Re: [PATCH v2 3/4] kthread: NUMA aware kthread_create_on_cpu()

Andrew Morton
2010-12-10T06:32:10Z

On Fri, 10 Dec 2010 06:59:58 +0100 Eric Dumazet <eric.dumazet@gmail.com> wrote:

> Le jeudi 09 d__cembre 2010 __ 16:44 -0800, Andrew Morton a __crit :
>
> > The name "kthread_create_on_cpu" is pretty misleading.
> >
> > One would expect such a function to create a kthread which is bound to
> > that CPU. But what it in fact does is to create a kthread which is
> > bound to all CPUs and whose stack, task_struct and thread_info were
> > allocated from the node which contains `cpu'.
> >
> > Also, a saner interface would be one which takes the numa_node_id, not
> > the cpu number.
> >
>
>
> > >
> > > ...
> > >
> > > /**
> > > - * kthread_create - create a kthread.
> > > + * kthread_create_on_cpu - create a kthread.
> > > * @threadfn: the function to run until signal_pending(current).
> > > * @data: data ptr for @threadfn.
> > > + * @cpu: cpu number.
> > > * @namefmt: printf-style name for the thread.
> > > *
> > > * Description: This helper function creates and names a kernel
> > > * thread. The thread will be stopped: use wake_up_process() to start
> > > * it. See also kthread_run().
> > > *
> > > + * If thread is going to be bound on a particular cpu, give its number
> > > + * in @cpu, to get NUMA affinity for kthread stack, or else give -1.
> >
> > This is a bit presumptuous. The caller might wish to later bind this
> > thread to some or all of the CPUs on the node, rather than to a single
> > CPU (eg, kswapd()).
> >
> >
> > So what to do? Maybe add a new kthread_create_node() which prepares a
> > kthread whose memory is bound to that node, then add a
> > kthread_create_cpu() convenience wrapper around that?
> >
>
> We probably can add the "bind to cpu" as a fifth patch, to avoid one
> kthread_bind(p, cpu); done by these callers.
>
> My reasoning not including this kthread_bind(p, cpu) in initial patch
> series that I was focusing on NUMA properties first, not on scheduling
> (this part already runs correctly as far as I know)
>
> Thanks for taking the patch series, I was about to resubmit it today :)
>

but but but. The name "kthread_create_on_cpu" sucks. It's plain wrong.