> On 3 Apr 2015, at 02:19, Ethan Jackson <et...@nicira.com> wrote:
>
> Do we have to use the ATOMIC(long long) syntax, or can we use
> atomic_ullong for most of the struct variable definitions? If we have
> to use the former, there should be a comment explaining why.
>
I’ve changed it to atomic_ullong
> In dp_netdev_flow_used() I don't think we need to read the time and do
> a MAX. time_msec() is monotonic, so just writing the value should be
> fine.
>
That’s an improvement, thanks
> There should be a comment in dp_netdev_flow_used() explaining why we
> do updates the way we do.
>
> Alternatively, the patch might benefit from a "relaxed add" helper
> local to dpif_netdev which does a relaxed read, add, relaxed write.
>
I’ve added the helper to save some lines of code.
> Acked-by: Ethan Jackson <et...@nicira.com>
>
Thanks for the review!
>
> On Wed, Apr 1, 2015 at 9:20 AM, Daniele Di Proietto
> <diproiet...@vmware.com> wrote:
>> A read operation from a non atomic shared value (without external
>> locking) can return incorrect values. Using the atomic semantics
>> prevents this from happening.
>>
>> However:
>> * No memory barriers are used. We don't need that kind of consistency
>> for statistics (we use relaxed operations).
>> * The updates are not atomic, just the loads and stores. This is ok
>> because there's a single writer.
>>
>> Suggested-by: Ethan Jackson <et...@nicira.com>
>> Signed-off-by: Daniele Di Proietto <diproiet...@vmware.com>
>> ---
>> lib/dpif-netdev.c | 69
>> ++++++++++++++++++++++++++++++++++++++++---------------
>> 1 file changed, 50 insertions(+), 19 deletions(-)
>>
>> diff --git a/lib/dpif-netdev.c b/lib/dpif-netdev.c
>> index b5cfdcb..a882e9c 100644
>> --- a/lib/dpif-netdev.c
>> +++ b/lib/dpif-netdev.c
>> @@ -239,10 +239,10 @@ struct dp_netdev_port {
>>
>> /* Contained by struct dp_netdev_flow's 'stats' member. */
>> struct dp_netdev_flow_stats {
>> - long long int used; /* Last used time, in monotonic msecs.
>> */
>> - long long int packet_count; /* Number of packets matched. */
>> - long long int byte_count; /* Number of bytes matched. */
>> - uint16_t tcp_flags; /* Bitwise-OR of seen tcp_flags values.
>> */
>> + ATOMIC(long long) used; /* Last used time, in monotonic msecs.
>> */
>> + ATOMIC(long long) packet_count; /* Number of packets matched. */
>> + ATOMIC(long long) byte_count; /* Number of bytes matched. */
>> + ATOMIC(uint16_t) tcp_flags; /* Bitwise-OR of seen tcp_flags values.
>> */
>> };
>>
>> /* A flow in 'dp_netdev_pmd_thread's 'flow_table'.
>> @@ -338,7 +338,7 @@ static void dp_netdev_actions_free(struct
>> dp_netdev_actions *);
>> /* Contained by struct dp_netdev_pmd_thread's 'stats' member. */
>> struct dp_netdev_pmd_stats {
>> /* Indexed by DP_STAT_*. */
>> - unsigned long long int n[DP_N_STATS];
>> + ATOMIC(unsigned long long) n[DP_N_STATS];
>> };
>>
>> /* PMD: Poll modes drivers. PMD accesses devices via polling to eliminate
>> @@ -754,10 +754,15 @@ dpif_netdev_get_stats(const struct dpif *dpif, struct
>> dpif_dp_stats *stats)
>>
>> stats->n_flows = stats->n_hit = stats->n_missed = stats->n_lost = 0;
>> CMAP_FOR_EACH (pmd, node, &dp->poll_threads) {
>> + unsigned long long n;
>> stats->n_flows += cmap_count(&pmd->flow_table);
>> - stats->n_hit += pmd->stats.n[DP_STAT_HIT];
>> - stats->n_missed += pmd->stats.n[DP_STAT_MISS];
>> - stats->n_lost += pmd->stats.n[DP_STAT_LOST];
>> +
>> + atomic_read_relaxed(&pmd->stats.n[DP_STAT_HIT], &n);
>> + stats->n_hit += n;
>> + atomic_read_relaxed(&pmd->stats.n[DP_STAT_MISS], &n);
>> + stats->n_missed += n;
>> + atomic_read_relaxed(&pmd->stats.n[DP_STAT_LOST], &n);
>> + stats->n_lost += n;
>> }
>> stats->n_masks = UINT32_MAX;
>> stats->n_mask_hit = UINT64_MAX;
>> @@ -1545,13 +1550,23 @@ dp_netdev_pmd_find_flow(const struct
>> dp_netdev_pmd_thread *pmd,
>> }
>>
>> static void
>> -get_dpif_flow_stats(const struct dp_netdev_flow *netdev_flow,
>> +get_dpif_flow_stats(const struct dp_netdev_flow *netdev_flow_,
>> struct dpif_flow_stats *stats)
>> {
>> - stats->n_packets = netdev_flow->stats.packet_count;
>> - stats->n_bytes = netdev_flow->stats.byte_count;
>> - stats->used = netdev_flow->stats.used;
>> - stats->tcp_flags = netdev_flow->stats.tcp_flags;
>> + struct dp_netdev_flow *netdev_flow;
>> + long long n;
>> + uint16_t flags;
>> +
>> + netdev_flow = CONST_CAST(struct dp_netdev_flow *, netdev_flow_);
>> +
>> + atomic_read_relaxed(&netdev_flow->stats.packet_count, &n);
>> + stats->n_packets = n;
>> + atomic_read_relaxed(&netdev_flow->stats.byte_count, &n);
>> + stats->n_bytes = n;
>> + atomic_read_relaxed(&netdev_flow->stats.used, &n);
>> + stats->used = n;
>> + atomic_read_relaxed(&netdev_flow->stats.tcp_flags, &flags);
>> + stats->tcp_flags = flags;
>> }
>>
>> /* Converts to the dpif_flow format, using 'key_buf' and 'mask_buf' for
>> @@ -2678,19 +2693,35 @@ static void
>> dp_netdev_flow_used(struct dp_netdev_flow *netdev_flow, int cnt, int size,
>> uint16_t tcp_flags)
>> {
>> - long long int now = time_msec();
>> + long long n, now = time_msec();
>> + uint16_t flags;
>>
>> - netdev_flow->stats.used = MAX(now, netdev_flow->stats.used);
>> - netdev_flow->stats.packet_count += cnt;
>> - netdev_flow->stats.byte_count += size;
>> - netdev_flow->stats.tcp_flags |= tcp_flags;
>> + atomic_read_relaxed(&netdev_flow->stats.used, &n);
>> + n = MAX(now, n);
>> + atomic_store_relaxed(&netdev_flow->stats.used, n);
>> +
>> + atomic_read_relaxed(&netdev_flow->stats.packet_count, &n);
>> + n += cnt;
>> + atomic_store_relaxed(&netdev_flow->stats.packet_count, n);
>> +
>> + atomic_read_relaxed(&netdev_flow->stats.byte_count, &n);
>> + n += size;
>> + atomic_store_relaxed(&netdev_flow->stats.byte_count, n);
>> +
>> + atomic_read_relaxed(&netdev_flow->stats.tcp_flags, &flags);
>> + flags |= tcp_flags;
>> + atomic_store_relaxed(&netdev_flow->stats.tcp_flags, flags);
>> }
>>
>> static void
>> dp_netdev_count_packet(struct dp_netdev_pmd_thread *pmd,
>> enum dp_stat_type type, int cnt)
>> {
>> - pmd->stats.n[type] += cnt;
>> + unsigned long long n;
>> +
>> + atomic_read_relaxed(&pmd->stats.n[type], &n);
>> + n += cnt;
>> + atomic_store_relaxed(&pmd->stats.n[type], n);
>> }
>>
>> static int
>> --
>> 2.1.4
>>
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