From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mails.dpdk.org (mails.dpdk.org [217.70.189.124]) by inbox.dpdk.org (Postfix) with ESMTP id 32AA046B81 for ; Tue, 15 Jul 2025 19:30:20 +0200 (CEST) Received: from mails.dpdk.org (localhost [127.0.0.1]) by mails.dpdk.org (Postfix) with ESMTP id D8E92402A9; Tue, 15 Jul 2025 19:30:19 +0200 (CEST) Received: from agw.arknetworks.am (agw.arknetworks.am [79.141.165.80]) by mails.dpdk.org (Postfix) with ESMTP id D13364028C for ; Tue, 15 Jul 2025 19:30:17 +0200 (CEST) Received: from debian (unknown [78.109.66.95]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by agw.arknetworks.am (Postfix) with ESMTPSA id 28CC3E0A8D; Tue, 15 Jul 2025 21:30:17 +0400 (+04) DKIM-Filter: OpenDKIM Filter v2.11.0 agw.arknetworks.am 28CC3E0A8D DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arknetworks.am; s=default; t=1752600617; bh=6yxoXTQ+8RkYAKUb61E/q9Qoba+68uPBjxEiPO4haQk=; h=Date:From:To:cc:Subject:In-Reply-To:References:From; b=D1a3etAJr/7mrIwA7JQ6KdixVyyTBV7CWkiucP49YDRZTqiqomaMP558/SawNWk7T 53bX8uTc36zXWy6IccGtwWZ69Vzk55Y6PRtFCt8JucN452oV/Tauge/4/9ZskIvlT1 VULeBudBqealSbtf9mGR12QgFrsBFyOH3G73+on80PwUlMj2SDEAxQ3CnBt7IUJi6K jYXxGX7pWC2+WJU3Dh+UvonA+JV1ID1d0VHTwdTctwbBh7pTjfDw6gCUmuOTw3GFDQ n4jbeRuGlWOY+KtXhqvFIhzqIi0lyMoOxnKvV98W8KNpVUIxSGoDQtI32XeOhlwzva twKlcHyJOLQSw== Date: Tue, 15 Jul 2025 21:30:09 +0400 (+04) From: Ivan Malov To: Scott Wasson cc: "users@dpdk.org" , Tom Barbette Subject: Re: rte_eth_dev_rss_reta_update() locking considerations? In-Reply-To: Message-ID: <642496a8-cf8a-ed62-d682-11794ed1738b@arknetworks.am> References: MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="8323328-1295376034-1752600617=:7701" X-BeenThere: users@dpdk.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: DPDK usage discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: users-bounces@dpdk.org This message is in MIME format. The first part should be readable text, while the remaining parts are likely unreadable without MIME-aware tools. --8323328-1295376034-1752600617=:7701 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8BIT Hi Scott, On Tue, 15 Jul 2025, Scott Wasson wrote: > > Hi, > >   > > We’re using multiqueue, and RSS doesn’t always balance the load very well.  I had a clever idea to periodically measure the load distribution (cpu load on the IO cores)  in the > background pthread, and use rte_eth_dev_rss_reta_update() to adjust the redirection table dynamically if the imbalance exceeds a given threshold.  In practice it seems to work nicely. As far as I remember, there has already been an academic project [1] that would do (almost) the same thing: dynamically reprogram the table based on the current load status. I vaguely remember mentions of DPDK RETA update in video [2], so perhaps Tom can shed some light on how the API is invoked lock-wise (Cc Tom). [1] https://dejankosticgithub.github.io/documents/publications/rsspp-conext19.pdf [2] https://www.youtube.com/watch?v=YV3aJOxjUqI >   But I’m concerned about: > > https://doc.dpdk.org/api/rte__ethdev_8h.html#a3c1540852c9cf1e576a883902c2e310d  > > Which states: > > > By default, all the functions of the Ethernet Device API exported by a PMD are lock-free functions which assume to not be invoked in parallel on different logical cores to work on the > same target object. For instance, the receive function of a PMD cannot be invoked in parallel on two logical cores to poll the same Rx queue [of the same port]. Of course, this function > can be invoked in parallel by different logical cores on different Rx queues. It is the responsibility of the upper level application to enforce this rule. > >   > > In this context, what is the “target object”?  The queue_id of the port?  Or the port itself?  Would I need to add port-level spinlocks around every invocation of rte_eth_dev_*()?  > That’s a hard no, it would destroy performance. My guess is that it mostly refers to the queue receive/transmit operations. So the target object might be the DMA queue (Rx/Tx) of the device in question. As for the control-plane APIs in 'ethdev', I would imagine the typical usage of these was meant to be via a dedicated control-plane core. So if one can gather load statistics from IO workers in some clever message-oriented way so that the main (control) lcore can read those periodically and invoke RETA update, that would ideally not create contention on the control-plane lock of the port. More to that, in particular, for the RETA update API, I guess many drivers have an implicit/internal port lock in place, so it is going to be leveraged anyway. > >   > > Alternatively, if I were to periodically call rte_eth_dev_rss_reta_update() from the IO cores instead of the background core, as the above paragraph suggests, that doesn’t seem correct > either.  The function takes a reta_conf[] array that affects all RETA entries for that port and maps them to a queue_id.  Is it safe to remap RETA entries for a given port on one IO core > while another IO core is potentially reading from its rx queue for that same port?  That problem seems not much different from remapping in the background core as I am now. That may not be desirable for the reason explained above: implicit port locks in vendor-specific implementations of the control-plane APIs like RETA update. Regarding the question on whether it is safe to reprogram RETA "on the fly" (we may assume this is done from the main/control lcore): it was my impression that doing so would affect some HW component sitting deep in the NIC that distributes packets across DMA queues (which are one level above), not the queues per se, which just contain packets that have been distributed so far. Reprogramming RSS in general might not have that much to do with DMA queues, if I'm not mistaken. > >   > > I’m starting to suspect this function was intended to be initialized once on startup before rte_eth_dev_start(), and/or the ports must be stopped before calling it.  If that’s the case, > then I’ll call this idea too clever by half and give it up now. Not really. There are PMDs that support RETA update in the started state. It should be fine to invoke this API from a single control-plane core. Why not? If the device does not support it in started state, it is the duty of the driver to either remember the table to be set on next port start or return an error. Also, while RETA update API of 'ethdev' is meant to control the "global" RSS setting, there is also RTE flow API's action 'RSS' [3] that can be shared among multiple flow rule objects by means of a container action, 'INDIRECT', [4]. For example, one can create a flow rule generic enough to target a wide subset of flows (or multiple flow rules targeting the same 'INDIRECT' RSS action) and then use an update API [5] to update specifically this shared action. That being said, once again, this update is better be done from some central place (main lcore), as even if the PMD says it supports thread-safe flow API (by setting flag [6]), this may just mean that the driver uses locks internally. However, there might be vendors who support so-called "asynchronous flow API", which uses slightly different flow management APIs ([7], for instance), but those are clearly designed to be invoked by IO workers when they see interesting traffic and either need to insert new flows or update some shared actions "on the fly". [3] https://doc.dpdk.org/api-25.03/rte__flow_8h.html#a78f0386e683cfc491462a771df8b971aa72428c7c1896fe4dfdc2dbed85214d27 [4] https://doc.dpdk.org/api-25.03/rte__flow_8h.html#a78f0386e683cfc491462a771df8b971aa47ea41707def29ff416e233434ab33a6 [5] https://doc.dpdk.org/api-25.03/rte__flow_8h.html#aea5b96385043898923f3b1690a72d2c0 [6] https://doc.dpdk.org/api-25.03/rte__ethdev_8h.html#a3c1540852c9cf1e576a883902c2e310d [7] https://doc.dpdk.org/api-25.03/rte__flow_8h.html#a5097c64396d74102d8f2ae119c9dc7d5 Thank you. > >   > > Thanks in advance for your help! > >   > > -Scott > >   > > > --8323328-1295376034-1752600617=:7701--