 @@ int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release,
else
start_count++;

atomic_long_set(&ref->count, start_count);
atomic_long_set(&data->count, start_count);

ref->release = release;
ref->confirm_switch = NULL;
data->release = release;
data->confirm_switch = NULL;
data->ref = ref;
ref->data = data;
return 0;
}
EXPORT_SYMBOL_GPL(percpu_ref_init);

static void __percpu_ref_exit(struct percpu_ref *ref)
{
unsigned long __percpu *percpu_count = percpu_count_ptr(ref);

if (percpu_count) {
/* non-NULL confirm_switch indicates switching in progress */
WARN_ON_ONCE(ref->data->confirm_switch);
free_percpu(percpu_count);
ref->percpu_count_ptr = __PERCPU_REF_ATOMIC_DEAD;
}
}

/**
* percpu_ref_exit - undo percpu_ref_init()
* @ref: percpu_ref to exit
Expand All @@ -105,35 +127,46 @@ EXPORT_SYMBOL_GPL(percpu_ref_init);
*/
void percpu_ref_exit(struct percpu_ref *ref)
{
unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
struct percpu_ref_data *data = ref->data;
unsigned long flags;

if (percpu_count) {
/* non-NULL confirm_switch indicates switching in progress */
WARN_ON_ONCE(ref->confirm_switch);
free_percpu(percpu_count);
ref->percpu_count_ptr = __PERCPU_REF_ATOMIC_DEAD;
}
__percpu_ref_exit(ref);

if (!data)
return;

spin_lock_irqsave(&percpu_ref_switch_lock, flags);
ref->percpu_count_ptr |= atomic_long_read(&ref->data->count) <<
__PERCPU_REF_FLAG_BITS;
ref->data = NULL;
spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);

kfree(data);
}
EXPORT_SYMBOL_GPL(percpu_ref_exit);

static void percpu_ref_call_confirm_rcu(struct rcu_head *rcu)
{
struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);
struct percpu_ref_data *data = container_of(rcu,
struct percpu_ref_data, rcu);
struct percpu_ref *ref = data->ref;

ref->confirm_switch(ref);
ref->confirm_switch = NULL;
data->confirm_switch(ref);
data->confirm_switch = NULL;
wake_up_all(&percpu_ref_switch_waitq);

if (!ref->allow_reinit)
percpu_ref_exit(ref);
if (!data->allow_reinit)
__percpu_ref_exit(ref);

/* drop ref from percpu_ref_switch_to_atomic() */
percpu_ref_put(ref);
}

static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu)
{
struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);
struct percpu_ref_data *data = container_of(rcu,
struct percpu_ref_data, rcu);
struct percpu_ref *ref = data->ref;
unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
unsigned long count = 0;
int cpu;
Expand All @@ -142,7 +175,7 @@ static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu)
count += *per_cpu_ptr(percpu_count, cpu);

pr_debug("global %lu percpu %lu\n",
atomic_long_read(&ref->count), count);
atomic_long_read(&data->count), count);

/*
* It's crucial that we sum the percpu counters _before_ adding the sum
Expand All @@ -156,11 +189,11 @@ static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu)
* reaching 0 before we add the percpu counts. But doing it at the same
* time is equivalent and saves us atomic operations:
*/
atomic_long_add((long)count - PERCPU_COUNT_BIAS, &ref->count);
atomic_long_add((long)count - PERCPU_COUNT_BIAS, &data->count);

WARN_ONCE(atomic_long_read(&ref->count) <= 0,
WARN_ONCE(atomic_long_read(&data->count) <= 0,
"percpu ref (%ps) <= 0 (%ld) after switching to atomic",
ref->release, atomic_long_read(&ref->count));
data->release, atomic_long_read(&data->count));

/* @ref is viewed as dead on all CPUs, send out switch confirmation */
percpu_ref_call_confirm_rcu(rcu);
Expand All @@ -186,10 +219,11 @@ static void __percpu_ref_switch_to_atomic(struct percpu_ref *ref,
* Non-NULL ->confirm_switch is used to indicate that switching is
* in progress. Use noop one if unspecified.
*/
ref->confirm_switch = confirm_switch ?: percpu_ref_noop_confirm_switch;
ref->data->confirm_switch = confirm_switch ?:
percpu_ref_noop_confirm_switch;

percpu_ref_get(ref); /* put after confirmation */
call_rcu(&ref->rcu, percpu_ref_switch_to_atomic_rcu);
call_rcu(&ref->data->rcu, percpu_ref_switch_to_atomic_rcu);
}

static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)
Expand All @@ -202,10 +236,10 @@ static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)
if (!(ref->percpu_count_ptr & __PERCPU_REF_ATOMIC))
return;

if (WARN_ON_ONCE(!ref->allow_reinit))
if (WARN_ON_ONCE(!ref->data->allow_reinit))
return;

atomic_long_add(PERCPU_COUNT_BIAS, &ref->count);
atomic_long_add(PERCPU_COUNT_BIAS, &ref->data->count);

/*
* Restore per-cpu operation. smp_store_release() is paired
Expand All @@ -223,17 +257,19 @@ static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)
static void __percpu_ref_switch_mode(struct percpu_ref *ref,
percpu_ref_func_t *confirm_switch)
{
struct percpu_ref_data *data = ref->data;

lockdep_assert_held(&percpu_ref_switch_lock);

/*
* If the previous ATOMIC switching hasn't finished yet, wait for
* its completion. If the caller ensures that ATOMIC switching
* isn't in progress, this function can be called from any context.
*/
wait_event_lock_irq(percpu_ref_switch_waitq, !ref->confirm_switch,
wait_event_lock_irq(percpu_ref_switch_waitq, !data->confirm_switch,
percpu_ref_switch_lock);

if (ref->force_atomic || (ref->percpu_count_ptr & __PERCPU_REF_DEAD))
if (data->force_atomic || (ref->percpu_count_ptr & __PERCPU_REF_DEAD))
__percpu_ref_switch_to_atomic(ref, confirm_switch);
else
__percpu_ref_switch_to_percpu(ref);
Expand Down Expand Up @@ -266,7 +302,7 @@ void percpu_ref_switch_to_atomic(struct percpu_ref *ref,

spin_lock_irqsave(&percpu_ref_switch_lock, flags);

ref->force_atomic = true;
ref->data->force_atomic = true;
__percpu_ref_switch_mode(ref, confirm_switch);

spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
Expand All @@ -284,7 +320,7 @@ EXPORT_SYMBOL_GPL(percpu_ref_switch_to_atomic);
void percpu_ref_switch_to_atomic_sync(struct percpu_ref *ref)
{
percpu_ref_switch_to_atomic(ref, NULL);
wait_event(percpu_ref_switch_waitq, !ref->confirm_switch);
wait_event(percpu_ref_switch_waitq, !ref->data->confirm_switch);
}
EXPORT_SYMBOL_GPL(percpu_ref_switch_to_atomic_sync);

Expand Down Expand Up @@ -312,7 +348,7 @@ void percpu_ref_switch_to_percpu(struct percpu_ref *ref)

spin_lock_irqsave(&percpu_ref_switch_lock, flags);

ref->force_atomic = false;
ref->data->force_atomic = false;
__percpu_ref_switch_mode(ref, NULL);

spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
Expand Down Expand Up @@ -344,7 +380,8 @@ void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
spin_lock_irqsave(&percpu_ref_switch_lock, flags);

WARN_ONCE(ref->percpu_count_ptr & __PERCPU_REF_DEAD,
"%s called more than once on %ps!", __func__, ref->release);
"%s called more than once on %ps!", __func__,
ref->data->release);

ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
__percpu_ref_switch_mode(ref, confirm_kill);
Expand All @@ -354,6 +391,34 @@ void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
}
EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm);

/**
* percpu_ref_is_zero - test whether a percpu refcount reached zero
* @ref: percpu_ref to test
*
* Returns %true if @ref reached zero.
*
* This function is safe to call as long as @ref is between init and exit.
*/
bool percpu_ref_is_zero(struct percpu_ref *ref)
{
unsigned long __percpu *percpu_count;
unsigned long count, flags;

if (__ref_is_percpu(ref, &percpu_count))
return false;

/* protect us from being destroyed */
spin_lock_irqsave(&percpu_ref_switch_lock, flags);
if (ref->data)
count = atomic_long_read(&ref->data->count);
else
count = ref->percpu_count_ptr >> __PERCPU_REF_FLAG_BITS;
spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);

return count == 0;
}
EXPORT_SYMBOL_GPL(percpu_ref_is_zero);

/**
* percpu_ref_reinit - re-initialize a percpu refcount
* @ref: perpcu_ref to re-initialize
Expand Down

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