From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from relay2-d.mail.gandi.net (relay2-d.mail.gandi.net [217.70.183.194]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 1655473478; Tue, 18 Jun 2024 10:22:27 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.70.183.194 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1718706150; cv=none; b=QcV6zEsbv+DTgM8z6DEGwwn5T1sBTqcxsrGqmjLy7zNfU4cGLJYJaEagQFOMfS1Sbu3eTDAv+/w+cxOytfCn6N3fDY+mrG1SZKNyG69nRCXECozTVGnbuWMPeWj4z0yO9QFzawxU2wKr//RTyZHY2Q11UzwMH6z95Q6HpTmQR/s= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1718706150; c=relaxed/simple; bh=PXEI59+bXZNPBfIyYhqKGAlfSY6LLbPzT3dOZE2Pgi0=; h=Message-ID:Date:MIME-Version:Cc:Subject:To:References:From: In-Reply-To:Content-Type; b=g6gC/1l0oMetPkCPieYQHrcCpn27lS6fIhxKnqApRHuoRsbZjoyafxVGrPYKq89O69lwdS5Afzsb9w01rljnsBeqM9anhVOE0uV6EaeV4UEpg47Ykfmk7F3Mc2nl8UMDrFfDklM0y788+H0RZp/9B8kBNjTrBJvYLM2RDxJ/AsE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=ovn.org; spf=pass smtp.mailfrom=ovn.org; arc=none smtp.client-ip=217.70.183.194 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=ovn.org Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=ovn.org Received: by mail.gandi.net (Postfix) with ESMTPSA id 469534000A; Tue, 18 Jun 2024 10:22:24 +0000 (UTC) Message-ID: <5f293bac-4117-4f93-8d3f-636d6ce236a4@ovn.org> Date: Tue, 18 Jun 2024 12:22:23 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Cc: i.maximets@ovn.org, netdev@vger.kernel.org, aconole@redhat.com, echaudro@redhat.com, horms@kernel.org, dev@openvswitch.org, Pravin B Shelar , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , linux-kernel@vger.kernel.org Subject: Re: [PATCH net-next v2 7/9] net: openvswitch: do not notify drops inside sample To: =?UTF-8?Q?Adri=C3=A1n_Moreno?= References: <20240603185647.2310748-1-amorenoz@redhat.com> <20240603185647.2310748-8-amorenoz@redhat.com> <8624ccf8-e9e2-4a95-a25c-7d3166bb3256@ovn.org> Content-Language: en-US From: Ilya Maximets Autocrypt: addr=i.maximets@ovn.org; keydata= xsFNBF77bOMBEADVZQ4iajIECGfH3hpQMQjhIQlyKX4hIB3OccKl5XvB/JqVPJWuZQRuqNQG /B70MP6km95KnWLZ4H1/5YOJK2l7VN7nO+tyF+I+srcKq8Ai6S3vyiP9zPCrZkYvhqChNOCF pNqdWBEmTvLZeVPmfdrjmzCLXVLi5De9HpIZQFg/Ztgj1AZENNQjYjtDdObMHuJQNJ6ubPIW cvOOn4WBr8NsP4a2OuHSTdVyAJwcDhu+WrS/Bj3KlQXIdPv3Zm5x9u/56NmCn1tSkLrEgi0i /nJNeH5QhPdYGtNzPixKgPmCKz54/LDxU61AmBvyRve+U80ukS+5vWk8zvnCGvL0ms7kx5sA tETpbKEV3d7CB3sQEym8B8gl0Ux9KzGp5lbhxxO995KWzZWWokVUcevGBKsAx4a/C0wTVOpP FbQsq6xEpTKBZwlCpxyJi3/PbZQJ95T8Uw6tlJkPmNx8CasiqNy2872gD1nN/WOP8m+cIQNu o6NOiz6VzNcowhEihE8Nkw9V+zfCxC8SzSBuYCiVX6FpgKzY/Tx+v2uO4f/8FoZj2trzXdLk BaIiyqnE0mtmTQE8jRa29qdh+s5DNArYAchJdeKuLQYnxy+9U1SMMzJoNUX5uRy6/3KrMoC/ 7zhn44x77gSoe7XVM6mr/mK+ViVB7v9JfqlZuiHDkJnS3yxKPwARAQABzSJJbHlhIE1heGlt ZXRzIDxpLm1heGltZXRzQG92bi5vcmc+wsGUBBMBCAA+AhsDBQsJCAcCBhUKCQgLAgQWAgMB Ah4BAheAFiEEh+ma1RKWrHCY821auffsd8gpv5YFAmP+Y/MFCQjFXhAACgkQuffsd8gpv5Yg OA//eEakvE7xTHNIMdLW5r3XnWSEY44dFDEWTLnS7FbZLLHxPNFXN0GSAA8ZsJ3fE26O5Pxe EEFTf7R/W6hHcSXNK4c6S8wR4CkTJC3XOFJchXCdgSc7xS040fLZwGBuO55WT2ZhQvZj1PzT 8Fco8QKvUXr07saHUaYk2Lv2mRhEPP9zsyy7C2T9zUzG04a3SGdP55tB5Adi0r/Ea+6VJoLI ctN8OaF6BwXpag8s76WAyDx8uCCNBF3cnNkQrCsfKrSE2jrvrJBmvlR3/lJ0OYv6bbzfkKvo 0W383EdxevzAO6OBaI2w+wxBK92SMKQB3R0ZI8/gqCokrAFKI7gtnyPGEKz6jtvLgS3PeOtf 5D7PTz+76F/X6rJGTOxR3bup+w1bP/TPHEPa2s7RyJISC07XDe24n9ZUlpG5ijRvfjbCCHb6 pOEijIj2evcIsniTKER2pL+nkYtx0bp7dZEK1trbcfglzte31ZSOsfme74u5HDxq8/rUHT01 51k/vvUAZ1KOdkPrVEl56AYUEsFLlwF1/j9mkd7rUyY3ZV6oyqxV1NKQw4qnO83XiaiVjQus K96X5Ea+XoNEjV4RdxTxOXdDcXqXtDJBC6fmNPzj4QcxxyzxQUVHJv67kJOkF4E+tJza+dNs 8SF0LHnPfHaSPBFrc7yQI9vpk1XBxQWhw6oJgy3OwU0EXvts4wEQANCXyDOic0j2QKeyj/ga OD1oKl44JQfOgcyLVDZGYyEnyl6b/tV1mNb57y/YQYr33fwMS1hMj9eqY6tlMTNz+ciGZZWV YkPNHA+aFuPTzCLrapLiz829M5LctB2448bsgxFq0TPrr5KYx6AkuWzOVq/X5wYEM6djbWLc VWgJ3o0QBOI4/uB89xTf7mgcIcbwEf6yb/86Cs+jaHcUtJcLsVuzW5RVMVf9F+Sf/b98Lzrr 2/mIB7clOXZJSgtV79Alxym4H0cEZabwiXnigjjsLsp4ojhGgakgCwftLkhAnQT3oBLH/6ix 87ahawG3qlyIB8ZZKHsvTxbWte6c6xE5dmmLIDN44SajAdmjt1i7SbAwFIFjuFJGpsnfdQv1 OiIVzJ44kdRJG8kQWPPua/k+AtwJt/gjCxv5p8sKVXTNtIP/sd3EMs2xwbF8McebLE9JCDQ1 RXVHceAmPWVCq3WrFuX9dSlgf3RWTqNiWZC0a8Hn6fNDp26TzLbdo9mnxbU4I/3BbcAJZI9p 9ELaE9rw3LU8esKqRIfaZqPtrdm1C+e5gZa2gkmEzG+WEsS0MKtJyOFnuglGl1ZBxR1uFvbU VXhewCNoviXxkkPk/DanIgYB1nUtkPC+BHkJJYCyf9Kfl33s/bai34aaxkGXqpKv+CInARg3 fCikcHzYYWKaXS6HABEBAAHCwXwEGAEIACYCGwwWIQSH6ZrVEpascJjzbVq59+x3yCm/lgUC Y/5kJAUJCMVeQQAKCRC59+x3yCm/lpF7D/9Lolx00uxqXz2vt/u9flvQvLsOWa+UBmWPGX9u oWhQ26GjtbVvIf6SECcnNWlu/y+MHhmYkz+h2VLhWYVGJ0q03XkktFCNwUvHp3bTXG3IcPIC eDJUVMMIHXFp7TcuRJhrGqnlzqKverlY6+2CqtCpGMEmPVahMDGunwqFfG65QubZySCHVYvX T9SNga0Ay/L71+eVwcuGChGyxEWhVkpMVK5cSWVzZe7C+gb6N1aTNrhu2dhpgcwe1Xsg4dYv dYzTNu19FRpfc+nVRdVnOto8won1SHGgYSVJA+QPv1x8lMYqKESOHAFE/DJJKU8MRkCeSfqs izFVqTxTk3VXOCMUR4t2cbZ9E7Qb/ZZigmmSgilSrOPgDO5TtT811SzheAN0PvgT+L1Gsztc Q3BvfofFv3OLF778JyVfpXRHsn9rFqxG/QYWMqJWi+vdPJ5RhDl1QUEFyH7ok/ZY60/85FW3 o9OQwoMf2+pKNG3J+EMuU4g4ZHGzxI0isyww7PpEHx6sxFEvMhsOp7qnjPsQUcnGIIiqKlTj H7i86580VndsKrRK99zJrm4s9Tg/7OFP1SpVvNvSM4TRXSzVF25WVfLgeloN1yHC5Wsqk33X XNtNovqA0TLFjhfyyetBsIOgpGakgBNieC9GnY7tC3AG+BqG5jnVuGqSTO+iM/d+lsoa+w== In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-GND-Sasl: i.maximets@ovn.org 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 >>>> --- >>>> 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.