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[204.195.22.127]) by smtp.gmail.com with ESMTPSA id 78sm765576pge.58.2020.02.21.08.38.36 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 21 Feb 2020 08:38:37 -0800 (PST) Date: Fri, 21 Feb 2020 08:38:28 -0800 From: Stephen Hemminger To: Aaron Conole Cc: David Marchand , "Song\, Keesang" , "ktraynor\@redhat.com" , "bluca\@debian.org" , Thomas Monjalon , "dev\@dpdk.org" , "ferruh.yigit\@intel.com" , "bruce.richardson\@intel.com" , "honnappa.nagarahalli\@arm.com" , "drc\@linux.vnet.ibm.com" , "stable\@dpdk.org" , "Grimm\, Jon" , "Hollingsworth\, Brent" Message-ID: <20200221083828.4b39a854@hermes.lan> In-Reply-To: References: <20191202153559.9709-1-david.marchand@redhat.com> <2076701.vBoWY3egPC@xps> <5572457.lOV4Wx5bFT@xps> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Subject: Re: [dpdk-dev] [PATCH 0/4] Extend --lcores to run on cores > RTE_MAX_LCORE X-BeenThere: dev@dpdk.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org Sender: "dev" On Fri, 21 Feb 2020 09:48:58 -0500 Aaron Conole wrote: > David Marchand writes: > > > On Fri, Feb 21, 2020 at 9:19 AM Song, Keesang wrote: > >> > >> [AMD Official Use Only - Internal Distribution Only] > > > > Please, get this header removed. > > This is a public mailing list. > > > > > >> Thanks Thomas for bringing this up. > >> I consider this is not a new feature, but rather a fix to address > >> the issue with statically assigned maximum lcore limit on > >> high-density CPU platform such as AMD Epyc. > >> As I see a lot of DPDK adopters are still using LTS 18.11 & 19.11, > >> and they have 1~2 yrs of lifetime left, we like to backport this to > >> LTS 18.11 & 19.11 at least. > > > > It is not a fix. > > > > The use of static arrays is a design choice that goes back to the > > early days in dpdk. > > The addition of --lcores came in after this, but it was introduced for > > a different use case than placing lcores on any physical core. > > And there was no claim that a core > RTE_MAX_LCORE would be usable. > > > > > > When testing on a new hardware, it is normal to observe some limitations. > > Running DPDK on those platforms should be possible: "should be" > > because I do not have access to this hardware and saw neither tests > > reports nor performance numbers. > > Before this patch, the limitation is that on Epyc, cores > > > RTE_MAX_LCORE are not usable. > > > > > > Now, this change is quite constrained. > > If we backport it, I don't expect issues in the main dpdk components > > (based on code review and ovs tests with a RTE_MAX_LCORE set to 16 on > > a 24 cores system). > > There might be issues in some examples or not widely used library > > which uses a physical core id instead of a lcore id. > > > > > > This is the same recurring question "do we allow new features in a > > stable branch?". > > Usually, the answer is 'no'. But we do allow some "new" things to be > backported (pci ids, etc) that might be required to enable older > functionality. Additionally, I'm sure if some feature were required to > mitigate a CVE, we'd rather favor backporting it. > > I guess we could pose a litmus test: > > 1. Is the problem this feature solves so widespread that it needs to > be addressed ASAP? > 2. Is there a known workaround to the problem this is solving? > 3. How intrusive is the feature? > 4. Is it shown to be stable in the mainline (number of fixes, testing, > etc)? > 5. Is it constrained enough that we know we can support it with even > higher priority than other things? > > Probably other questions that will need to be asked. > > And even in that list of question, I'm not sure I'd be able to advocate > backporting this in the upstream branches - it hasn't had much testing. > It's unstable. It's "difficult" to use. It is not widespread that > people have so many cores. The workaround is much simpler than > supporting this (recompile). > > > > > -- > > David Marchand > RTE_MAX_LCORES is exposed in API/ABI to application. Many applications use that to size internal data structures. Having rte_lcore_id() potentially return a larger value would cause out of bounds access (and crash) in that application.