[dpdk-dev] [PATCH] eal: add option --avail-cores to detect lcores

Ananyev, Konstantin konstantin.ananyev at intel.com
Wed Mar 9 15:44:09 CET 2016



> -----Original Message-----
> From: Tan, Jianfeng
> Sent: Wednesday, March 09, 2016 2:17 PM
> To: Ananyev, Konstantin; Panu Matilainen; dev at dpdk.org
> Subject: Re: [dpdk-dev] [PATCH] eal: add option --avail-cores to detect lcores
> 
> 
> 
> On 3/9/2016 10:01 PM, Ananyev, Konstantin wrote:
> >
> >> -----Original Message-----
> >> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Tan, Jianfeng
> >> Sent: Wednesday, March 09, 2016 1:53 PM
> >> To: Panu Matilainen; dev at dpdk.org
> >> Subject: Re: [dpdk-dev] [PATCH] eal: add option --avail-cores to detect lcores
> >>
> >>
> >>
> >> On 3/9/2016 9:05 PM, Panu Matilainen wrote:
> >>> On 03/08/2016 07:38 PM, Tan, Jianfeng wrote:
> >>>> Hi Panu,
> >>>>
> >>>> On 3/8/2016 4:54 PM, Panu Matilainen wrote:
> >>>>> On 03/04/2016 12:05 PM, Jianfeng Tan wrote:
> >>>>>> This patch adds option, --avail-cores, to use lcores which are
> >>>>>> available
> >>>>>> by calling pthread_getaffinity_np() to narrow down detected cores
> >>>>>> before
> >>>>>> parsing coremask (-c), corelist (-l), and coremap (--lcores).
> >>>>>>
> >>>>>> Test example:
> >>>>>> $ taskset 0xc0000 ./examples/helloworld/build/helloworld \
> >>>>>>          --avail-cores -m 1024
> >>>>>>
> >>>>>> Signed-off-by: Jianfeng Tan <jianfeng.tan at intel.com>
> >>>>>> Acked-by: Neil Horman <nhorman at tuxdriver.com>
> >>>>> Hmm, to me this sounds like something that should be done always so
> >>>>> there's no need for an option. Or if there's a chance it might do the
> >>>>> wrong thing in some rare circumstance then perhaps there should be a
> >>>>> disabler option instead?
> >>>> Thanks for comments.
> >>>>
> >>>> Yes, there's a use case that we cannot handle.
> >>>>
> >>>> If we make it as default, DPDK applications may fail to start, when user
> >>>> specifies a core in isolcpus and its parent process (say bash) has a
> >>>> cpuset affinity that excludes isolcpus. Originally, DPDK applications
> >>>> just blindly do pthread_setaffinity_np() and it always succeeds because
> >>>> it always has root privilege to change any cpu affinity.
> >>>>
> >>>> Now, if we do the checking in rte_eal_cpu_init(), those lcores will be
> >>>> flagged as undetected (in my older implementation) and leads to failure.
> >>>> To make it correct, we would always add "taskset mask" (or other ways)
> >>>> before DPDK application cmd lines.
> >>>>
> >>>> How do you think?
> >>> I still think it sounds like something that should be done by default
> >>> and maybe be overridable with some flag, rather than the other way
> >>> around. Another alternative might be detecting the cores always but if
> >>> running as root, override but with a warning.
> >> For your second solution, only root can setaffinity to isolcpus?
> >> Your first solution seems like a promising way for me.
> >>
> >>> But I dont know, just wondering. To look at it from another angle: why
> >>> would somebody use this new --avail-cores option and in what
> >>> situation, if things "just work" otherwise anyway?
> >> For DPDK applications, the most common case to initialize DPDK is like
> >> this: "$dpdk-app [options for DPDK] -- [options for app]", so users need
> >> to specify which cores to run and how much hugepages are used. Suppose
> >> we need this dpdk-app to run in a container, users already give those
> >> information when they build up the cgroup for it to run inside, this
> >> option or this patch is to make DPDK more smart to discover how much
> >> resource will be used. Make sense?
> > But then, all we need might be just a script that would extract this information from the system
> > and form a proper cmdline parameter for DPDK?
> 
> Yes, a script will work. Or to construct (argc, argv) to call
> rte_eal_init() in the application. But as Neil Horman once suggested, a
> simple pthread_getaffinity_np() will get all things done. So if it worth
> a patch here?

Don't know...
Personally I would prefer not to put extra logic inside EAL.
For me - there are too many different options already.
From other side looking at the patch itself:
You are updating lcore_count and lcore_config[],based on physical cpu availability,
but these days it is not always one-to-one mapping between EAL lcore and physical cpu. 
Shouldn't that be taken into account?
Konstantin
 




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