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

[Jens Axboe]

On Tue, Apr 22 2008, Jens Axboe wrote:
> 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@xxxxxx> 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.

We don't need any extra statically allocated data, we can just reuse the
'csd' element of the existing call_data_fallback. So that is what I did.
Once all archs are converted, we can now change
smp_call_function_single() to a void return, as it always succeeds now.

-- 
Jens Axboe

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