[RFC] irqbalance: Mark in-kernel irqbalance as obsolete, set to N by default

[Auke Kok <auke-jan.h.kok@intel.com>]

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We've recently seen a number of user bug reports against e1000 that the 
in-kernel irqbalance code is detrimental to network latency. The algorithm 
keeps swapping irq's for NICs from cpu to cpu causing extremely high network 
latency (>1000ms). Another NIC driver (cxgb) already has severe warnings in 
their documentation file against using CONFIG_IRQBALANCE, but this is a 
general problem for all NIC drivers and other subsystems. This is especially 
so with cpufreq scaling where the system is slowed down and the migrations 
take much longer.

I suggest that the in-kernel irqbalance is phased out, by marking it OBSOLETE 
first and (perhaps) removing the code later. The userspace irqbalance daemon 
written by Arjan van de Ven does a wonderful job and should be used instead.

Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>

---

  Kconfig |   15 +++++++++++----
  1 file changed, 11 insertions(+), 4 deletions(-)
---

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diff --git a/arch/i386/Kconfig b/arch/i386/Kconfig
index daa75ce..5a40cfe 100644
--- a/arch/i386/Kconfig
+++ b/arch/i386/Kconfig
@@ -690,12 +690,19 @@ config EFI
 	kernel should continue to boot on existing non-EFI platforms.
 
 config IRQBALANCE
- 	bool "Enable kernel irq balancing"
+	bool "Enable kernel irq balancing (obsolete)"
 	depends on SMP && X86_IO_APIC
-	default y
+	default n
 	help
- 	  The default yes will allow the kernel to do irq load balancing.
-	  Saying no will keep the kernel from doing irq load balancing.
+	  The kernel irq balance will migrate interrupts between cpu's
+	  constantly, which may help reduce load in some cases. It is not
+	  beneficial for latency however, and a user-space daemon is available
+	  that does a much better job.
+
+	  The default no will keep the kernel from doing irq load balancing.
+	  Say yes will allow the kernel to do irq load balancing.
+
+	  If unsure, say N.
 
 # turning this on wastes a bunch of space.
 # Summit needs it only when NUMA is on