Re: [dpdk-dev] [PATCH] vdpa/mlx5: use master core Id for vhost thread

[Matan Azrad <matan@mellanox.com>]

Hi David

We found that our kernel driver  doesn't really need it and we didn't understand
the internal API correctly.

We will create v2 as fix to the vdpa code. 

Thank you for the review!

Matan

From: Xueming(Steven) Li:
> > -----Original Message-----
> > From: David Marchand <david.marchand@redhat.com>
> > Sent: Tuesday, July 28, 2020 3:53 PM
> > To: Matan Azrad <matan@mellanox.com>
> > Cc: Xueming(Steven) Li <xuemingl@mellanox.com>; dev <dev@dpdk.org>;
> > Asaf Penso <asafp@mellanox.com>
> > Subject: Re: [dpdk-dev] [PATCH] vdpa/mlx5: use master core Id for
> > vhost thread
> >
> > On Tue, Jul 28, 2020 at 9:30 AM Matan Azrad <matan@mellanox.com>
> wrote:
> > > > > > rte_lcore_to_cpu_id(-1) is supposed to return the current lcore id.
> > > > > > As explained in the commitlog of 67ae5936c4fc36, for a non-EAL
> > > > > > thread (which has no associated lcore by default) calling this
> > > > > > function means lcore_config[-1].core_id.
> > > > >
> > > > > Deosn't lcore mean core?
> > > >
> > > > No.
> > >
> > > So, it may be that we have more than one lcore working on a specific
> cpu_id?
> > > Can you extend here?
> >
> > A control thread is a non-EAL thread.
> 
> Is the default thread a control thread? After eal init, rte_lcore_to_cpu_id(-1)
> returns 0.
> 
> In function rte_ctrl_thread_create(), the new thread is set cpu affinity with
> control cpu set, bind it same core of default control thread.
> 
> > A non-EAL thread has no lcore associated (unless you call the newly
> > added rte_thread_register API).
> >
> > Not sure why you need the physical core identifier in the driver.
> >
> > It is possible that non-EAL threads run on one physical core if you
> > start your application with the right cpuset.
> > But without checking, you can't make such an assumption.
> >
> >
> > >
> > > >
> > > > > So, if the private thread run on a core that opened by EAL,
> > > > > Shouldn't the API return the lcore of the private thread core?
> > > >
> > > > vhost-events is a control thread.
> > > > Was it working just by chance and returning 0 before?
> > >
> > > Probably yes.
> >
> > (gdb) info symbol &lcore_config[-1].core_id vfio_cfgs + 444820 in
> > section .bss of
> > /home/dmarchan/git/pub/dpdk.org/build/app/../lib/librte_eal.so.20.0
> >
> > Please, update the commitlog.
> > Thanks.
> >
> >
> > --
> > David Marchand