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Linville" , "Thomas Monjalon" , , Mattias =?iso-8859-1?Q?R=F6nnblom?= Subject: Re: [RFC v3] net/af_packet: make stats reset reliable Message-ID: References: <20240425174617.2126159-1-ferruh.yigit@amd.com> <20240503154547.392069-1-ferruh.yigit@amd.com> <20240503150011.55681b97@hermes.local> <432fb280-96e3-4079-b987-990d509b8c79@lysator.liu.se> <20240508082339.6bf74202@hermes.local> <834102e5-8f4b-4b67-9f62-8e8f61403b39@amd.com> <20240508135418.55505105@hermes.local> <98CBD80474FA8B44BF855DF32C47DC35E9F431@smartserver.smartshare.dk> Content-Type: text/plain; charset="iso-8859-1" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <98CBD80474FA8B44BF855DF32C47DC35E9F431@smartserver.smartshare.dk> X-ClientProxiedBy: DU6P191CA0046.EURP191.PROD.OUTLOOK.COM (2603:10a6:10:53f::20) To DS0PR11MB7309.namprd11.prod.outlook.com (2603:10b6:8:13e::17) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS0PR11MB7309:EE_|BL1PR11MB6027:EE_ X-MS-Office365-Filtering-Correlation-Id: 8c2d6dc4-cd4b-434d-73bd-08dc700a9a12 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230031|366007|1800799015|376005; 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It would > > break so many things. > > > > It is the kind of stupid compiler thing that would send Linus on a > > rant and get > > > > the GCC compiler writers in trouble. > > > > > > > > The DPDK has always favored performance over strict safety guard > > rails everywhere. > > > > Switching to making every statistic an atomic operation is not in > > the spirit of > > > > what is required. There is no strict guarantee necessary here. > > > > > > > > > > I kind of agree with Stephen. > > > > > > Thanks Mattias, Morten & Stephen, it was informative discussion. But > > for > > > *SW drivers* stats update and reset is not core functionality and I > > > think we can be OK to get hit on corner cases, instead of > > > over-engineering or making code more complex. > > > > > > I forgot the case of 64 bit values on 32 bit platforms! > > Mostly because haven't cared about 32 bit for years... > > > > The Linux kernel uses some wrappers to handle this. > > On 64 bit platforms they become noop. > > On 32 bit platform, they are protected by a seqlock and updates are > > wrapped by the sequence count. > > > > If we go this way, then doing similar Noop on 64 bit and atomic or > > seqlock > > on 32 bit should be done, but in common helper. > > > > Looking inside FreeBSD, it looks like that has changed over the years as > > well. > > > > if_inc_counter > > counter_u64_add > > atomic_add_64 > > But the counters are always per-cpu in this case. So although it does > > use > > locked operation, will always be uncontended. > > > > > > PS: Does DPDK still actually support 32 bit on x86? Can it be dropped > > this cycle? > > We cannot drop 32 bit architecture support altogether. > > But, unlike the Linux kernel, DPDK doesn't need to support ancient 32 bit architectures. > If the few 32 bit architectures supported by DPDK provide non-tearing 64 bit loads/stores, we don't need locks (in the fast path) for 64 bit counters. > > In addition to 32 bit x86, DPDK supports ARMv7-A (a 32 bit architecture) and 32 bit ARMv8. > I don't think DPDK support any other 32 bit architectures. > > > As Mattias mentioned, 32 bit x86 can use xmm registers to provide 64 bit non-tearing load/store. > Testing this a little in godbolt, I see gcc using xmm registers on 32-bit when updating 64-bit counters, but clang doesn't seem to do so, but instead does 2 stores when writing back the 64 value. (I tried with both volatile and non-volatile 64-bit values, just to see if volatile would encourage clang to do a single store). GCC: https://godbolt.org/z/9eqKfT3hz Clang: https://godbolt.org/z/PT5EqKn4c /Bruce