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tg3: Improve small packet performance
smp_mb() inside tg3_tx_avail() is used twice in the normal
tg3_start_xmit() path (see illustration below). The full memory
barrier is only necessary during race conditions with tx completion.
We can speed up the tx path by replacing smp_mb() in tg3_tx_avail()
with a compiler barrier. The compiler barrier is to force the
compiler to fetch the tx_prod and tx_cons from memory.
In the race condition between tg3_start_xmit() and tg3_tx(),
we have the following situation:
tg3_start_xmit() tg3_tx()
if (!tg3_tx_avail())
BUG();
...
if (!tg3_tx_avail())
netif_tx_stop_queue(); update_tx_index();
smp_mb(); smp_mb();
if (tg3_tx_avail()) if (netif_tx_queue_stopped() &&
netif_tx_wake_queue(); tg3_tx_avail())
With smp_mb() removed from tg3_tx_avail(), we need to add smp_mb() to
tg3_start_xmit() as shown above to properly order netif_tx_stop_queue()
and tg3_tx_avail() to check the ring index. If it is not strictly
ordered, the tx queue can be stopped forever.
This improves performance by about 3% with 2 ports running
bi-directional 64-byte packets.
Reviewed-by: Benjamin Li <benli@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>