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

["Tan, Jianfeng" <jianfeng.tan@intel.com>]

Hi Konstantin,

On 3/9/2016 10:44 PM, Ananyev, Konstantin wrote:
>
>> -----Original Message-----
>> From: Tan, Jianfeng
>> Sent: Wednesday, March 09, 2016 2:17 PM
>> To: Ananyev, Konstantin; Panu Matilainen; dev@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@dpdk.org] On Behalf Of Tan, Jianfeng
>>>> Sent: Wednesday, March 09, 2016 1:53 PM
>>>> To: Panu Matilainen; dev@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@intel.com>
>>>>>>>> Acked-by: Neil Horman <nhorman@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.

Then how about make it default in rte_eal_cpu_init()? And it is already 
known it will bring trouble to those use isolcpus users, they need to 
add "taskset [mask]" before starting a DPDK app.

>  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?

I have not see the problem so far, because this work is done before 
parsing coremask (-c), corelist (-l), and coremap (--lcores). If a core 
is disabled here, it's like it is not detected in rte_eal_cpu_init(). Or 
could you please give more hints?

Thanks,
Jianfeng

> Konstantin
>   
>
>