Re: [dpdk-dev] [PATCH] eal: restrict ctrl threads to startup cpu affinity

["Burakov, Anatoly" <anatoly.burakov@intel.com>]

On 14-Feb-19 9:53 AM, David Marchand wrote:
> On Thu, Feb 14, 2019 at 10:39 AM Burakov, Anatoly 
> <anatoly.burakov@intel.com <mailto:anatoly.burakov@intel.com>> wrote:
> 
>     On 13-Feb-19 4:13 PM, David Marchand wrote:
>      > Spawning the ctrl threads on anything that is not part of the eal
>      > coremask is not that polite to the rest of the system.
>      >
>      > Rather than introduce yet another eal options for this, let's take
>      > the startup cpu affinity as a reference and remove the eal coremask
>      > from it.
>      > If no cpu is left, then we default to the master core.
>      >
>      > The cpuset is computed once at init before the original cpu affinity.
>      >
>      > Fixes: d651ee4919cd ("eal: set affinity for control threads")
>      > Signed-off-by: David Marchand <david.marchand@redhat.com
>     <mailto:david.marchand@redhat.com>>
>      > ---
> 
>     Hi David,
> 
>     Maybe i didn't have enough coffee today and i'm missing something here,
>     but how is this different? Removing the coremask cores from the cpuset
>     will effectively "spawn the ctrl threads on anything that is not
>     part of
>     the EAL coremask" (which is "not that polite to the rest of the
>     system"), unless the application was run with taskset.
> 
>     Is "taskset" the key point here? I.e. by default, we're still "not
>     polite", unless the user asks nicely? :)
> 
> 
> Eheh, sorry, yes.
> A bit more context then, if you want to clearly pin cpu resources for 
> the processes on your system (let's say having virtual machines and a 
> popular vswitch), I can only think of two solutions.
> Either you have something to configure your processes to have them call 
> sched_setaffinity/pthread_set_affinity_np, or you use taskset to get 
> them "jailed" without them caring.
> 
> Before the incriminated commit, we were keeping all threads on the 
> coremask that had been passed, but as Olivier said, we would end up with 
> ctrl threads spanwed on core running dataplane threads as well.
> 
> Now, the ctrl threads can be spawned anywhere on all & ~coremask, with 
> no way to configure this.
> I considered adding a new eal option, but I think relying on the current 
> cpu affinity is a better default behavior and I can't see drawbacks at 
> the moment.
> 
> 
> -- 
> David Marchand
> 

OK, that makes sense. However, i feel this behavior (both old and new, 
for that matter) should be better documented somewhere in the EAL docs.

-- 
Thanks,
Anatoly