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locking/percpu-rwsem: Optimize readers and reduce global impact

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Currently the percpu-rwsem switches to (global) atomic ops while a
writer is waiting; which could be quite a while and slows down
releasing the readers.

This patch cures this problem by ordering the reader-state vs
reader-count (see the comments in __percpu_down_read() and
percpu_down_write()). This changes a global atomic op into a full
memory barrier, which doesn't have the global cacheline contention.

This also enables using the percpu-rwsem with rcu_sync disabled in order
to bias the implementation differently, reducing the writer latency by
adding some cost to readers.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
[ Fixed modular build. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra 2016-07-14 20:08:46 +02:00 committed by Ingo Molnar
parent 08be8f63c4
commit 80127a3968
3 changed files with 226 additions and 124 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

84
include/linux/percpu-rwsem.h
View file

@ -10,30 +10,96 @@
struct percpu_rw_semaphore {
struct rcu_sync rss;
unsigned int __percpu *fast_read_ctr;
unsigned int __percpu *read_count;
struct rw_semaphore rw_sem;
atomic_t slow_read_ctr;
wait_queue_head_t write_waitq;
wait_queue_head_t writer;
int readers_block;
};
extern void percpu_down_read(struct percpu_rw_semaphore *);
extern int percpu_down_read_trylock(struct percpu_rw_semaphore *);
extern void percpu_up_read(struct percpu_rw_semaphore *);
extern int __percpu_down_read(struct percpu_rw_semaphore *, int);
extern void __percpu_up_read(struct percpu_rw_semaphore *);
static inline void percpu_down_read(struct percpu_rw_semaphore *sem)
{
might_sleep();
rwsem_acquire_read(&sem->rw_sem.dep_map, 0, 0, _RET_IP_);
preempt_disable();
/*
* We are in an RCU-sched read-side critical section, so the writer
* cannot both change sem->state from readers_fast and start checking
* counters while we are here. So if we see !sem->state, we know that
* the writer won't be checking until we're past the preempt_enable()
* and that one the synchronize_sched() is done, the writer will see
* anything we did within this RCU-sched read-size critical section.
*/
__this_cpu_inc(*sem->read_count);
if (unlikely(!rcu_sync_is_idle(&sem->rss)))
__percpu_down_read(sem, false); /* Unconditional memory barrier */
preempt_enable();
/*
* The barrier() from preempt_enable() prevents the compiler from
* bleeding the critical section out.
*/
}
static inline int percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
{
int ret = 1;
preempt_disable();
/*
* Same as in percpu_down_read().
*/
__this_cpu_inc(*sem->read_count);
if (unlikely(!rcu_sync_is_idle(&sem->rss)))
ret = __percpu_down_read(sem, true); /* Unconditional memory barrier */
preempt_enable();
/*
* The barrier() from preempt_enable() prevents the compiler from
* bleeding the critical section out.
*/
if (ret)
rwsem_acquire_read(&sem->rw_sem.dep_map, 0, 1, _RET_IP_);
return ret;
}
static inline void percpu_up_read(struct percpu_rw_semaphore *sem)
{
/*
* The barrier() in preempt_disable() prevents the compiler from
* bleeding the critical section out.
*/
preempt_disable();
/*
* Same as in percpu_down_read().
*/
if (likely(rcu_sync_is_idle(&sem->rss)))
__this_cpu_dec(*sem->read_count);
else
__percpu_up_read(sem); /* Unconditional memory barrier */
preempt_enable();
rwsem_release(&sem->rw_sem.dep_map, 1, _RET_IP_);
}
extern void percpu_down_write(struct percpu_rw_semaphore *);
extern void percpu_up_write(struct percpu_rw_semaphore *);
extern int __percpu_init_rwsem(struct percpu_rw_semaphore *,
const char *, struct lock_class_key *);
extern void percpu_free_rwsem(struct percpu_rw_semaphore *);
#define percpu_init_rwsem(brw) \
#define percpu_init_rwsem(sem) \
({ \
static struct lock_class_key rwsem_key; \
__percpu_init_rwsem(brw, #brw, &rwsem_key); \
__percpu_init_rwsem(sem, #sem, &rwsem_key); \
})
#define percpu_rwsem_is_held(sem) lockdep_is_held(&(sem)->rw_sem)
static inline void percpu_rwsem_release(struct percpu_rw_semaphore *sem,

264
kernel/locking/percpu-rwsem.c
View file

@ -8,152 +8,186 @@
#include <linux/sched.h>
#include <linux/errno.h>
int __percpu_init_rwsem(struct percpu_rw_semaphore *brw,
int __percpu_init_rwsem(struct percpu_rw_semaphore *sem,
const char *name, struct lock_class_key *rwsem_key)
{
brw->fast_read_ctr = alloc_percpu(int);
if (unlikely(!brw->fast_read_ctr))
sem->read_count = alloc_percpu(int);
if (unlikely(!sem->read_count))
return -ENOMEM;
/* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */
__init_rwsem(&brw->rw_sem, name, rwsem_key);
rcu_sync_init(&brw->rss, RCU_SCHED_SYNC);
atomic_set(&brw->slow_read_ctr, 0);
init_waitqueue_head(&brw->write_waitq);
rcu_sync_init(&sem->rss, RCU_SCHED_SYNC);
__init_rwsem(&sem->rw_sem, name, rwsem_key);
init_waitqueue_head(&sem->writer);
sem->readers_block = 0;
return 0;
}
EXPORT_SYMBOL_GPL(__percpu_init_rwsem);
void percpu_free_rwsem(struct percpu_rw_semaphore *brw)
void percpu_free_rwsem(struct percpu_rw_semaphore *sem)
{
/*
* XXX: temporary kludge. The error path in alloc_super()
* assumes that percpu_free_rwsem() is safe after kzalloc().
*/
if (!brw->fast_read_ctr)
if (!sem->read_count)
return;
rcu_sync_dtor(&brw->rss);
free_percpu(brw->fast_read_ctr);
brw->fast_read_ctr = NULL; /* catch use after free bugs */
rcu_sync_dtor(&sem->rss);
free_percpu(sem->read_count);
sem->read_count = NULL; /* catch use after free bugs */
}
EXPORT_SYMBOL_GPL(percpu_free_rwsem);
/*
* This is the fast-path for down_read/up_read. If it succeeds we rely
* on the barriers provided by rcu_sync_enter/exit; see the comments in
* percpu_down_write() and percpu_up_write().
*
* If this helper fails the callers rely on the normal rw_semaphore and
* atomic_dec_and_test(), so in this case we have the necessary barriers.
*/
static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val)
{
bool success;
preempt_disable();
success = rcu_sync_is_idle(&brw->rss);
if (likely(success))
__this_cpu_add(*brw->fast_read_ctr, val);
preempt_enable();
return success;
}
/*
* Like the normal down_read() this is not recursive, the writer can
* come after the first percpu_down_read() and create the deadlock.
*
* Note: returns with lock_is_held(brw->rw_sem) == T for lockdep,
* percpu_up_read() does rwsem_release(). This pairs with the usage
* of ->rw_sem in percpu_down/up_write().
*/
void percpu_down_read(struct percpu_rw_semaphore *brw)
{
might_sleep();
rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 0, _RET_IP_);
if (likely(update_fast_ctr(brw, +1)))
return;
/* Avoid rwsem_acquire_read() and rwsem_release() */
__down_read(&brw->rw_sem);
atomic_inc(&brw->slow_read_ctr);
__up_read(&brw->rw_sem);
}
EXPORT_SYMBOL_GPL(percpu_down_read);
int percpu_down_read_trylock(struct percpu_rw_semaphore *brw)
{
if (unlikely(!update_fast_ctr(brw, +1))) {
if (!__down_read_trylock(&brw->rw_sem))
return 0;
atomic_inc(&brw->slow_read_ctr);
__up_read(&brw->rw_sem);
}
rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 1, _RET_IP_);
return 1;
}
void percpu_up_read(struct percpu_rw_semaphore *brw)
{
rwsem_release(&brw->rw_sem.dep_map, 1, _RET_IP_);
if (likely(update_fast_ctr(brw, -1)))
return;
/* false-positive is possible but harmless */
if (atomic_dec_and_test(&brw->slow_read_ctr))
wake_up_all(&brw->write_waitq);
}
EXPORT_SYMBOL_GPL(percpu_up_read);
static int clear_fast_ctr(struct percpu_rw_semaphore *brw)
{
unsigned int sum = 0;
int cpu;
for_each_possible_cpu(cpu) {
sum += per_cpu(*brw->fast_read_ctr, cpu);
per_cpu(*brw->fast_read_ctr, cpu) = 0;
}
return sum;
}
void percpu_down_write(struct percpu_rw_semaphore *brw)
int __percpu_down_read(struct percpu_rw_semaphore *sem, int try)
{
/*
* Make rcu_sync_is_idle() == F and thus disable the fast-path in
* percpu_down_read() and percpu_up_read(), and wait for gp pass.
* Due to having preemption disabled the decrement happens on
* the same CPU as the increment, avoiding the
* increment-on-one-CPU-and-decrement-on-another problem.
*
* The latter synchronises us with the preceding readers which used
* the fast-past, so we can not miss the result of __this_cpu_add()
* or anything else inside their criticial sections.
* If the reader misses the writer's assignment of readers_block, then
* the writer is guaranteed to see the reader's increment.
*
* Conversely, any readers that increment their sem->read_count after
* the writer looks are guaranteed to see the readers_block value,
* which in turn means that they are guaranteed to immediately
* decrement their sem->read_count, so that it doesn't matter that the
* writer missed them.
*/
rcu_sync_enter(&brw->rss);
/* exclude other writers, and block the new readers completely */
down_write(&brw->rw_sem);
smp_mb(); /* A matches D */
/* nobody can use fast_read_ctr, move its sum into slow_read_ctr */
atomic_add(clear_fast_ctr(brw), &brw->slow_read_ctr);
/*
* If !readers_block the critical section starts here, matched by the
* release in percpu_up_write().
*/
if (likely(!smp_load_acquire(&sem->readers_block)))
return 1;
/* wait for all readers to complete their percpu_up_read() */
wait_event(brw->write_waitq, !atomic_read(&brw->slow_read_ctr));
/*
* Per the above comment; we still have preemption disabled and
* will thus decrement on the same CPU as we incremented.
*/
__percpu_up_read(sem);
if (try)
return 0;
/*
* We either call schedule() in the wait, or we'll fall through
* and reschedule on the preempt_enable() in percpu_down_read().
*/
preempt_enable_no_resched();
/*
* Avoid lockdep for the down/up_read() we already have them.
*/
__down_read(&sem->rw_sem);
this_cpu_inc(*sem->read_count);
__up_read(&sem->rw_sem);
preempt_disable();
return 1;
}
EXPORT_SYMBOL_GPL(__percpu_down_read);
void __percpu_up_read(struct percpu_rw_semaphore *sem)
{
smp_mb(); /* B matches C */
/*
* In other words, if they see our decrement (presumably to aggregate
* zero, as that is the only time it matters) they will also see our
* critical section.
*/
__this_cpu_dec(*sem->read_count);
/* Prod writer to recheck readers_active */
wake_up(&sem->writer);
}
EXPORT_SYMBOL_GPL(__percpu_up_read);
#define per_cpu_sum(var) \
({ \
typeof(var) __sum = 0; \
int cpu; \
compiletime_assert_atomic_type(__sum); \
for_each_possible_cpu(cpu) \
__sum += per_cpu(var, cpu); \
__sum; \
})
/*
* Return true if the modular sum of the sem->read_count per-CPU variable is
* zero. If this sum is zero, then it is stable due to the fact that if any
* newly arriving readers increment a given counter, they will immediately
* decrement that same counter.
*/
static bool readers_active_check(struct percpu_rw_semaphore *sem)
{
if (per_cpu_sum(*sem->read_count) != 0)
return false;
/*
* If we observed the decrement; ensure we see the entire critical
* section.
*/
smp_mb(); /* C matches B */
return true;
}
void percpu_down_write(struct percpu_rw_semaphore *sem)
{
/* Notify readers to take the slow path. */
rcu_sync_enter(&sem->rss);
down_write(&sem->rw_sem);
/*
* Notify new readers to block; up until now, and thus throughout the
* longish rcu_sync_enter() above, new readers could still come in.
*/
WRITE_ONCE(sem->readers_block, 1);
smp_mb(); /* D matches A */
/*
* If they don't see our writer of readers_block, then we are
* guaranteed to see their sem->read_count increment, and therefore
* will wait for them.
*/
/* Wait for all now active readers to complete. */
wait_event(sem->writer, readers_active_check(sem));
}
EXPORT_SYMBOL_GPL(percpu_down_write);
void percpu_up_write(struct percpu_rw_semaphore *brw)
void percpu_up_write(struct percpu_rw_semaphore *sem)
{
/* release the lock, but the readers can't use the fast-path */
up_write(&brw->rw_sem);
/*
* Enable the fast-path in percpu_down_read() and percpu_up_read()
* but only after another gp pass; this adds the necessary barrier
* to ensure the reader can't miss the changes done by us.
* Signal the writer is done, no fast path yet.
*
* One reason that we cannot just immediately flip to readers_fast is
* that new readers might fail to see the results of this writer's
* critical section.
*
* Therefore we force it through the slow path which guarantees an
* acquire and thereby guarantees the critical section's consistency.
*/
rcu_sync_exit(&brw->rss);
smp_store_release(&sem->readers_block, 0);
/*
* Release the write lock, this will allow readers back in the game.
*/
up_write(&sem->rw_sem);
/*
* Once this completes (at least one RCU-sched grace period hence) the
* reader fast path will be available again. Safe to use outside the
* exclusive write lock because its counting.
*/
rcu_sync_exit(&sem->rss);
}
EXPORT_SYMBOL_GPL(percpu_up_write);

2
kernel/rcu/sync.c
View file

@ -68,6 +68,8 @@ void rcu_sync_lockdep_assert(struct rcu_sync *rsp)
RCU_LOCKDEP_WARN(!gp_ops[rsp->gp_type].held(),
"suspicious rcu_sync_is_idle() usage");
}
EXPORT_SYMBOL_GPL(rcu_sync_lockdep_assert);
#endif
/**