[RFC PATCH 0/10] apic_wait_icr_idle issues and possible solutions

Vivek Goyal vgoyal at in.ibm.com
Thu Apr 26 02:48:06 EDT 2007

On Wed, Apr 25, 2007 at 08:03:04PM +0900, Fernando Luis Vázquez Cao wrote:
> static __inline__ void apic_wait_icr_idle(void)
> {
>   while (apic_read(APIC_ICR) & APIC_ICR_BUSY)
>     cpu_relax();
> }
> The busy loop in this function would not be problematic if the
> corresponding status bit in the ICR were always updated, but that does
> not seem to be the case under certain crash scenarios. As an example,
> when the other CPUs are locked-up inside the NMI handler the CPU that
> sends the IPI will end up looping forever in the ICR check, effectively
> hard-locking the whole system.
> Quoting from Intel's "MultiProcessor Specification" (Version 1.4), B-3:
> "A local APIC unit indicates successful dispatch of an IPI by
> resetting the Delivery Status bit in the Interrupt Command
> Register (ICR). The operating system polls the delivery status
> bit after sending an INIT or STARTUP IPI until the command has
> been dispatched.
> A period of 20 microseconds should be sufficient for IPI dispatch
> to complete under normal operating conditions. If the IPI is not
> successfully dispatched, the operating system can abort the
> command. Alternatively, the operating system can retry the IPI by
> writing the lower 32-bit double word of the ICR. This “time-out”
> mechanism can be implemented through an external interrupt, if
> interrupts are enabled on the processor, or through execution of
> an instruction or time-stamp counter spin loop."
> Intel's documentation suggests the implementation of a time-out
> mechanism, which, by the way, is already being open-coded in some parts
> of the kernel that tinker with ICR.
> --- Possible solutions
> * Solution A: Implement the time-out mechanism in apic_wait_icr_idle.
> The problem with this approach is that introduces a performance penalty
> that may not be acceptable for some callers of apic_wait_icr_idle.
> Besides, during normal operation delivery errors should not occur. This
> brings us to solution B.

Hi Fernando,

How much is the performance penalty? Is it really significant. My point
is that, to me changing apic_wait_icr_dle() itself seems to be the simple
approach instead of introducing another function.

Original implementation is:

static __inline__ void apic_wait_icr_idle(void)
	while (apic_read(APIC_ICR) & APIC_ICR_BUSY)

And new one will look something like.

        do {
                send_status = apic_read(APIC_ICR) & APIC_ICR_BUSY;
                if (!send_status)
        } while (timeout++ < 1000);

There will be at max 100 microsecond delay before you realize that IPI has
been dispatched. To optimize it further we can change it to 10 microsecond

        do {
                send_status = apic_read(APIC_ICR) & APIC_ICR_BUSY;
                if (!send_status)
        } while (timeout++ < 10000);
or may be

        do {
                send_status = apic_read(APIC_ICR) & APIC_ICR_BUSY;
                if (!send_status)
        } while (timeout++ < 100000);

I don't know if 1 micro second delay is supported. I do see it being
used in kernel/hpet.c

Is it too much of performance overhead? Somebody who knows more about it
needs to tell. To me changing apic_wait_icr_idle() seems simple instead
of introducing a new function and then making a special case for NMI.


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