Message-ID: <5f293bac-4117-4f93-8d3f-636d6ce236a4@ovn.org>
Date: Tue, 18 Jun 2024 12:22:23 +0200
From: Ilya Maximets <i.maximets@....org>
To: Adrián Moreno <amorenoz@...hat.com>
Cc: i.maximets@....org, netdev@...r.kernel.org, aconole@...hat.com,
 echaudro@...hat.com, horms@...nel.org, dev@...nvswitch.org,
 Pravin B Shelar <pshelar@....org>, "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v2 7/9] net: openvswitch: do not notify drops
 inside sample

On 6/18/24 09:00, Adrián Moreno wrote:
> On Mon, Jun 17, 2024 at 02:10:37PM GMT, Ilya Maximets wrote:
>> On 6/17/24 13:55, Ilya Maximets wrote:
>>> On 6/3/24 20:56, Adrian Moreno wrote:
>>>> The OVS_ACTION_ATTR_SAMPLE action is, in essence,
>>>> observability-oriented.
>>>>
>>>> Apart from some corner case in which it's used a replacement of clone()
>>>> for old kernels, it's really only used for sFlow, IPFIX and now,
>>>> local emit_sample.
>>>>
>>>> With this in mind, it doesn't make much sense to report
>>>> OVS_DROP_LAST_ACTION inside sample actions.
>>>>
>>>> For instance, if the flow:
>>>>
>>>>   actions:sample(..,emit_sample(..)),2
>>>>
>>>> triggers a OVS_DROP_LAST_ACTION skb drop event, it would be extremely
>>>> confusing for users since the packet did reach its destination.
>>>>
>>>> This patch makes internal action execution silently consume the skb
>>>> instead of notifying a drop for this case.
>>>>
>>>> Unfortunately, this patch does not remove all potential sources of
>>>> confusion since, if the sample action itself is the last action, e.g:
>>>>
>>>>     actions:sample(..,emit_sample(..))
>>>>
>>>> we actually _should_ generate a OVS_DROP_LAST_ACTION event, but we aren't.
>>>>
>>>> Sadly, this case is difficult to solve without breaking the
>>>> optimization by which the skb is not cloned on last sample actions.
>>>> But, given explicit drop actions are now supported, OVS can just add one
>>>> after the last sample() and rewrite the flow as:
>>>>
>>>>     actions:sample(..,emit_sample(..)),drop
>>>>
>>>> Signed-off-by: Adrian Moreno <amorenoz@...hat.com>
>>>> ---
>>>>  net/openvswitch/actions.c | 13 +++++++++++--
>>>>  1 file changed, 11 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/net/openvswitch/actions.c b/net/openvswitch/actions.c
>>>> index 33f6d93ba5e4..54fc1abcff95 100644
>>>> --- a/net/openvswitch/actions.c
>>>> +++ b/net/openvswitch/actions.c
>>>> @@ -82,6 +82,15 @@ static struct action_fifo __percpu *action_fifos;
>>>>  static struct action_flow_keys __percpu *flow_keys;
>>>>  static DEFINE_PER_CPU(int, exec_actions_level);
>>>>
>>>> +static inline void ovs_drop_skb_last_action(struct sk_buff *skb)
>>>> +{
>>>> +	/* Do not emit packet drops inside sample(). */
>>>> +	if (OVS_CB(skb)->probability)
>>>> +		consume_skb(skb);
>>>> +	else
>>>> +		ovs_kfree_skb_reason(skb, OVS_DROP_LAST_ACTION);
>>>> +}
>>>> +
>>>>  /* Make a clone of the 'key', using the pre-allocated percpu 'flow_keys'
>>>>   * space. Return NULL if out of key spaces.
>>>>   */
>>>> @@ -1061,7 +1070,7 @@ static int sample(struct datapath *dp, struct sk_buff *skb,
>>>>  	if ((arg->probability != U32_MAX) &&
>>>>  	    (!arg->probability || get_random_u32() > arg->probability)) {
>>>>  		if (last)
>>>> -			ovs_kfree_skb_reason(skb, OVS_DROP_LAST_ACTION);
>>>> +			ovs_drop_skb_last_action(skb);
>>
>> Always consuming the skb at this point makes sense, since having smaple()
>> as a last action is a reasonable thing to have.  But this looks more like
>> a fix for the original drop reason patch set.
>>
> 
> I don't think consuming the skb at this point makes sense. It was very
> intentionally changed to a drop since a very common use-case for
> sampling is drop-sampling, i.e: replacing an empty action list (that
> triggers OVS_DROP_LAST_ACTION) with a sample(emit_sample()). Ideally,
> that replacement should not have any effect on the number of
> OVS_DROP_LAST_ACTION being reported as the packets are being treated in
> the same way (only observed in one case).
> 
> 
>>>>  		return 0;
>>>>  	}
>>>>
>>>> @@ -1579,7 +1588,7 @@ static int do_execute_actions(struct datapath *dp, struct sk_buff *skb,
>>>>  		}
>>>>  	}
>>>>
>>>> -	ovs_kfree_skb_reason(skb, OVS_DROP_LAST_ACTION);
>>>> +	ovs_drop_skb_last_action(skb);
>>>
>>> I don't think I agree with this one.  If we have a sample() action with
>>> a lot of different actions inside and we reached the end while the last
>>> action didn't consume the skb, then we should report that.  E.g.
>>> "sample(emit_sample(),push_vlan(),set(eth())),2"  should report that the
>>> cloned skb was dropped.  "sample(push_vlan(),emit_sample())" should not.
>>>
> 
> What is the use case for such action list? Having an action branch
> executed randomly doesn't make sense to me if it's not some
> observability thing (which IMHO should not trigger drops).

It is exactly my point.  A list of actions that doesn't end is some sort
of a terminal action (output, drop, etc) does not make a lot of sense and
hence should be signaled as an unexpected drop, so users can re-check the
pipeline in case they missed the terminal action somehow.

> 
>>> The only actions that are actually consuming the skb are "output",
>>> "userspace", "recirc" and now "emit_sample".  "output" and "recirc" are
>>> consuming the skb "naturally" by stealing it when it is the last action.
>>> "userspace" has an explicit check to consume the skb if it is the last
>>> action.  "emit_sample" should have the similar check.  It should likely
>>> be added at the point of action introduction instead of having a separate
>>> patch.
>>>
> 
> Unlinke "output", "recirc", "userspace", etc. with emit_sample the
> packet does not continue it's way through the datapath.

After "output" the packet leaves the datapath too, i.e. does not continue
it's way through OVS datapath.

> 
> It would be very confusing if OVS starts monitoring drops and adds a bunch
> of flows such as "actions:emit_sample()" and suddently it stops reporting such
> drops via standard kfree_skb_reason. Packets _are_ being dropped here,
> we are just observing them.

This might make sense from the higher logic in user space application, but
it doesn't from the datapath perspective.  And also, if the user adds the
'emit_sample' action for drop monitring, they already know where to find
packet samples, they don't need to use tools like dropwatch anymore.
This packet is not dropped from the datapath perspective, it is sampled.

> 
> And if we change emit_sample to trigger a drop if it's the last action,
> then "sample(50%, emit_sample()),2" will trigger a drop half of the times
> which is also terribly confusing.

If emit_sample is the last action, then skb should be consumed silently.
The same as for "output" and "userspace".

> 
> I think we should try to be clear and informative with what we
> _actually_ drop and not require the user that is just running
> "dropwatch" to understand the internals of the OVS module.

If someone is already using sampling to watch their packet drops, why would
they use dropwatch?

> 
> So if you don't want to accept the "observational" nature of sample(),
> the only other solution that does not bring even more confusion to OVS
> drops would be to have userspace add explicit drop actions. WDYT?
> 

These are not drops from the datapath perspective.  Users can add explicit
drop actions if they want to, but I'm really not sure why they would do that
if they are already capturing all these packets in psample, sFlow or IPFIX.

Best regards, Ilya Maximets.

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