mirror of
https://github.com/FEX-Emu/linux.git
synced 2024-12-19 15:39:07 +00:00
a4244454df
percpu-refcount was incorrectly using preempt_disable/enable() for RCU
critical sections against call_rcu(). 6a24474da8
("percpu-refcount:
consistently use plain (non-sched) RCU") fixed it by converting the
preepmtion operations with rcu_read_[un]lock() citing that there isn't
any advantage in using sched-RCU over using the usual one; however,
rcu_read_[un]lock() for the preemptible RCU implementation -
CONFIG_TREE_PREEMPT_RCU, chosen when CONFIG_PREEMPT - are slightly
more expensive than preempt_disable/enable().
In a contrived microbench which repeats the followings,
- percpu_ref_get()
- copy 32 bytes of data into percpu buffer
- percpu_put_get()
- copy 32 bytes of data into percpu buffer
rcu_read_[un]lock() used in percpu_ref_get/put() makes it go slower by
about 15% when compared to using sched-RCU.
As the RCU critical sections are extremely short, using sched-RCU
shouldn't have any latency implications. Convert to RCU-sched.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Kent Overstreet <koverstreet@google.com>
Acked-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Rusty Russell <rusty@rustcorp.com.au>
159 lines
5.5 KiB
C
159 lines
5.5 KiB
C
#define pr_fmt(fmt) "%s: " fmt "\n", __func__
|
|
|
|
#include <linux/kernel.h>
|
|
#include <linux/percpu-refcount.h>
|
|
|
|
/*
|
|
* Initially, a percpu refcount is just a set of percpu counters. Initially, we
|
|
* don't try to detect the ref hitting 0 - which means that get/put can just
|
|
* increment or decrement the local counter. Note that the counter on a
|
|
* particular cpu can (and will) wrap - this is fine, when we go to shutdown the
|
|
* percpu counters will all sum to the correct value
|
|
*
|
|
* (More precisely: because moduler arithmatic is commutative the sum of all the
|
|
* pcpu_count vars will be equal to what it would have been if all the gets and
|
|
* puts were done to a single integer, even if some of the percpu integers
|
|
* overflow or underflow).
|
|
*
|
|
* The real trick to implementing percpu refcounts is shutdown. We can't detect
|
|
* the ref hitting 0 on every put - this would require global synchronization
|
|
* and defeat the whole purpose of using percpu refs.
|
|
*
|
|
* What we do is require the user to keep track of the initial refcount; we know
|
|
* the ref can't hit 0 before the user drops the initial ref, so as long as we
|
|
* convert to non percpu mode before the initial ref is dropped everything
|
|
* works.
|
|
*
|
|
* Converting to non percpu mode is done with some RCUish stuff in
|
|
* percpu_ref_kill. Additionally, we need a bias value so that the atomic_t
|
|
* can't hit 0 before we've added up all the percpu refs.
|
|
*/
|
|
|
|
#define PCPU_COUNT_BIAS (1U << 31)
|
|
|
|
/**
|
|
* percpu_ref_init - initialize a percpu refcount
|
|
* @ref: percpu_ref to initialize
|
|
* @release: function which will be called when refcount hits 0
|
|
*
|
|
* Initializes the refcount in single atomic counter mode with a refcount of 1;
|
|
* analagous to atomic_set(ref, 1).
|
|
*
|
|
* Note that @release must not sleep - it may potentially be called from RCU
|
|
* callback context by percpu_ref_kill().
|
|
*/
|
|
int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release)
|
|
{
|
|
atomic_set(&ref->count, 1 + PCPU_COUNT_BIAS);
|
|
|
|
ref->pcpu_count = alloc_percpu(unsigned);
|
|
if (!ref->pcpu_count)
|
|
return -ENOMEM;
|
|
|
|
ref->release = release;
|
|
return 0;
|
|
}
|
|
|
|
/**
|
|
* percpu_ref_cancel_init - cancel percpu_ref_init()
|
|
* @ref: percpu_ref to cancel init for
|
|
*
|
|
* Once a percpu_ref is initialized, its destruction is initiated by
|
|
* percpu_ref_kill() and completes asynchronously, which can be painful to
|
|
* do when destroying a half-constructed object in init failure path.
|
|
*
|
|
* This function destroys @ref without invoking @ref->release and the
|
|
* memory area containing it can be freed immediately on return. To
|
|
* prevent accidental misuse, it's required that @ref has finished
|
|
* percpu_ref_init(), whether successful or not, but never used.
|
|
*
|
|
* The weird name and usage restriction are to prevent people from using
|
|
* this function by mistake for normal shutdown instead of
|
|
* percpu_ref_kill().
|
|
*/
|
|
void percpu_ref_cancel_init(struct percpu_ref *ref)
|
|
{
|
|
unsigned __percpu *pcpu_count = ref->pcpu_count;
|
|
int cpu;
|
|
|
|
WARN_ON_ONCE(atomic_read(&ref->count) != 1 + PCPU_COUNT_BIAS);
|
|
|
|
if (pcpu_count) {
|
|
for_each_possible_cpu(cpu)
|
|
WARN_ON_ONCE(*per_cpu_ptr(pcpu_count, cpu));
|
|
free_percpu(ref->pcpu_count);
|
|
}
|
|
}
|
|
|
|
static void percpu_ref_kill_rcu(struct rcu_head *rcu)
|
|
{
|
|
struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);
|
|
unsigned __percpu *pcpu_count = ref->pcpu_count;
|
|
unsigned count = 0;
|
|
int cpu;
|
|
|
|
/* Mask out PCPU_REF_DEAD */
|
|
pcpu_count = (unsigned __percpu *)
|
|
(((unsigned long) pcpu_count) & ~PCPU_STATUS_MASK);
|
|
|
|
for_each_possible_cpu(cpu)
|
|
count += *per_cpu_ptr(pcpu_count, cpu);
|
|
|
|
free_percpu(pcpu_count);
|
|
|
|
pr_debug("global %i pcpu %i", atomic_read(&ref->count), (int) count);
|
|
|
|
/*
|
|
* It's crucial that we sum the percpu counters _before_ adding the sum
|
|
* to &ref->count; since gets could be happening on one cpu while puts
|
|
* happen on another, adding a single cpu's count could cause
|
|
* @ref->count to hit 0 before we've got a consistent value - but the
|
|
* sum of all the counts will be consistent and correct.
|
|
*
|
|
* Subtracting the bias value then has to happen _after_ adding count to
|
|
* &ref->count; we need the bias value to prevent &ref->count from
|
|
* reaching 0 before we add the percpu counts. But doing it at the same
|
|
* time is equivalent and saves us atomic operations:
|
|
*/
|
|
|
|
atomic_add((int) count - PCPU_COUNT_BIAS, &ref->count);
|
|
|
|
/* @ref is viewed as dead on all CPUs, send out kill confirmation */
|
|
if (ref->confirm_kill)
|
|
ref->confirm_kill(ref);
|
|
|
|
/*
|
|
* Now we're in single atomic_t mode with a consistent refcount, so it's
|
|
* safe to drop our initial ref:
|
|
*/
|
|
percpu_ref_put(ref);
|
|
}
|
|
|
|
/**
|
|
* percpu_ref_kill_and_confirm - drop the initial ref and schedule confirmation
|
|
* @ref: percpu_ref to kill
|
|
* @confirm_kill: optional confirmation callback
|
|
*
|
|
* Equivalent to percpu_ref_kill() but also schedules kill confirmation if
|
|
* @confirm_kill is not NULL. @confirm_kill, which may not block, will be
|
|
* called after @ref is seen as dead from all CPUs - all further
|
|
* invocations of percpu_ref_tryget() will fail. See percpu_ref_tryget()
|
|
* for more details.
|
|
*
|
|
* Due to the way percpu_ref is implemented, @confirm_kill will be called
|
|
* after at least one full RCU grace period has passed but this is an
|
|
* implementation detail and callers must not depend on it.
|
|
*/
|
|
void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
|
|
percpu_ref_func_t *confirm_kill)
|
|
{
|
|
WARN_ONCE(REF_STATUS(ref->pcpu_count) == PCPU_REF_DEAD,
|
|
"percpu_ref_kill() called more than once!\n");
|
|
|
|
ref->pcpu_count = (unsigned __percpu *)
|
|
(((unsigned long) ref->pcpu_count)|PCPU_REF_DEAD);
|
|
ref->confirm_kill = confirm_kill;
|
|
|
|
call_rcu_sched(&ref->rcu, percpu_ref_kill_rcu);
|
|
}
|