linux-kernel - Re: smp_call_function

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Date:	Thu, 19 Feb 2015 08:59:17 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Rafael David Tinoco <inaddy@...ntu.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Jens Axboe <axboe@...nel.dk>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Gema Gomez <gema.gomez-solano@...onical.com>,
	chris.j.arges@...onical.com,
	"the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: smp_call_function_single lockups

On Thu, Feb 19, 2015 at 8:32 AM, Rafael David Tinoco <inaddy@...ntu.com> wrote:
> Feb 19 08:21:28 derain kernel: [    3.637682] Switched APIC routing to
> cluster x2apic.

Ok. That "cluster x2apic" mode is just about the nastiest mode when it
comes to sending a single ipi. We do that insane dance where we

 - turn single cpu number into cpumask
 - copy the cpumask to a percpu temporary storage
 - walk each cpu in the cpumask
 - for each cpu, look up the cluster siblings
 - for each cluster sibling that is also in the cpumask, look up the
logical apic mask and add it to the actual ipi destination mask
 - send an ipi to that final mask.

which is just insane. It's complicated, it's fragile, and it's unnecessary.

If we had a simple "send_IPI()" function, we could do this all with
something much saner, and it would look sopmething like

  static void x2apic_send_IPI(int cpu, int vector)
  {
        u32 dest = per_cpu(x86_cpu_to_logical_apicid, cpu);
        x2apic_wrmsr_fence();
        __x2apic_send_IPI_dest(dest, vector, APIC_DEST_LOGICAL);
  }

and then 'void native_send_call_func_single_ipi()' would just look like

  void native_send_call_func_single_ipi(int cpu)
  {
        apic->send_IPI(cpu, CALL_FUNCTION_SINGLE_VECTOR);
  }

but I might have missed something (and we might want to have a wrapper
that says "if the apic doesn't have a 'send_IPI' function, use
"send_IPI_mask(cpumask_of(cpu, vector) instead"

The fact that you need that no_x2apic_optout (which in turn means that
your ACPI tables seem to say "don't use x2apic") also makes me worry.

Are there known errata for the x2apic?

                         Linus
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