| Message ID | 20130214205304.993009840@linutronix.de (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Thu, Feb 14, 2013 at 08:53:53PM -0000, Thomas Gleixner wrote: > All timer interrupts and the perf interrupt are marked NO_THREAD, so > its safe to allow forced interrupt threading. What effect does this have on IRQ latency, particularly on the slower CPUs? If it adds to the IRQ latency, then it must be conditional; there are platforms where IRQ latency is important for things like network devices to keep up (particularly the SMC NICs).
On Thu, 14 Feb 2013, Russell King - ARM Linux wrote: > On Thu, Feb 14, 2013 at 08:53:53PM -0000, Thomas Gleixner wrote: > > All timer interrupts and the perf interrupt are marked NO_THREAD, so > > its safe to allow forced interrupt threading. > > What effect does this have on IRQ latency, particularly on the slower CPUs? > > If it adds to the IRQ latency, then it must be conditional; there are > platforms where IRQ latency is important for things like network devices > to keep up (particularly the SMC NICs). It does not affect anything unless you add "threadirqs" to the kernel command line. This command line argument puts the kernel into forced irq threading mode. Otherwise it just works as now. And yes, if enabled on the command line it adds latency due to the delayed invocation of the handler in the thread. Though for mainline this is primary a debug mechanism where you can morph the kernel explosion in the interrupt handler into a takedown of the handler thread instead of crashing the machine completely. Now on RT we force this and I can tell you that even on slow machines with SMC NICs the overhead seems to be in a range where these systems can cope with. Of course, if you tweak the scheduling params of the irq thread low enough you can run into problematic situations. But that's not an ARM specific problem, that's a general problem of configuring a RT system. Thanks, tglx
Index: linux-2.6/arch/arm/Kconfig =================================================================== --- linux-2.6.orig/arch/arm/Kconfig +++ linux-2.6/arch/arm/Kconfig @@ -17,6 +17,7 @@ config ARM select GENERIC_STRNCPY_FROM_USER select GENERIC_STRNLEN_USER select HARDIRQS_SW_RESEND + select IRQ_FORCED_THREADING select HAVE_AOUT select HAVE_ARCH_JUMP_LABEL if !XIP_KERNEL select HAVE_ARCH_KGDB
All timer interrupts and the perf interrupt are marked NO_THREAD, so its safe to allow forced interrupt threading. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> --- arch/arm/Kconfig | 1 + 1 file changed, 1 insertion(+)