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[146.241.44.112]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4318b58b7e8sm182571125e9.48.2024.10.29.09.08.38 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 29 Oct 2024 09:08:40 -0700 (PDT) Message-ID: <1201dfde-c552-4f61-b2bd-b3415eb7b4ed@redhat.com> Date: Tue, 29 Oct 2024 17:08:37 +0100 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 net-next 1/1] sched: Add dualpi2 qdisc To: "Chia-Yu Chang (Nokia)" , "netdev@vger.kernel.org" , "davem@davemloft.net" , "stephen@networkplumber.org" , "jhs@mojatatu.com" , "edumazet@google.com" , "kuba@kernel.org" , "dsahern@kernel.org" , "ij@kernel.org" , "ncardwell@google.com" , "Koen De Schepper (Nokia)" , "g.white@CableLabs.com" , "ingemar.s.johansson@ericsson.com" , "mirja.kuehlewind@ericsson.com" , "cheshire@apple.com" , "rs.ietf@gmx.at" , "Jason_Livingood@comcast.com" , "vidhi_goel@apple.com" Cc: Olga Albisser , "Olivier Tilmans (Nokia)" , Henrik Steen , Bob Briscoe References: <20241021221248.60378-1-chia-yu.chang@nokia-bell-labs.com> <20241021221248.60378-2-chia-yu.chang@nokia-bell-labs.com> Content-Language: en-US From: Paolo Abeni In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/29/24 16:27, Chia-Yu Chang (Nokia) wrote: > On Tuesday, October 29, 2024 1:56 PM Paolo Abeni wrote: >> On 10/22/24 00:12, chia-yu.chang@nokia-bell-labs.com wrote: >>> +/* Default alpha/beta values give a 10dB stability margin with >>> +max_rtt=100ms. */ static void dualpi2_reset_default(struct >>> +dualpi2_sched_data *q) { >>> + q->sch->limit = 10000; /* Max 125ms at 1Gbps */ >>> + >>> + q->pi2.target = 15 * NSEC_PER_MSEC; >>> + q->pi2.tupdate = 16 * NSEC_PER_MSEC; >>> + q->pi2.alpha = dualpi2_scale_alpha_beta(41); /* ~0.16 Hz * 256 */ >>> + q->pi2.beta = dualpi2_scale_alpha_beta(819); /* ~3.20 Hz * 256 */ >>> + >>> + q->step.thresh = 1 * NSEC_PER_MSEC; >>> + q->step.in_packets = false; >>> + >>> + dualpi2_calculate_c_protection(q->sch, q, 10); /* wc=10%, >>> + wl=90% */ >>> + >>> + q->ecn_mask = INET_ECN_ECT_1; >>> + q->coupling_factor = 2; /* window fairness for equal RTTs */ >>> + q->drop_overload = true; /* Preserve latency by dropping */ >>> + q->drop_early = false; /* PI2 drops on dequeue */ >>> + q->split_gso = true; > >> This is a very unexpected default. Splitting GSO packets earlier WRT the H/W constaints definitely impact performances in a bad way. > >> Under which condition this is expected to give better results? >> It should be at least documented clearly. > > I see a similar operation exists in other qdisc (e.g., sch_tbf.c and sch_cake). They both walk through segs of skb_list. > Instead, I see other qdisc use "skb_list_walk_safe" macro, so I was thinking to follow a similar approach in dualpi2 (or other comments please let me know). > Or do you suggest we should force gso-splitting like in other qdisc? The main point is not traversing an skb list, but the segmentation this scheduler performs by default. Note that the sch_tbf case is slightly different, as it segments skbs as a last resort to avoid dropping packets exceeding the burst limit. You should provide some more wording or test showing when and how such splitting is advantageous - i.e. as done in the cake scheduler in commit 2db6dc2662bab14e59517ab4b86a164cc4d2db42. The reason for the above is that performing unneeded S/W segmentation is, generally speaking, a huge loss. Thanks, Paolo