linux/kernel/locking/qspinlock.c
Waiman Long cd0272fab7 locking/pvqspinlock: Queue node adaptive spinning
In an overcommitted guest where some vCPUs have to be halted to make
forward progress in other areas, it is highly likely that a vCPU later
in the spinlock queue will be spinning while the ones earlier in the
queue would have been halted. The spinning in the later vCPUs is then
just a waste of precious CPU cycles because they are not going to
get the lock soon as the earlier ones have to be woken up and take
their turn to get the lock.

This patch implements an adaptive spinning mechanism where the vCPU
will call pv_wait() if the previous vCPU is not running.

Linux kernel builds were run in KVM guest on an 8-socket, 4
cores/socket Westmere-EX system and a 4-socket, 8 cores/socket
Haswell-EX system. Both systems are configured to have 32 physical
CPUs. The kernel build times before and after the patch were:

		    Westmere			Haswell
  Patch		32 vCPUs    48 vCPUs	32 vCPUs    48 vCPUs
  -----		--------    --------    --------    --------
  Before patch   3m02.3s     5m00.2s     1m43.7s     3m03.5s
  After patch    3m03.0s     4m37.5s	 1m43.0s     2m47.2s

For 32 vCPUs, this patch doesn't cause any noticeable change in
performance. For 48 vCPUs (over-committed), there is about 8%
performance improvement.

Signed-off-by: Waiman Long <Waiman.Long@hpe.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Douglas Hatch <doug.hatch@hpe.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Scott J Norton <scott.norton@hpe.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1447114167-47185-8-git-send-email-Waiman.Long@hpe.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
2015-12-04 11:39:51 +01:00

520 lines
14 KiB
C

/*
* Queued spinlock
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* (C) Copyright 2013-2015 Hewlett-Packard Development Company, L.P.
* (C) Copyright 2013-2014 Red Hat, Inc.
* (C) Copyright 2015 Intel Corp.
* (C) Copyright 2015 Hewlett-Packard Enterprise Development LP
*
* Authors: Waiman Long <waiman.long@hpe.com>
* Peter Zijlstra <peterz@infradead.org>
*/
#ifndef _GEN_PV_LOCK_SLOWPATH
#include <linux/smp.h>
#include <linux/bug.h>
#include <linux/cpumask.h>
#include <linux/percpu.h>
#include <linux/hardirq.h>
#include <linux/mutex.h>
#include <asm/byteorder.h>
#include <asm/qspinlock.h>
/*
* The basic principle of a queue-based spinlock can best be understood
* by studying a classic queue-based spinlock implementation called the
* MCS lock. The paper below provides a good description for this kind
* of lock.
*
* http://www.cise.ufl.edu/tr/DOC/REP-1992-71.pdf
*
* This queued spinlock implementation is based on the MCS lock, however to make
* it fit the 4 bytes we assume spinlock_t to be, and preserve its existing
* API, we must modify it somehow.
*
* In particular; where the traditional MCS lock consists of a tail pointer
* (8 bytes) and needs the next pointer (another 8 bytes) of its own node to
* unlock the next pending (next->locked), we compress both these: {tail,
* next->locked} into a single u32 value.
*
* Since a spinlock disables recursion of its own context and there is a limit
* to the contexts that can nest; namely: task, softirq, hardirq, nmi. As there
* are at most 4 nesting levels, it can be encoded by a 2-bit number. Now
* we can encode the tail by combining the 2-bit nesting level with the cpu
* number. With one byte for the lock value and 3 bytes for the tail, only a
* 32-bit word is now needed. Even though we only need 1 bit for the lock,
* we extend it to a full byte to achieve better performance for architectures
* that support atomic byte write.
*
* We also change the first spinner to spin on the lock bit instead of its
* node; whereby avoiding the need to carry a node from lock to unlock, and
* preserving existing lock API. This also makes the unlock code simpler and
* faster.
*
* N.B. The current implementation only supports architectures that allow
* atomic operations on smaller 8-bit and 16-bit data types.
*
*/
#include "mcs_spinlock.h"
#ifdef CONFIG_PARAVIRT_SPINLOCKS
#define MAX_NODES 8
#else
#define MAX_NODES 4
#endif
/*
* Per-CPU queue node structures; we can never have more than 4 nested
* contexts: task, softirq, hardirq, nmi.
*
* Exactly fits one 64-byte cacheline on a 64-bit architecture.
*
* PV doubles the storage and uses the second cacheline for PV state.
*/
static DEFINE_PER_CPU_ALIGNED(struct mcs_spinlock, mcs_nodes[MAX_NODES]);
/*
* We must be able to distinguish between no-tail and the tail at 0:0,
* therefore increment the cpu number by one.
*/
static inline u32 encode_tail(int cpu, int idx)
{
u32 tail;
#ifdef CONFIG_DEBUG_SPINLOCK
BUG_ON(idx > 3);
#endif
tail = (cpu + 1) << _Q_TAIL_CPU_OFFSET;
tail |= idx << _Q_TAIL_IDX_OFFSET; /* assume < 4 */
return tail;
}
static inline struct mcs_spinlock *decode_tail(u32 tail)
{
int cpu = (tail >> _Q_TAIL_CPU_OFFSET) - 1;
int idx = (tail & _Q_TAIL_IDX_MASK) >> _Q_TAIL_IDX_OFFSET;
return per_cpu_ptr(&mcs_nodes[idx], cpu);
}
#define _Q_LOCKED_PENDING_MASK (_Q_LOCKED_MASK | _Q_PENDING_MASK)
/*
* By using the whole 2nd least significant byte for the pending bit, we
* can allow better optimization of the lock acquisition for the pending
* bit holder.
*
* This internal structure is also used by the set_locked function which
* is not restricted to _Q_PENDING_BITS == 8.
*/
struct __qspinlock {
union {
atomic_t val;
#ifdef __LITTLE_ENDIAN
struct {
u8 locked;
u8 pending;
};
struct {
u16 locked_pending;
u16 tail;
};
#else
struct {
u16 tail;
u16 locked_pending;
};
struct {
u8 reserved[2];
u8 pending;
u8 locked;
};
#endif
};
};
#if _Q_PENDING_BITS == 8
/**
* clear_pending_set_locked - take ownership and clear the pending bit.
* @lock: Pointer to queued spinlock structure
*
* *,1,0 -> *,0,1
*
* Lock stealing is not allowed if this function is used.
*/
static __always_inline void clear_pending_set_locked(struct qspinlock *lock)
{
struct __qspinlock *l = (void *)lock;
WRITE_ONCE(l->locked_pending, _Q_LOCKED_VAL);
}
/*
* xchg_tail - Put in the new queue tail code word & retrieve previous one
* @lock : Pointer to queued spinlock structure
* @tail : The new queue tail code word
* Return: The previous queue tail code word
*
* xchg(lock, tail)
*
* p,*,* -> n,*,* ; prev = xchg(lock, node)
*/
static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
{
struct __qspinlock *l = (void *)lock;
/*
* Use release semantics to make sure that the MCS node is properly
* initialized before changing the tail code.
*/
return (u32)xchg_release(&l->tail,
tail >> _Q_TAIL_OFFSET) << _Q_TAIL_OFFSET;
}
#else /* _Q_PENDING_BITS == 8 */
/**
* clear_pending_set_locked - take ownership and clear the pending bit.
* @lock: Pointer to queued spinlock structure
*
* *,1,0 -> *,0,1
*/
static __always_inline void clear_pending_set_locked(struct qspinlock *lock)
{
atomic_add(-_Q_PENDING_VAL + _Q_LOCKED_VAL, &lock->val);
}
/**
* xchg_tail - Put in the new queue tail code word & retrieve previous one
* @lock : Pointer to queued spinlock structure
* @tail : The new queue tail code word
* Return: The previous queue tail code word
*
* xchg(lock, tail)
*
* p,*,* -> n,*,* ; prev = xchg(lock, node)
*/
static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail)
{
u32 old, new, val = atomic_read(&lock->val);
for (;;) {
new = (val & _Q_LOCKED_PENDING_MASK) | tail;
/*
* Use release semantics to make sure that the MCS node is
* properly initialized before changing the tail code.
*/
old = atomic_cmpxchg_release(&lock->val, val, new);
if (old == val)
break;
val = old;
}
return old;
}
#endif /* _Q_PENDING_BITS == 8 */
/**
* set_locked - Set the lock bit and own the lock
* @lock: Pointer to queued spinlock structure
*
* *,*,0 -> *,0,1
*/
static __always_inline void set_locked(struct qspinlock *lock)
{
struct __qspinlock *l = (void *)lock;
WRITE_ONCE(l->locked, _Q_LOCKED_VAL);
}
/*
* Generate the native code for queued_spin_unlock_slowpath(); provide NOPs for
* all the PV callbacks.
*/
static __always_inline void __pv_init_node(struct mcs_spinlock *node) { }
static __always_inline void __pv_wait_node(struct mcs_spinlock *node,
struct mcs_spinlock *prev) { }
static __always_inline void __pv_kick_node(struct qspinlock *lock,
struct mcs_spinlock *node) { }
static __always_inline u32 __pv_wait_head_or_lock(struct qspinlock *lock,
struct mcs_spinlock *node)
{ return 0; }
#define pv_enabled() false
#define pv_init_node __pv_init_node
#define pv_wait_node __pv_wait_node
#define pv_kick_node __pv_kick_node
#define pv_wait_head_or_lock __pv_wait_head_or_lock
#ifdef CONFIG_PARAVIRT_SPINLOCKS
#define queued_spin_lock_slowpath native_queued_spin_lock_slowpath
#endif
#endif /* _GEN_PV_LOCK_SLOWPATH */
/**
* queued_spin_lock_slowpath - acquire the queued spinlock
* @lock: Pointer to queued spinlock structure
* @val: Current value of the queued spinlock 32-bit word
*
* (queue tail, pending bit, lock value)
*
* fast : slow : unlock
* : :
* uncontended (0,0,0) -:--> (0,0,1) ------------------------------:--> (*,*,0)
* : | ^--------.------. / :
* : v \ \ | :
* pending : (0,1,1) +--> (0,1,0) \ | :
* : | ^--' | | :
* : v | | :
* uncontended : (n,x,y) +--> (n,0,0) --' | :
* queue : | ^--' | :
* : v | :
* contended : (*,x,y) +--> (*,0,0) ---> (*,0,1) -' :
* queue : ^--' :
*/
void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
{
struct mcs_spinlock *prev, *next, *node;
u32 new, old, tail;
int idx;
BUILD_BUG_ON(CONFIG_NR_CPUS >= (1U << _Q_TAIL_CPU_BITS));
if (pv_enabled())
goto queue;
if (virt_spin_lock(lock))
return;
/*
* wait for in-progress pending->locked hand-overs
*
* 0,1,0 -> 0,0,1
*/
if (val == _Q_PENDING_VAL) {
while ((val = atomic_read(&lock->val)) == _Q_PENDING_VAL)
cpu_relax();
}
/*
* trylock || pending
*
* 0,0,0 -> 0,0,1 ; trylock
* 0,0,1 -> 0,1,1 ; pending
*/
for (;;) {
/*
* If we observe any contention; queue.
*/
if (val & ~_Q_LOCKED_MASK)
goto queue;
new = _Q_LOCKED_VAL;
if (val == new)
new |= _Q_PENDING_VAL;
/*
* Acquire semantic is required here as the function may
* return immediately if the lock was free.
*/
old = atomic_cmpxchg_acquire(&lock->val, val, new);
if (old == val)
break;
val = old;
}
/*
* we won the trylock
*/
if (new == _Q_LOCKED_VAL)
return;
/*
* we're pending, wait for the owner to go away.
*
* *,1,1 -> *,1,0
*
* this wait loop must be a load-acquire such that we match the
* store-release that clears the locked bit and create lock
* sequentiality; this is because not all clear_pending_set_locked()
* implementations imply full barriers.
*/
while ((val = smp_load_acquire(&lock->val.counter)) & _Q_LOCKED_MASK)
cpu_relax();
/*
* take ownership and clear the pending bit.
*
* *,1,0 -> *,0,1
*/
clear_pending_set_locked(lock);
return;
/*
* End of pending bit optimistic spinning and beginning of MCS
* queuing.
*/
queue:
node = this_cpu_ptr(&mcs_nodes[0]);
idx = node->count++;
tail = encode_tail(smp_processor_id(), idx);
node += idx;
node->locked = 0;
node->next = NULL;
pv_init_node(node);
/*
* We touched a (possibly) cold cacheline in the per-cpu queue node;
* attempt the trylock once more in the hope someone let go while we
* weren't watching.
*/
if (queued_spin_trylock(lock))
goto release;
/*
* We have already touched the queueing cacheline; don't bother with
* pending stuff.
*
* p,*,* -> n,*,*
*/
old = xchg_tail(lock, tail);
next = NULL;
/*
* if there was a previous node; link it and wait until reaching the
* head of the waitqueue.
*/
if (old & _Q_TAIL_MASK) {
prev = decode_tail(old);
WRITE_ONCE(prev->next, node);
pv_wait_node(node, prev);
arch_mcs_spin_lock_contended(&node->locked);
/*
* While waiting for the MCS lock, the next pointer may have
* been set by another lock waiter. We optimistically load
* the next pointer & prefetch the cacheline for writing
* to reduce latency in the upcoming MCS unlock operation.
*/
next = READ_ONCE(node->next);
if (next)
prefetchw(next);
}
/*
* we're at the head of the waitqueue, wait for the owner & pending to
* go away.
*
* *,x,y -> *,0,0
*
* this wait loop must use a load-acquire such that we match the
* store-release that clears the locked bit and create lock
* sequentiality; this is because the set_locked() function below
* does not imply a full barrier.
*
* The PV pv_wait_head_or_lock function, if active, will acquire
* the lock and return a non-zero value. So we have to skip the
* smp_load_acquire() call. As the next PV queue head hasn't been
* designated yet, there is no way for the locked value to become
* _Q_SLOW_VAL. So both the set_locked() and the
* atomic_cmpxchg_relaxed() calls will be safe.
*
* If PV isn't active, 0 will be returned instead.
*
*/
if ((val = pv_wait_head_or_lock(lock, node)))
goto locked;
smp_cond_acquire(!((val = atomic_read(&lock->val)) & _Q_LOCKED_PENDING_MASK));
locked:
/*
* claim the lock:
*
* n,0,0 -> 0,0,1 : lock, uncontended
* *,0,0 -> *,0,1 : lock, contended
*
* If the queue head is the only one in the queue (lock value == tail),
* clear the tail code and grab the lock. Otherwise, we only need
* to grab the lock.
*/
for (;;) {
/* In the PV case we might already have _Q_LOCKED_VAL set */
if ((val & _Q_TAIL_MASK) != tail) {
set_locked(lock);
break;
}
/*
* The smp_load_acquire() call above has provided the necessary
* acquire semantics required for locking. At most two
* iterations of this loop may be ran.
*/
old = atomic_cmpxchg_relaxed(&lock->val, val, _Q_LOCKED_VAL);
if (old == val)
goto release; /* No contention */
val = old;
}
/*
* contended path; wait for next if not observed yet, release.
*/
if (!next) {
while (!(next = READ_ONCE(node->next)))
cpu_relax();
}
arch_mcs_spin_unlock_contended(&next->locked);
pv_kick_node(lock, next);
release:
/*
* release the node
*/
this_cpu_dec(mcs_nodes[0].count);
}
EXPORT_SYMBOL(queued_spin_lock_slowpath);
/*
* Generate the paravirt code for queued_spin_unlock_slowpath().
*/
#if !defined(_GEN_PV_LOCK_SLOWPATH) && defined(CONFIG_PARAVIRT_SPINLOCKS)
#define _GEN_PV_LOCK_SLOWPATH
#undef pv_enabled
#define pv_enabled() true
#undef pv_init_node
#undef pv_wait_node
#undef pv_kick_node
#undef pv_wait_head_or_lock
#undef queued_spin_lock_slowpath
#define queued_spin_lock_slowpath __pv_queued_spin_lock_slowpath
#include "qspinlock_paravirt.h"
#include "qspinlock.c"
#endif