Re: [PATCH 1/11] Add generic helpers for arch IPI function calls

Jens Axboe
2008-04-22T09:22:46Z

On Tue, Apr 22 2008, Avi Kivity wrote:
> Jens Axboe wrote:
> >This adds kernel/smp.c which contains helpers for IPI function calls. In
> >addition to supporting the existing smp_call_function() in a more efficient
> >manner, it also adds a more scalable variant called
> >smp_call_function_single()
> >for calling a given function on a single CPU only.
> >
> >The core of this is based on the x86-64 patch from Nick Piggin, lots of
> >changes since then. "Alan D. Brunelle" <Alan.Brunelle@hp.com> has
> >contributed lots of fixes and suggestions as well.
> >
> >+int smp_call_function_single(int cpu, void (*func) (void *info), void
> >*info,
> >+ int retry, int wait)
> >+{
> >+ unsigned long flags;
> >+ /* prevent preemption and reschedule on another processor */
> >+ int me = get_cpu();
> >+ int ret = 0;
> >+
> >+ /* Can deadlock when called with interrupts disabled */
> >+ WARN_ON(wait && irqs_disabled());
> >+
> >+ if (cpu == me) {
> >+ local_irq_save(flags);
> >+ func(info);
> >+ local_irq_restore(flags);
> >+ } else {
> >+ struct call_single_data d;
> >+ struct call_single_data *data;
> >+
> >+ if (!wait) {
> >+ data = kmalloc(sizeof(*data), GFP_ATOMIC);
> >+ if (unlikely(!data)) {
> >+ ret = -ENOMEM;
> >+ goto out;
> >+ }
> >+ data->flags = CSD_FLAG_ALLOC;
> >+ } else {
> >+ data = &d;
> >+ data->flags = CSD_FLAG_WAIT;
> >+ }
> >+
> >
>
> Instead of introducing a rare error case, how about falling back to the
> wait case if the allocation fails?
>
> Of course, if the called function relies on the calling cpu doing
> something else, then this fails, but I don't think anybody would do
> that? On the other hand, there is at least one use of
> smp_call_function_single() with !wait, which doesn't check the error return.

Sure, either failling back to waiting, or add a static call_single_data
like it exists for smp_call_function(). In reality it'll never happen,
so the fallback static structure appeals the most to me.

--
Jens Axboe