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

[David Marchand <david.marchand@6wind.com>]

Hello Jianfeng,

On Wed, Mar 9, 2016 at 2:05 PM, Panu Matilainen <pmatilai@redhat.com> 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.
>
> 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?

+1 and I don't even see why we should have an option to disable this,
since taskset would do the job.

Looking at your special case, if the user did set an isolcpus option
for another use, with no -c/-l, I understand the dpdk application
won't care too much about it.
So, this seems like somehow rude to the rest of the system and unwanted.

We can still help the user starting its application as root (without
taskset) by adding a warning message if a requested cpu (-c / -l ..)
is not part of the available cpus.


-- 
David Marchand