Re: [ovs-dev] [PATCH net-next v3 1/1] net:openvswitch:reduce cpu_used_mask memory

From: Eelco Chaudron
Date: Thu Feb 02 2023 - 06:45:46 EST



On 2 Feb 2023, at 12:26, 陶 缘 wrote:

> Hi, Eelco:
>
> Thanks for your time going through the detail.
> The thing is: sizeof(struct cpumask), with default CONFIG_NR_CPUS 8192, has a size of 1024 bytes even on a system with only 4 cpus.
> While in practice the cpumask APIs like cpumask_next and cpumask_set_cpu never access more than cpumask_size() of bytes in the bitmap
> My change used cpumask_size() (in above example, consume 8 bytes after alignement for the cpumask, it saved 1016 bytes for every flow.

I looked at the wrong nr_cpumask_bits definition, so thanks for this education :)

> Your question reminded me to revisit the description "as well as the iteration of bits in cpu_used_mask", after a second think, this statement is not valid and should be removed.
> since the iteration API will not access the number of bytes decided by nr_cpu_ids(running CPUs)
>
> I will remove this statement after solving a final style issue in the next submission.

Thanks!


> Thanks
> eddy
> ________________________________
> 发件人: Eelco Chaudron <echaudro@xxxxxxxxxx>
> 发送时间: 2023年2月2日 11:05
> 收件人: Eddy Tao <taoyuan_eddy@xxxxxxxxxxx>
> 抄送: netdev@xxxxxxxxxxxxxxx <netdev@xxxxxxxxxxxxxxx>; dev@xxxxxxxxxxxxxxx <dev@xxxxxxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx <linux-kernel@xxxxxxxxxxxxxxx>; Eric Dumazet <edumazet@xxxxxxxxxx>; Jakub Kicinski <kuba@xxxxxxxxxx>; Paolo Abeni <pabeni@xxxxxxxxxx>; David S. Miller <davem@xxxxxxxxxxxxx>
> 主题: Re: [ovs-dev] [PATCH net-next v3 1/1] net:openvswitch:reduce cpu_used_mask memory
>
>
>
> On 2 Feb 2023, at 11:32, Eddy Tao wrote:
>
>> Use actual CPU number instead of hardcoded value to decide the size
>> of 'cpu_used_mask' in 'struct sw_flow'. Below is the reason.
>>
>> 'struct cpumask cpu_used_mask' is embedded in struct sw_flow.
>> Its size is hardcoded to CONFIG_NR_CPUS bits, which can be
>> 8192 by default, it costs memory and slows down ovs_flow_alloc
>> as well as the iteration of bits in cpu_used_mask when handling
>> netlink message from ofproto
>
> I’m trying to understand how this will decrease memory usage. The size of the flow_cache stayed the same (actually it’s large due to the extra pointer).
>
> Also do not understand why the iteration is less, as the mask is initialized the same.
>
> Cheers,
>
> Eelco
>
>> To address this, redefine cpu_used_mask to pointer
>> append cpumask_size() bytes after 'stat' to hold cpumask
>>
>> cpumask APIs like cpumask_next and cpumask_set_cpu never access
>> bits beyond cpu count, cpumask_size() bytes of memory is enough
>>
>> Signed-off-by: Eddy Tao <taoyuan_eddy@xxxxxxxxxxx>
>> ---
>> net/openvswitch/flow.c | 8 +++++---
>> net/openvswitch/flow.h | 2 +-
>> net/openvswitch/flow_table.c | 8 +++++---
>> 3 files changed, 11 insertions(+), 7 deletions(-)
>>
>> diff --git a/net/openvswitch/flow.c b/net/openvswitch/flow.c
>> index e20d1a973417..0109a5f86f6a 100644
>> --- a/net/openvswitch/flow.c
>> +++ b/net/openvswitch/flow.c
>> @@ -107,7 +107,7 @@ void ovs_flow_stats_update(struct sw_flow *flow, __be16 tcp_flags,
>>
>> rcu_assign_pointer(flow->stats[cpu],
>> new_stats);
>> - cpumask_set_cpu(cpu, &flow->cpu_used_mask);
>> + cpumask_set_cpu(cpu, flow->cpu_used_mask);
>> goto unlock;
>> }
>> }
>> @@ -135,7 +135,8 @@ void ovs_flow_stats_get(const struct sw_flow *flow,
>> memset(ovs_stats, 0, sizeof(*ovs_stats));
>>
>> /* We open code this to make sure cpu 0 is always considered */
>> - for (cpu = 0; cpu < nr_cpu_ids; cpu = cpumask_next(cpu, &flow->cpu_used_mask)) {
>> + for (cpu = 0; cpu < nr_cpu_ids;
>> + cpu = cpumask_next(cpu, flow->cpu_used_mask)) {
>> struct sw_flow_stats *stats = rcu_dereference_ovsl(flow->stats[cpu]);
>>
>> if (stats) {
>> @@ -159,7 +160,8 @@ void ovs_flow_stats_clear(struct sw_flow *flow)
>> int cpu;
>>
>> /* We open code this to make sure cpu 0 is always considered */
>> - for (cpu = 0; cpu < nr_cpu_ids; cpu = cpumask_next(cpu, &flow->cpu_used_mask)) {
>> + for (cpu = 0; cpu < nr_cpu_ids;
>> + cpu = cpumask_next(cpu, flow->cpu_used_mask)) {
>> struct sw_flow_stats *stats = ovsl_dereference(flow->stats[cpu]);
>>
>> if (stats) {
>> diff --git a/net/openvswitch/flow.h b/net/openvswitch/flow.h
>> index 073ab73ffeaa..b5711aff6e76 100644
>> --- a/net/openvswitch/flow.h
>> +++ b/net/openvswitch/flow.h
>> @@ -229,7 +229,7 @@ struct sw_flow {
>> */
>> struct sw_flow_key key;
>> struct sw_flow_id id;
>> - struct cpumask cpu_used_mask;
>> + struct cpumask *cpu_used_mask;
>> struct sw_flow_mask *mask;
>> struct sw_flow_actions __rcu *sf_acts;
>> struct sw_flow_stats __rcu *stats[]; /* One for each CPU. First one
>> diff --git a/net/openvswitch/flow_table.c b/net/openvswitch/flow_table.c
>> index 0a0e4c283f02..dc6a174c3194 100644
>> --- a/net/openvswitch/flow_table.c
>> +++ b/net/openvswitch/flow_table.c
>> @@ -87,11 +87,12 @@ struct sw_flow *ovs_flow_alloc(void)
>> if (!stats)
>> goto err;
>>
>> + flow->cpu_used_mask = (struct cpumask *)&flow->stats[nr_cpu_ids];
>> spin_lock_init(&stats->lock);
>>
>> RCU_INIT_POINTER(flow->stats[0], stats);
>>
>> - cpumask_set_cpu(0, &flow->cpu_used_mask);
>> + cpumask_set_cpu(0, flow->cpu_used_mask);
>>
>> return flow;
>> err:
>> @@ -115,7 +116,7 @@ static void flow_free(struct sw_flow *flow)
>> flow->sf_acts);
>> /* We open code this to make sure cpu 0 is always considered */
>> for (cpu = 0; cpu < nr_cpu_ids;
>> - cpu = cpumask_next(cpu, &flow->cpu_used_mask)) {
>> + cpu = cpumask_next(cpu, flow->cpu_used_mask)) {
>> if (flow->stats[cpu])
>> kmem_cache_free(flow_stats_cache,
>> (struct sw_flow_stats __force *)flow->stats[cpu]);
>> @@ -1196,7 +1197,8 @@ int ovs_flow_init(void)
>>
>> flow_cache = kmem_cache_create("sw_flow", sizeof(struct sw_flow)
>> + (nr_cpu_ids
>> - * sizeof(struct sw_flow_stats *)),
>> + * sizeof(struct sw_flow_stats *))
>> + + cpumask_size(),
>> 0, 0, NULL);
>> if (flow_cache == NULL)
>> return -ENOMEM;
>> --
>> 2.27.0
>>
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