Re: [PATCH bpf v2 6/9] bpf: Switch to bpf mem allocator for LPM trie

[Hou Tao <houtao@huaweicloud.com>]

Hi,

On 11/27/2024 1:51 PM, Alexei Starovoitov wrote:
> On Tue, Nov 26, 2024 at 4:34 PM Hou Tao <houtao@huaweicloud.com> wrote:
>> 2. nodes are freed before invoking spin_unlock_irqrestore(). Therefore,
>> there is no need to add paired migrate_{disable|enable}() calls for
>> these free operations.
> ...
>
>>         if (ret)
>> -               kfree(new_node);
>> +               bpf_mem_cache_free(&trie->ma, new_node);
>> +       bpf_mem_cache_free_rcu(&trie->ma, free_node);
>>         spin_unlock_irqrestore(&trie->lock, irq_flags);
>> -       kfree_rcu(free_node, rcu);
> ...
>
>> +       bpf_mem_cache_free_rcu(&trie->ma, free_parent);
>> +       bpf_mem_cache_free_rcu(&trie->ma, free_node);
>>         spin_unlock_irqrestore(&trie->lock, irq_flags);
>> -       kfree_rcu(free_parent, rcu);
>> -       kfree_rcu(free_node, rcu);
> going back to under lock wasn't obvious.
> I only understood after reading the commit log for the 2nd time.
>
> Probably a code comment would be good.

Missed that. Will be alert next time.
>
> Though I wonder whether we should add migrate_disable/enable
> in the syscall path of these callbacks.
> We already wrapped them with rcu_read_lock().
> Extra migrate_disable() won't hurt.

It seems that bpf program has already been running with migration
disabled. I think we could also do the similar thing for the syscall path.
>
> And it will help this patch. bpf_mem_cache_free_rcu() can be
> done outside of bucket lock.
> bpf_ma can easily exhaust the free list in irq disabled region,
> so the more operations outside of the known irq region the better.
>
> Also it will help remove migrate_disable/enable from a bunch
> of places in kernel/bpf where we added them due to syscall path
> or map free path.
>
> It's certainly a follow up, if you agree.
> This patch set will go through bpf tree
> (after hopefully few more acks from reviewers)
Thanks for the suggestion. Will try it.