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[23.128.96.18]) by mx.google.com with ESMTP id e20si6279789pfn.170.2021.05.07.03.24.23; Fri, 07 May 2021 03:24:35 -0700 (PDT) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18; Authentication-Results: mx.google.com; dkim=pass header.i=@walle.cc header.s=mail2016061301 header.b=ARtBpiG2; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235147AbhEGHgU (ORCPT + 99 others); Fri, 7 May 2021 03:36:20 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48946 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233280AbhEGHgT (ORCPT ); Fri, 7 May 2021 03:36:19 -0400 Received: from ssl.serverraum.org (ssl.serverraum.org [IPv6:2a01:4f8:151:8464::1:2]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7EB23C061574; Fri, 7 May 2021 00:35:19 -0700 (PDT) Received: from ssl.serverraum.org (web.serverraum.org [172.16.0.2]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ssl.serverraum.org (Postfix) with ESMTPSA id D20BA2224B; Fri, 7 May 2021 09:35:12 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=walle.cc; s=mail2016061301; t=1620372915; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=xxzUiiFuJ/9Edm/UU/hesO2aF750sfl0VQkiGiRSOrM=; b=ARtBpiG2s/alm7WolB54kzz7ze/VXFEy11dW09cq102bbar3Vu/JhnMAK8M9YJE86/yOui zJvCvYD/UGiouodhZYpW+dJ+IhWBA3zDKkGmEu48zBvb20ii8889ejTS23ajWZvLU/f4WT JB4n9mPb9PT9oOqW76C+D7K1q+yYlnM= MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 7bit Date: Fri, 07 May 2021 09:35:12 +0200 From: Michael Walle To: Xiaoliang Yang Cc: Vladimir Oltean , Vladimir Oltean , UNGLinuxDriver@microchip.com, alexandre.belloni@bootlin.com, allan.nielsen@microchip.com, Claudiu Manoil , davem@davemloft.net, idosch@mellanox.com, joergen.andreasen@microchip.com, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, Po Liu , vinicius.gomes@intel.com Subject: Re: [EXT] Re: [net-next] net: dsa: felix: disable always guard band bit for TAS config In-Reply-To: References: <20210419102530.20361-1-xiaoliang.yang_1@nxp.com> <20210504170514.10729-1-michael@walle.cc> <20210504181833.w2pecbp2qpuiactv@skbuf> <20210504185040.ftkub3ropuacmyel@skbuf> <20210504191739.73oejybqb6z7dlxr@skbuf> <20210504213259.l5rbnyhxrrbkykyg@skbuf> User-Agent: Roundcube Webmail/1.4.11 Message-ID: <2898c3ae1319756e13b95da2b74ccacc@walle.cc> X-Sender: michael@walle.cc Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Xiaoliang, Am 2021-05-07 09:16, schrieb Xiaoliang Yang: > On 2021-05-06 21:25, Michael Walle wrote: >> Am 2021-05-04 23:33, schrieb Vladimir Oltean: >> > [ trimmed the CC list, as this is most likely spam for most people ] >> > >> > On Tue, May 04, 2021 at 10:23:11PM +0200, Michael Walle wrote: >> >> Am 2021-05-04 21:17, schrieb Vladimir Oltean: >> >> > On Tue, May 04, 2021 at 09:08:00PM +0200, Michael Walle wrote: >> >> > > > > > > As explained in another mail in this thread, all queues >> >> > > > > > > are marked as scheduled. So this is actually a no-op, >> >> > > > > > > correct? It doesn't matter if it set or not set for now. Dunno why >> we even care for this bit then. >> >> > > > > > >> >> > > > > > It matters because ALWAYS_GUARD_BAND_SCH_Q reduces the >> >> > > > > > available throughput when set. >> >> > > > > >> >> > > > > Ahh, I see now. All queues are "scheduled" but the guard band >> >> > > > > only applies for "non-scheduled" -> "scheduled" transitions. >> >> > > > > So the guard band is never applied, right? Is that really >> >> > > > > what we want? >> >> > > > >> >> > > > Xiaoliang explained that yes, this is what we want. If the end >> >> > > > user wants a guard band they can explicitly add a "sched-entry >> >> > > > 00" in the tc-taprio config. >> >> > > >> >> > > You're disabling the guard band, then. I figured, but isn't that >> >> > > suprising for the user? Who else implements taprio? Do they do it >> >> > > in the same way? I mean this behavior is passed right to the >> >> > > userspace and have a direct impact on how it is configured. Of >> >> > > course a user can add it manually, but I'm not sure that is what >> >> > > we want here. At least it needs to be documented somewhere. Or >> >> > > maybe it should be a switchable option. >> >> > > >> >> > > Consider the following: >> >> > > sched-entry S 01 25000 >> >> > > sched-entry S fe 175000 >> >> > > basetime 0 >> >> > > >> >> > > Doesn't guarantee, that queue 0 is available at the beginning of >> >> > > the cycle, in the worst case it takes up to + >> >> > > ~12.5us until the frame makes it through (given gigabit and 1518b >> >> > > frames). >> >> > > >> >> > > Btw. there are also other implementations which don't need a >> >> > > guard band (because they are store-and-forward and cound the >> >> > > remaining bytes). So yes, using a guard band and scheduling is >> >> > > degrading the performance. >> >> > >> >> > What is surprising for the user, and I mentioned this already in >> >> > another thread on this patch, is that the Felix switch overruns the >> >> > time gate (a packet taking 2 us to transmit will start transmission >> >> > even if there is only 1 us left of its time slot, delaying the >> >> > packets from the next time slot by 1 us). I guess that this is why >> >> > the ALWAYS_GUARD_BAND_SCH_Q bit exists, as a way to avoid these >> >> > overruns, but it is a bit of a poor tool for that job. Anyway, >> >> > right now we disable it and live with the overruns. >> >> >> >> We are talking about the same thing here. Why is that a poor tool? >> > >> > It is a poor tool because it revolves around the idea of "scheduled >> > queues" and "non-scheduled queues". >> > >> > Consider the following tc-taprio schedule: >> > >> > sched-entry S 81 2000 # TC 7 and 0 open, all others closed >> > sched-entry S 82 2000 # TC 7 and 1 open, all others closed >> > sched-entry S 84 2000 # TC 7 and 2 open, all others closed >> > sched-entry S 88 2000 # TC 7 and 3 open, all others closed >> > sched-entry S 90 2000 # TC 7 and 4 open, all others closed >> > sched-entry S a0 2000 # TC 7 and 5 open, all others closed >> > sched-entry S c0 2000 # TC 7 and 6 open, all others closed >> > >> > Otherwise said, traffic class 7 should be able to send any time it >> > wishes. >> >> What is the use case behind that? TC7 (with the highest priority) may >> always >> take precedence of the other TCs, thus what is the point of having a >> dedicated >> window for the others. >> >> Anyway, I've tried it and there are no hiccups. I've meassured the >> delta >> between the start of successive packets and they are always ~12370ns >> for a >> 1518b frame. TC7 is open all the time, which makes sense. It only >> happens if >> you actually close the gate, eg. you have a sched-entry where a TC7 >> bit is not >> set. In this case, I can see a difference between >> ALWAYS_GUARD_BAND_SCH_Q >> set and not set. If it is set, there is up to a ~12.5us delay added >> (of course it >> depends on when the former frame was scheduled). >> >> It seems that also needs to be 1->0 transition. >> >> You've already mentioned that the switch violates the Qbv standard. >> What makes you think so? IMHO before that patch, it wasn't violated. >> Now it likely is (still have to confirm that). How can this be >> reasonable? >> >> If you have a look at the initial commit message, it is about making >> it possible >> to have a smaller gate window, but that is not possible because of the >> current >> guard band of ~12.5us. It seems to be a shortcut for not having the >> MAXSDU >> (and thus the length of the guard band) configurable. Yes (static) >> guard bands >> will have a performance impact, also described in [1]. You are trading >> the >> correctness of the TAS for performance. And it is the sole purpose of >> Qbv to >> have a determisitc way (in terms of timing) of sending the frames. >> >> And telling the user, hey, we know we violate the Qbv standard, please >> insert >> the guard bands yourself if you really need them is not a real >> solution. As >> already mentioned, (1) it is not documented anywhere, (2) can't be >> shared >> among other switches (unless they do the same workaround) and (3) what >> am >> I supposed to do for TSN compliance testing. Modifying the schedule >> that is >> about to be checked (and thus given by the compliance suite)? >> > I disable the always guard band bit because each gate control list > needs to reserve a guard band slot, which affects performance. The > user does not need to set a guard band for each queue transmission. > For example, "sched-entry S 01 2000 sched-entry S fe 98000". Queue 0 > is protected traffic and has smaller frames, so there is no need to > reserve a guard band during the open time of queue 0. The user can add > the following guard band before protected traffic: "sched-entry S 00 > 25000 sched-entry S 01 2000 sched-entry S fe 73000" Again, you're passing the handling of the guard band to the user, which is an implementation detail for this switch (unless there is a new switch for it on the qdisc IMHO). And (1), (2) and (3) from above is still valid. Consider the entry sched-entry S 01 2000 sched-entry S 02 20000 A user assumes that TC0 is open for 2us. But with your change it is bascially open for 2us + 12.5us. And even worse, it is not deterministic. A frame in the subsequent queue (ie TC1) can be scheduled anywhere beteeen 0us and 12.5us after opening the gate, depending on wether there is still a frame transmitting on TC0. > I checked other devices such as ENETC and it can calculate how long > the frame transmission will take and determine whether to transmit > before the gate is closed. The VSC9959 device does not have this > hardware function. If we implement guard bands on each queue, we need > to reserve a 12.5us guard band for each GCL, even if it only needs to > send a small packet. This confuses customers. How about getting it right and working on how we can set the MAXSDU per queue and thus making the guard band smaller? > actually, I'm not sure if this will violate the Qbv standard. I looked > up the Qbv standard spec, and found it only introduce the guard band > before protected window (Annex Q (informative)Traffic scheduling). It > allows to design the schedule to accommodate the intended pattern of > transmission without overrunning the next gate-close event for the > traffic classes concerned. Vladimir already quoted "IEEE 802.1Q-2018 clause 8.6.8.4". I didn't check it, though. A static guard band is one of the options you have to fulfill that. Granted, it is not that efficient, but it is how the switch handles it. -michael