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

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Date:	Tue, 22 Apr 2008 11:22:30 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Avi Kivity <avi@...ranet.com>
Cc:	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	npiggin@...e.de, torvalds@...ux-foundation.org
Subject: Re: [PATCH 1/11] Add generic helpers for arch IPI function calls

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@...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

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