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dql: dql_queued() should write first to reduce bus transactions

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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>
This commit is contained in:
Eric Dumazet 2014-09-25 23:04:56 -07:00 committed by David S. Miller
parent 68f6a7c6c9
commit 3d9a0d2f82
1 changed files with 10 additions and 2 deletions
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12
include/linux/dynamic_queue_limits.h
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@ -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. */