Floating point in the kernel
benh at kernel.crashing.org
Fri Dec 11 07:19:39 EST 2009
On Thu, 2009-12-10 at 13:13 -0500, Sean MacLennan wrote:
> 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".
Right, you must not use floating point in the kernel -and- expect it to
survive schedule. You should use preempt_disable() and ensure you don't
schedule() around a block using the FP.
Note that you may also lose the FP register content if you schedule.
> 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.
I'm not sure that will work in all cases, you are playing a bit with
fire :-) I suppose I could think it through after breakfast but my first
thought is "don't do that !". Among other things you may not have a
pt_regs to save the registers to.
> 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)
> + /* Under heavy RTP load the hsp thread can have a NULL pt_regs. */
> + PPC_LCMPI 0,r5,0
> + beq 1f
Right and that means you just lost the content of your FP registers.
> 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?
Just don't schedule when you enable_kernel_fp() or move your workload to
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