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Message-ID: <562E376E.40904@solarflare.com>
Date: Mon, 26 Oct 2015 14:23:42 +0000
From: Shradha Shah <sshah@...arflare.com>
To: David Miller <davem@...emloft.net>
CC: <netdev@...r.kernel.org>, <linux-net-drivers@...arflare.com>
Subject: [PATCH net-next v2 1/1] sfc: replace spinlocks with bit ops for busy
poll locking
From: Bert Kenward <bkenward@...arflare.com>
This patch reduces the overhead of locking for busy poll.
Previously the state was protected by a lock, whereas now
it's manipulated solely with atomic operations.
Signed-off-by: Shradha Shah <sshah@...arflare.com>
---
drivers/net/ethernet/sfc/efx.c | 4 +-
drivers/net/ethernet/sfc/net_driver.h | 129 +++++++++++++++-------------------
2 files changed, 58 insertions(+), 75 deletions(-)
diff --git a/drivers/net/ethernet/sfc/efx.c b/drivers/net/ethernet/sfc/efx.c
index 974637d..6e11ee6 100644
--- a/drivers/net/ethernet/sfc/efx.c
+++ b/drivers/net/ethernet/sfc/efx.c
@@ -2062,7 +2062,7 @@ static void efx_init_napi_channel(struct efx_channel *channel)
netif_napi_add(channel->napi_dev, &channel->napi_str,
efx_poll, napi_weight);
napi_hash_add(&channel->napi_str);
- efx_channel_init_lock(channel);
+ efx_channel_busy_poll_init(channel);
}
static void efx_init_napi(struct efx_nic *efx)
@@ -2125,7 +2125,7 @@ static int efx_busy_poll(struct napi_struct *napi)
if (!netif_running(efx->net_dev))
return LL_FLUSH_FAILED;
- if (!efx_channel_lock_poll(channel))
+ if (!efx_channel_try_lock_poll(channel))
return LL_FLUSH_BUSY;
old_rx_packets = channel->rx_queue.rx_packets;
diff --git a/drivers/net/ethernet/sfc/net_driver.h b/drivers/net/ethernet/sfc/net_driver.h
index ad56231..229e68c 100644
--- a/drivers/net/ethernet/sfc/net_driver.h
+++ b/drivers/net/ethernet/sfc/net_driver.h
@@ -431,21 +431,8 @@ struct efx_channel {
struct net_device *napi_dev;
struct napi_struct napi_str;
#ifdef CONFIG_NET_RX_BUSY_POLL
- unsigned int state;
- spinlock_t state_lock;
-#define EFX_CHANNEL_STATE_IDLE 0
-#define EFX_CHANNEL_STATE_NAPI (1 << 0) /* NAPI owns this channel */
-#define EFX_CHANNEL_STATE_POLL (1 << 1) /* poll owns this channel */
-#define EFX_CHANNEL_STATE_DISABLED (1 << 2) /* channel is disabled */
-#define EFX_CHANNEL_STATE_NAPI_YIELD (1 << 3) /* NAPI yielded this channel */
-#define EFX_CHANNEL_STATE_POLL_YIELD (1 << 4) /* poll yielded this channel */
-#define EFX_CHANNEL_OWNED \
- (EFX_CHANNEL_STATE_NAPI | EFX_CHANNEL_STATE_POLL)
-#define EFX_CHANNEL_LOCKED \
- (EFX_CHANNEL_OWNED | EFX_CHANNEL_STATE_DISABLED)
-#define EFX_CHANNEL_USER_PEND \
- (EFX_CHANNEL_STATE_POLL | EFX_CHANNEL_STATE_POLL_YIELD)
-#endif /* CONFIG_NET_RX_BUSY_POLL */
+ unsigned long busy_poll_state;
+#endif
struct efx_special_buffer eventq;
unsigned int eventq_mask;
unsigned int eventq_read_ptr;
@@ -480,98 +467,94 @@ struct efx_channel {
};
#ifdef CONFIG_NET_RX_BUSY_POLL
-static inline void efx_channel_init_lock(struct efx_channel *channel)
+enum efx_channel_busy_poll_state {
+ EFX_CHANNEL_STATE_IDLE = 0,
+ EFX_CHANNEL_STATE_NAPI = BIT(0),
+ EFX_CHANNEL_STATE_NAPI_REQ_BIT = 1,
+ EFX_CHANNEL_STATE_NAPI_REQ = BIT(1),
+ EFX_CHANNEL_STATE_POLL_BIT = 2,
+ EFX_CHANNEL_STATE_POLL = BIT(2),
+ EFX_CHANNEL_STATE_DISABLE_BIT = 3,
+};
+
+static inline void efx_channel_busy_poll_init(struct efx_channel *channel)
{
- spin_lock_init(&channel->state_lock);
+ WRITE_ONCE(channel->busy_poll_state, EFX_CHANNEL_STATE_IDLE);
}
/* Called from the device poll routine to get ownership of a channel. */
static inline bool efx_channel_lock_napi(struct efx_channel *channel)
{
- bool rc = true;
-
- spin_lock_bh(&channel->state_lock);
- if (channel->state & EFX_CHANNEL_LOCKED) {
- WARN_ON(channel->state & EFX_CHANNEL_STATE_NAPI);
- channel->state |= EFX_CHANNEL_STATE_NAPI_YIELD;
- rc = false;
- } else {
- /* we don't care if someone yielded */
- channel->state = EFX_CHANNEL_STATE_NAPI;
+ unsigned long prev, old = READ_ONCE(channel->busy_poll_state);
+
+ while (1) {
+ switch (old) {
+ case EFX_CHANNEL_STATE_POLL:
+ /* Ensure efx_channel_try_lock_poll() wont starve us */
+ set_bit(EFX_CHANNEL_STATE_NAPI_REQ_BIT,
+ &channel->busy_poll_state);
+ /* fallthrough */
+ case EFX_CHANNEL_STATE_POLL | EFX_CHANNEL_STATE_NAPI_REQ:
+ return false;
+ default:
+ break;
+ }
+ prev = cmpxchg(&channel->busy_poll_state, old,
+ EFX_CHANNEL_STATE_NAPI);
+ if (unlikely(prev != old)) {
+ /* This is likely to mean we've just entered polling
+ * state. Go back round to set the REQ bit.
+ */
+ old = prev;
+ continue;
+ }
+ return true;
}
- spin_unlock_bh(&channel->state_lock);
- return rc;
}
static inline void efx_channel_unlock_napi(struct efx_channel *channel)
{
- spin_lock_bh(&channel->state_lock);
- WARN_ON(channel->state &
- (EFX_CHANNEL_STATE_POLL | EFX_CHANNEL_STATE_NAPI_YIELD));
-
- channel->state &= EFX_CHANNEL_STATE_DISABLED;
- spin_unlock_bh(&channel->state_lock);
+ /* Make sure write has completed from efx_channel_lock_napi() */
+ smp_wmb();
+ WRITE_ONCE(channel->busy_poll_state, EFX_CHANNEL_STATE_IDLE);
}
/* Called from efx_busy_poll(). */
-static inline bool efx_channel_lock_poll(struct efx_channel *channel)
+static inline bool efx_channel_try_lock_poll(struct efx_channel *channel)
{
- bool rc = true;
-
- spin_lock_bh(&channel->state_lock);
- if ((channel->state & EFX_CHANNEL_LOCKED)) {
- channel->state |= EFX_CHANNEL_STATE_POLL_YIELD;
- rc = false;
- } else {
- /* preserve yield marks */
- channel->state |= EFX_CHANNEL_STATE_POLL;
- }
- spin_unlock_bh(&channel->state_lock);
- return rc;
+ return cmpxchg(&channel->busy_poll_state, EFX_CHANNEL_STATE_IDLE,
+ EFX_CHANNEL_STATE_POLL) == EFX_CHANNEL_STATE_IDLE;
}
-/* Returns true if NAPI tried to get the channel while it was locked. */
static inline void efx_channel_unlock_poll(struct efx_channel *channel)
{
- spin_lock_bh(&channel->state_lock);
- WARN_ON(channel->state & EFX_CHANNEL_STATE_NAPI);
-
- /* will reset state to idle, unless channel is disabled */
- channel->state &= EFX_CHANNEL_STATE_DISABLED;
- spin_unlock_bh(&channel->state_lock);
+ clear_bit_unlock(EFX_CHANNEL_STATE_POLL_BIT, &channel->busy_poll_state);
}
-/* True if a socket is polling, even if it did not get the lock. */
static inline bool efx_channel_busy_polling(struct efx_channel *channel)
{
- WARN_ON(!(channel->state & EFX_CHANNEL_OWNED));
- return channel->state & EFX_CHANNEL_USER_PEND;
+ return test_bit(EFX_CHANNEL_STATE_POLL_BIT, &channel->busy_poll_state);
}
static inline void efx_channel_enable(struct efx_channel *channel)
{
- spin_lock_bh(&channel->state_lock);
- channel->state = EFX_CHANNEL_STATE_IDLE;
- spin_unlock_bh(&channel->state_lock);
+ clear_bit_unlock(EFX_CHANNEL_STATE_DISABLE_BIT,
+ &channel->busy_poll_state);
}
-/* False if the channel is currently owned. */
+/* Stop further polling or napi access.
+ * Returns false if the channel is currently busy polling.
+ */
static inline bool efx_channel_disable(struct efx_channel *channel)
{
- bool rc = true;
-
- spin_lock_bh(&channel->state_lock);
- if (channel->state & EFX_CHANNEL_OWNED)
- rc = false;
- channel->state |= EFX_CHANNEL_STATE_DISABLED;
- spin_unlock_bh(&channel->state_lock);
-
- return rc;
+ set_bit(EFX_CHANNEL_STATE_DISABLE_BIT, &channel->busy_poll_state);
+ /* Implicit barrier in efx_channel_busy_polling() */
+ return !efx_channel_busy_polling(channel);
}
#else /* CONFIG_NET_RX_BUSY_POLL */
-static inline void efx_channel_init_lock(struct efx_channel *channel)
+static inline void efx_channel_busy_poll_init(struct efx_channel *channel)
{
}
@@ -584,7 +567,7 @@ static inline void efx_channel_unlock_napi(struct efx_channel *channel)
{
}
-static inline bool efx_channel_lock_poll(struct efx_channel *channel)
+static inline bool efx_channel_try_lock_poll(struct efx_channel *channel)
{
return false;
}
--
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