[bpf-next,v3] bpf: using rcu_read_lock for bpf_sk_storage_map iterator

If a bucket contains a lot of sockets, during bpf_iter traversing
a bucket, concurrent userspace bpf_map_update_elem() and
bpf program bpf_sk_storage_{get,delete}() may experience
some undesirable delays as they will compete with bpf_iter
for bucket lock.

Note that the number of buckets for bpf_sk_storage_map
is roughly the same as the number of cpus. So if there
are lots of sockets in the system, each bucket could
contain lots of sockets.

Different actual use cases may experience different delays.
Here, using selftest bpf_iter subtest bpf_sk_storage_map,
I hacked the kernel with ktime_get_mono_fast_ns()
to collect the time when a bucket was locked
during bpf_iter prog traversing that bucket. This way,
the maximum incurred delay was measured w.r.t. the
number of elements in a bucket.
    # elems in each bucket          delay(ns)
      64                            17000
      256                           72512
      2048                          875246

The potential delays will be further increased if
we have even more elemnts in a bucket. Using rcu_read_lock()
is a reasonable compromise here. It may lose some precision, e.g.,
access stale sockets, but it will not hurt performance of
bpf program or user space application which also tries
to get/delete or update map elements.

Cc: Martin KaFai Lau <kafai@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Yonghong Song <yhs@fb.com>
---
 net/core/bpf_sk_storage.c | 29 ++++++++++++-----------------
 1 file changed, 12 insertions(+), 17 deletions(-)

Changelog:
  v2 -> v3:
     - fix a bug hlist_for_each_entry() => hlist_for_each_entry_rcu(). (Martin)
     - use rcu_dereference() instead of rcu_dereference_raw() for lockdep checking. (Martin)
  v1 -> v2:
    - added some performance number. (Song)
    - tried to silence some sparse complains. but still has some left like
        context imbalance in "..." - different lock contexts for basic block
      which the code is too hard for sparse to analyze. (Jakub)

On Tue, Sep 15, 2020 at 11:16:49PM -0700, Yonghong Song wrote:
[ ... ]

> diff --git a/net/core/bpf_sk_storage.c b/net/core/bpf_sk_storage.c

> index 4a86ea34f29e..d43c3d6d0693 100644

> --- a/net/core/bpf_sk_storage.c

> +++ b/net/core/bpf_sk_storage.c

> @@ -678,6 +678,7 @@ struct bpf_iter_seq_sk_storage_map_info {

>  static struct bpf_local_storage_elem *

>  bpf_sk_storage_map_seq_find_next(struct bpf_iter_seq_sk_storage_map_info *info,

>  				 struct bpf_local_storage_elem *prev_selem)

> +	__acquires(RCU) __releases(RCU)

>  {

>  	struct bpf_local_storage *sk_storage;

>  	struct bpf_local_storage_elem *selem;

In the while loop earlier in this function, if I read it correctly,
it is sort of continuing the earlier hlist_for_each_entry_rcu() for the
same bucket, so the hlist_entry_safe() needs to be changed also.
Something like this (uncompiled code):

        while (selem) {
-               selem = hlist_entry_safe(selem->map_node.next,
+               selem = hlist_entry_safe(rcu_dereference(hlist_next_rcu(&selem->map_node)),
                                         struct bpf_local_storage_elem, map_node);
                if (!selem) {
                        /* not found, unlock and go to the next bucket */

> @@ -701,11 +702,11 @@ bpf_sk_storage_map_seq_find_next(struct bpf_iter_seq_sk_storage_map_info *info,

>  		if (!selem) {

>  			/* not found, unlock and go to the next bucket */

>  			b = &smap->buckets[bucket_id++];

> -			raw_spin_unlock_bh(&b->lock);

> +			rcu_read_unlock();

>  			skip_elems = 0;

>  			break;

>  		}

> -		sk_storage = rcu_dereference_raw(selem->local_storage);

> +		sk_storage = rcu_dereference(selem->local_storage);

>  		if (sk_storage) {

>  			info->skip_elems = skip_elems + count;

>  			return selem;

On 9/16/20 10:55 AM, Martin KaFai Lau wrote:
> On Tue, Sep 15, 2020 at 11:16:49PM -0700, Yonghong Song wrote:
> [ ... ]
> 
>> diff --git a/net/core/bpf_sk_storage.c b/net/core/bpf_sk_storage.c
>> index 4a86ea34f29e..d43c3d6d0693 100644
>> --- a/net/core/bpf_sk_storage.c
>> +++ b/net/core/bpf_sk_storage.c
>> @@ -678,6 +678,7 @@ struct bpf_iter_seq_sk_storage_map_info {
>>   static struct bpf_local_storage_elem *
>>   bpf_sk_storage_map_seq_find_next(struct bpf_iter_seq_sk_storage_map_info *info,
>>   				 struct bpf_local_storage_elem *prev_selem)
>> +	__acquires(RCU) __releases(RCU)
>>   {
>>   	struct bpf_local_storage *sk_storage;
>>   	struct bpf_local_storage_elem *selem;
> In the while loop earlier in this function, if I read it correctly,
> it is sort of continuing the earlier hlist_for_each_entry_rcu() for the
> same bucket, so the hlist_entry_safe() needs to be changed also.
> Something like this (uncompiled code):
> 
>          while (selem) {
> -               selem = hlist_entry_safe(selem->map_node.next,
> +               selem = hlist_entry_safe(rcu_dereference(hlist_next_rcu(&selem->map_node)),
>                                           struct bpf_local_storage_elem, map_node);
>                  if (!selem) {
>                          /* not found, unlock and go to the next bucket */

Thanks and Ack. Will send v4 shortly.

> 
>> @@ -701,11 +702,11 @@ bpf_sk_storage_map_seq_find_next(struct bpf_iter_seq_sk_storage_map_info *info,
>>   		if (!selem) {
>>   			/* not found, unlock and go to the next bucket */
>>   			b = &smap->buckets[bucket_id++];
>> -			raw_spin_unlock_bh(&b->lock);
>> +			rcu_read_unlock();
>>   			skip_elems = 0;
>>   			break;
>>   		}
>> -		sk_storage = rcu_dereference_raw(selem->local_storage);
>> +		sk_storage = rcu_dereference(selem->local_storage);
>>   		if (sk_storage) {
>>   			info->skip_elems = skip_elems + count;
>>   			return selem;