diff options
author | Eric Dumazet <edumazet@google.com> | 2014-09-25 23:04:56 -0700 |
---|---|---|
committer | David S. Miller <davem@davemloft.net> | 2014-09-29 00:04:55 -0400 |
commit | 3d9a0d2f8212879407e58d67f460d8920eb6543d (patch) | |
tree | 2f21d1f2173c017fabddec9009de8aba855b7c22 /include/linux | |
parent | 68f6a7c6c9817f2e6a66b59893de3c901ae5608c (diff) | |
download | linux-stable-3d9a0d2f8212879407e58d67f460d8920eb6543d.tar.gz linux-stable-3d9a0d2f8212879407e58d67f460d8920eb6543d.tar.bz2 linux-stable-3d9a0d2f8212879407e58d67f460d8920eb6543d.zip |
dql: dql_queued() should write first to reduce bus transactions
While doing high throughput test on a BQL enabled NIC,
I found a very high cost in ndo_start_xmit() when accessing BQL data.
It turned out the problem was caused by compiler trying to be
smart, but involving a bad MESI transaction :
0.05 │ mov 0xc0(%rax),%edi // LOAD dql->num_queued
0.48 │ mov %edx,0xc8(%rax) // STORE dql->last_obj_cnt = count
58.23 │ add %edx,%edi
0.58 │ cmp %edi,0xc4(%rax)
0.76 │ mov %edi,0xc0(%rax) // STORE dql->num_queued += count
0.72 │ js bd8
I got an incredible 10 % gain [1] by making sure cpu do not attempt
to get the cache line in Shared mode, but directly requests for
ownership.
New code :
mov %edx,0xc8(%rax) // STORE dql->last_obj_cnt = count
add %edx,0xc0(%rax) // RMW dql->num_queued += count
mov 0xc4(%rax),%ecx // LOAD dql->adj_limit
mov 0xc0(%rax),%edx // LOAD dql->num_queued
cmp %edx,%ecx
The TX completion was running from another cpu, with high interrupts
rate.
Note that I am using barrier() as a soft hint, as mb() here could be
too heavy cost.
[1] This was a netperf TCP_STREAM with TSO disabled, but GSO enabled.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'include/linux')
-rw-r--r-- | include/linux/dynamic_queue_limits.h | 12 |
1 files changed, 10 insertions, 2 deletions
diff --git a/include/linux/dynamic_queue_limits.h b/include/linux/dynamic_queue_limits.h index 5621547d631b..a4be70398ce1 100644 --- a/include/linux/dynamic_queue_limits.h +++ b/include/linux/dynamic_queue_limits.h @@ -73,14 +73,22 @@ static inline void dql_queued(struct dql *dql, unsigned int count) { BUG_ON(count > DQL_MAX_OBJECT); - dql->num_queued += count; dql->last_obj_cnt = count; + + /* We want to force a write first, so that cpu do not attempt + * to get cache line containing last_obj_cnt, num_queued, adj_limit + * in Shared state, but directly does a Request For Ownership + * It is only a hint, we use barrier() only. + */ + barrier(); + + dql->num_queued += count; } /* Returns how many objects can be queued, < 0 indicates over limit. */ static inline int dql_avail(const struct dql *dql) { - return dql->adj_limit - dql->num_queued; + return ACCESS_ONCE(dql->adj_limit) - ACCESS_ONCE(dql->num_queued); } /* Record number of completed objects and recalculate the limit. */ |