netdev - Re: [PATCH 2/2 v4] xps: Transmit Packet Steering

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Date:	Mon, 1 Nov 2010 18:15:46 -0700
From:	Tom Herbert <therbert@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [PATCH 2/2 v4] xps: Transmit Packet Steering

>> +struct xps_map {
>> +     unsigned int len;
>> +     unsigned int alloc_len;
>> +     struct rcu_head rcu;
>> +     u16 queues[0];
>> +};
>
> OK, so its a 'small' structure. And we dont want it to share a cache
> line with an other user in the kernel, or false sharing might happen.
>
> Make sure you allocate big enough ones to fill a full cache line.
>
> alloc_len = (L1_CACHE_BYTES - sizeof(struct xps_map)) / sizeof(u16);
>
> I see you allocate ones with alloc_len = 1. Thats not good.
>
Okay.

>> +
>> +/*
>> + * This structure holds all XPS maps for device.  Maps are indexed by CPU.
>> + */
>> +struct xps_dev_maps {
>> +     struct rcu_head rcu;
>> +     struct xps_map *cpu_map[0];
>
> Hmm... per_cpu maybe, instead of an array ?
>
The cpu_map is an array of pointers to the actual maps, and I wouldn't
expect those pointers to be changing so maybe that's okay for the
cache.  We still need the rcu head, and making cpu_map per-cpu (struct
xps_map **) would add another level of indirection.  Is there a
compelling reason to use per-cpu here?

> Also make sure this xps_dev_maps use a full cache line, to avoid false
> sharing.
>
Okay.



> Really I am not sure we need this array and smp_processor_id().
> Please consider alloc_percpu().
> Then, use __this_cpu_ptr() and avoid the preempt_disable()/enable()
> thing. Its a hint we want to use, because as soon as we leave
> get_xps_queue() we might migrate to another cpu ?

If we don't use per-cpu, how about raw_smp_processor_id() here?


>> +     if (dev_maps) {
>> +             for (i = 0; i < num_possible_cpus(); i++) {
>
> The use of num_possible_cpus() seems wrong to me.
> Dont you meant nr_cpu_ids ?
>

Okay, thanks for clarifying.

> Some machines have two possible cpus, numbered 0 and 8 :
> num_possible_cpus = 2
> nr_cpu_ids = 8
>
>
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