One of our drivers has code that was originally running on a DSP. The
code makes heavy use of floating point. We have isolated all the
floating point to one kthread in the driver. Using enable_kernel_fp()
this has worked well.
But under a specific heavy RTP load, we started getting kernel panics.
To make a long story short, the scheduler disables FP when you are
context switched out. When you come back and access a FP instruction,
you trap and call load_up_fpu() and everything is fine..... unless you
are in the kernel. If you are in the kernel, like our kthread is, you
get a "kernel FP unavailable exception".
Basically we got away with it for two years because the thread is at
high priority (-20) and tries very hard to finish within 1ms. But the
RTP high load causes us to context switch out and crash. The following
patch fixes this:
diff --git a/arch/powerpc/kernel/head_booke.h b/arch/powerpc/kernel/head_booke.h
index 50504ae..3476de9 100644
--- a/arch/powerpc/kernel/head_booke.h
+++ b/arch/powerpc/kernel/head_booke.h
@@ -383,7 +383,7 @@ label:
#define FP_UNAVAILABLE_EXCEPTION \
START_EXCEPTION(FloatingPointUnavailable) \
NORMAL_EXCEPTION_PROLOG; \
- beq 1f; \
+ /* SAM beq 1f; */ \
bl load_up_fpu; /* if from user, just load it up */ \
b fast_exception_return; \
1: addi r3,r1,STACK_FRAME_OVERHEAD; \
With the patch we run fine, at the expense that we lose the ability to
catch real FP unavailable exceptions in the kernel. It is because of
this loss that I have not submitted this patch.
We also hit another problem under high RTP load... and this is the
patch that fixes it:
diff --git a/arch/powerpc/kernel/fpu.S b/arch/powerpc/kernel/fpu.S
index fc8f5b1..051a02c 100644
--- a/arch/powerpc/kernel/fpu.S
+++ b/arch/powerpc/kernel/fpu.S
@@ -83,6 +83,11 @@ END_FTR_SECTION_IFSET(CPU_FTR_VSX)
stfd fr0,THREAD_FPSCR(r4)
PPC_LL r5,PT_REGS(r4)
toreal(r5)
+
+ /* Under heavy RTP load the hsp thread can have a NULL pt_regs. */
+ PPC_LCMPI 0,r5,0
+ beq 1f
+
PPC_LL r4,_MSR-STACK_FRAME_OVERHEAD(r5)
li r10,MSR_FP|MSR_FE0|MSR_FE1
andc r4,r4,r10 /* disable FP for previous task */
So, if you are still reading this far, I am just looking for any
suggestions. Are there better ways of handling this? Have I
missed something? Anybody know why pt_regs might be NULL?
Cheers,
Sean
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