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Date: Mon, 17 Mar 2008 20:53:01 +0100
From: Jens Axboe <jens.axboe@...cle.com>
To: "Alan D. Brunelle" <Alan.Brunelle@...com>
Cc: linux-kernel@...r.kernel.org, npiggin@...e.de, dgc@....com
Subject: Re: [Patch (block.git) 2/2] Ensure single IPI generation for SMP call single
On Mon, Mar 17 2008, Jens Axboe wrote:
> On Mon, Mar 17 2008, Alan D. Brunelle wrote:
> > diff --git a/kernel/smp.c b/kernel/smp.c
> > index 7232e1c..f7ec401 100644
> > --- a/kernel/smp.c
> > +++ b/kernel/smp.c
> > @@ -23,28 +23,34 @@ void __cpuinit generic_init_call_single_data(void)
> >
> > void generic_smp_call_function_single_interrupt(void)
> > {
> > - struct call_single_queue *q;
> > unsigned long flags;
> > LIST_HEAD(list);
> > + struct call_single_queue *q = &__get_cpu_var(call_single_queue);
> >
> > - q = &__get_cpu_var(call_single_queue);
> > spin_lock_irqsave(&q->lock, flags);
> > - list_replace_init(&q->list, &list);
> > - spin_unlock_irqrestore(&q->lock, flags);
> > + while (!list_empty(&q->list)) {
> > + list_replace_init(&q->list, &list);
> > + spin_unlock_irqrestore(&q->lock, flags);
> >
> > - while (!list_empty(&list)) {
> > - struct call_single_data *data;
> > + do {
> > + struct call_single_data *data;
> >
> > - data = list_entry(list.next, struct call_single_data, list);
> > - list_del(&data->list);
> > + data = list_entry(list.next, struct call_single_data,
> > + list);
> > + list_del(&data->list);
> >
> > - data->func(data->info);
> > - if (data->flags & CSD_FLAG_WAIT) {
> > - smp_wmb();
> > - data->flags = 0;
> > - } else if (data->flags & CSD_FLAG_ALLOC)
> > - kfree(data);
> > + data->func(data->info);
> > + if (data->flags & CSD_FLAG_WAIT) {
> > + smp_wmb();
> > + data->flags = 0;
> > + } else if (data->flags & CSD_FLAG_ALLOC)
> > + kfree(data);
> > + } while (!list_empty(&list));
> > +
> > + spin_lock_irqsave(&q->lock, flags);
> > }
> > + q->activated = 0;
> > + spin_unlock_irqrestore(&q->lock, flags);
> > }
>
> I agree with doing it this way, re-checking and doing another run (or
> more). However I think we can improve it a bit so we don't always have
> to grab the dst lock at least twice - it should be safe enough to
> include the lock only inside the first loop, doing an smp_mb() before
> the list_empty() check and again at the bottom before looping around and
> doing the list_empty() check again.
>
> I've rolled a new patch series here:
>
> http://git.kernel.dk/?p=linux-2.6-block.git;a=shortlog;h=io-cpu-affinity
>
> (or just pull the io-cpu-affinity branch), it also includes a bunch of
> other cleanups like porting the faster smp_call_function() to ia64 and
> powerpc as well. So the kernel/smp.c generic helpers have grown a bit,
> while the arch bits are smaller.
>
> I'm curious if it now boots on ia64, since I killed the hack to manually
> call the __init manually there. If you could check, I would appreciate
> it ;-)
>
> I've built all 4 supported archs and they compile and link fine, but
> nothing has been booted yet.
x86 and x86-64 boot just fine, so powerpc should work as well (will do
performance testing on a 4-way ppc tomorrow). ia64 should also work, as
long as the init_call_single_data() gets called correctly.
If it doesn't work on tha ia64, try and change the core_initcall() to a
postcore_initcall() or even an arch_initcall(). It now resides in
kernel/smp.c.
--
Jens Axboe
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