From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from linux.microsoft.com (linux.microsoft.com [13.77.154.182]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 18A81307AD0; Fri, 17 Oct 2025 17:41:21 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=13.77.154.182 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1760722883; cv=none; b=UB/1RbfStKHCGcIlv91fk/aZUfgu5NLMDlB7DgOFe545bpFk+NaPi27aliBBe5eBBNLRvTtceel9Gi4cKOiU8Ty3E1hx/klxK9jz2KVZjyGuFmCIgqErJbgOEDvvz0dKNpV7xjylfVd8wNL5XbuHCxaQgByrm8kFHdvgxNjKUw4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1760722883; c=relaxed/simple; bh=PRfVQxJjy6Vjp+xyCypyGyhNOHUhx/dYPG4ShHWopEk=; h=Message-ID:Date:MIME-Version:Subject:From:To:Cc:References: In-Reply-To:Content-Type; b=ht1N+pMWLEZTGq0CwFFqBRgLSwkOQUXENLiFRtB/Iyq6BwPQXtb+PqfxOh7RVLvu9TnZnSwCHLFfDXi3b0CR73c9izQKMtgEAXBRcQAlo+gC/O/i+RyMjsMig5VtGvPSbdnYjXCfCmss51Rg4FCiY3EKU/zlDZPqhzfxY/DVnCs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.microsoft.com; spf=pass smtp.mailfrom=linux.microsoft.com; dkim=pass (1024-bit key) header.d=linux.microsoft.com header.i=@linux.microsoft.com header.b=mcZxpF2N; arc=none smtp.client-ip=13.77.154.182 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.microsoft.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.microsoft.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.microsoft.com header.i=@linux.microsoft.com header.b="mcZxpF2N" Received: from [192.168.1.19] (unknown [103.212.145.76]) by linux.microsoft.com (Postfix) with ESMTPSA id A03B9201724E; Fri, 17 Oct 2025 10:41:14 -0700 (PDT) DKIM-Filter: OpenDKIM Filter v2.11.0 linux.microsoft.com A03B9201724E DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.microsoft.com; s=default; t=1760722881; bh=Vj1gk3aMFO+P9nUqeopKE4AuncD7z4KeVlwI818/kuQ=; h=Date:Subject:From:To:Cc:References:In-Reply-To:From; b=mcZxpF2N174Av/F08Hahi5sOnNi4wbANuLMTrs8jeuc7aGqKcDFhDpjSjqKb/Cuta K8Oi1Yc9cgeAM45PzaA1ouFsjnbdCIc/eG2aVfLh1CKIvY71FNf3rxs6E+1OiGQ+pA Jx9qOYEy9pHXZKqfKMxFYpTW8gYTkP4Vp5G49D+w= Message-ID: <1d3ac973-7bc7-4abe-9fe2-6b17dbba223b@linux.microsoft.com> Date: Fri, 17 Oct 2025 23:11:11 +0530 Precedence: bulk X-Mailing-List: linux-rdma@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net-next] net: mana: Linearize SKB if TX SGEs exceeds hardware limit From: Aditya Garg To: Eric Dumazet Cc: kys@microsoft.com, haiyangz@microsoft.com, wei.liu@kernel.org, decui@microsoft.com, andrew+netdev@lunn.ch, davem@davemloft.net, kuba@kernel.org, pabeni@redhat.com, longli@microsoft.com, kotaranov@microsoft.com, horms@kernel.org, shradhagupta@linux.microsoft.com, ernis@linux.microsoft.com, dipayanroy@linux.microsoft.com, shirazsaleem@microsoft.com, linux-hyperv@vger.kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, linux-rdma@vger.kernel.org, gargaditya@microsoft.com, ssengar@linux.microsoft.com References: <20251003154724.GA15670@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net> <9d886861-2e1f-4ea8-9f2c-604243bd751b@linux.microsoft.com> <7bc327ba-0050-4d9e-86b6-1b7427a96f53@linux.microsoft.com> Content-Language: en-US In-Reply-To: <7bc327ba-0050-4d9e-86b6-1b7427a96f53@linux.microsoft.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 08-10-2025 20:58, Aditya Garg wrote: > On 08-10-2025 20:51, Eric Dumazet wrote: >> On Wed, Oct 8, 2025 at 8:16 AM Aditya Garg >> wrote: >>> >>> On 03-10-2025 21:45, Eric Dumazet wrote: >>>> On Fri, Oct 3, 2025 at 8:47 AM Aditya Garg >>>> wrote: >>>>> >>>>> The MANA hardware supports a maximum of 30 scatter-gather entries >>>>> (SGEs) >>>>> per TX WQE. In rare configurations where MAX_SKB_FRAGS + 2 exceeds >>>>> this >>>>> limit, the driver drops the skb. Add a check in mana_start_xmit() to >>>>> detect such cases and linearize the SKB before transmission. >>>>> >>>>> Return NETDEV_TX_BUSY only for -ENOSPC from >>>>> mana_gd_post_work_request(), >>>>> send other errors to free_sgl_ptr to free resources and record the tx >>>>> drop. >>>>> >>>>> Signed-off-by: Aditya Garg >>>>> Reviewed-by: Dipayaan Roy >>>>> --- >>>>>    drivers/net/ethernet/microsoft/mana/mana_en.c | 26 +++++++++++++ >>>>> ++---- >>>>>    include/net/mana/gdma.h                       |  8 +++++- >>>>>    include/net/mana/mana.h                       |  1 + >>>>>    3 files changed, 29 insertions(+), 6 deletions(-) >>>>> >>>>> diff --git a/drivers/net/ethernet/microsoft/mana/mana_en.c b/ >>>>> drivers/net/ethernet/microsoft/mana/mana_en.c >>>>> index f4fc86f20213..22605753ca84 100644 >>>>> --- a/drivers/net/ethernet/microsoft/mana/mana_en.c >>>>> +++ b/drivers/net/ethernet/microsoft/mana/mana_en.c >>>>> @@ -20,6 +20,7 @@ >>>>> >>>>>    #include >>>>>    #include >>>>> +#include >>>>> >>>>>    static DEFINE_IDA(mana_adev_ida); >>>>> >>>>> @@ -289,6 +290,19 @@ netdev_tx_t mana_start_xmit(struct sk_buff >>>>> *skb, struct net_device *ndev) >>>>>           cq = &apc->tx_qp[txq_idx].tx_cq; >>>>>           tx_stats = &txq->stats; >>>>> >>>>> +       BUILD_BUG_ON(MAX_TX_WQE_SGL_ENTRIES != >>>>> MANA_MAX_TX_WQE_SGL_ENTRIES); >>>>> +       #if (MAX_SKB_FRAGS + 2 > MANA_MAX_TX_WQE_SGL_ENTRIES) >>>>> +               if (skb_shinfo(skb)->nr_frags + 2 > >>>>> MANA_MAX_TX_WQE_SGL_ENTRIES) { >>>>> +                       netdev_info_once(ndev, >>>>> +                                        "nr_frags %d exceeds max >>>>> supported sge limit. Attempting skb_linearize\n", >>>>> +                                        skb_shinfo(skb)->nr_frags); >>>>> +                       if (skb_linearize(skb)) { >>>> >>>> This will fail in many cases. >>>> >>>> This sort of check is better done in ndo_features_check() >>>> >>>> Most probably this would occur for GSO packets, so can ask a software >>>> segmentation >>>> to avoid this big and risky kmalloc() by all means. >>>> >>>> Look at idpf_features_check()  which has something similar. >>> >>> Hi Eric, >>> Thank you for your review. I understand your concerns regarding the use >>> of skb_linearize() in the xmit path, as it can fail under memory >>> pressure and introduces additional overhead in the transmit path. Based >>> on your input, I will work on a v2 that will move the SGE limit check to >>> the ndo_features_check() path and for GSO skbs exceding the hw limit >>> will disable the NETIF_F_GSO_MASK to enforce software segmentation in >>> kernel before the call to xmit. >>> Also for non GSO skb exceeding the SGE hw limit should we go for using >>> skb_linearize only then or would you suggest some other approach here? >> >> I think that for non GSO, the linearization attempt is fine. >> >> Note that this is extremely unlikely for non malicious users, >> and MTU being usually small (9K or less), >> the allocation will be much smaller than a GSO packet. > > Okay. Will send a v2 Hi Eric, I tested the code by disabling GSO in ndo_features_check when the number of SGEs exceeds the hardware limit, using iperf for a single TCP connection with zerocopy enabled. I noticed a significant difference in throughput compared to when we linearize the skbs. For reference, the throughput is 35.6 Gbits/sec when using skb_linearize, but drops to 6.75 Gbits/sec when disabling GSO per skb. Hence, We propose to linearizing skbs until the first failure occurs. After that, we switch to a fail-safe mode by disabling GSO for SKBs with sge > hw limit using the ndo_feature_check implementation, while continuing to apply skb_linearize() for non-GSO packets that exceed the hardware limit. This ensures we remain on the optimal performance path initially, and only transition to the fail-safe path after encountering a failure. Regards, Aditya