Re: [RFC][PATCH] i386 x86-64 Eliminate Local APIC timer interrupt

From: Zwane Mwaikambo
Date: Fri Apr 29 2005 - 19:47:15 EST

On Fri, 29 Apr 2005, Venkatesh Pallipadi wrote:

> Background:
> Local APIC timer stops functioning when CPU is in C3 state. As a
> result the local APIC timer interrupt will fire at uncertain times, depending
> on how long we spend in C3 state. And this has two side effects
> * Idle balancing will not happen as we expect it to.
> * Kernel statistics for idle time will not be proper (as we get less LAPIC
> interrupts when we are idle). This can result in confusing other parts of
> kernel (like ondemand cpufreq governor) which depends on this idle stats.
> Proposed Fix:
> Attached is a prototype patch, that tries to eliminate the dependency on
> local APIC timer for update_process_times(). The patch gets rid of Local APIC
> timer altogether. We use the timer interrupt (IRQ 0) configured in
> broadcast mode in IOAPIC instead (Doesn't work with 8259).
> As changing anything related to basic timer interrupt is a little bit risky,
> I have a boot parameter currently ("useapictimer") to switch back to original
> local APIC timer way of doing things.

This all looks like it'll contend on irq0 related locks really badly, have
you profiled this?


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