Subject: Re: [RT,RFC] Hacks allowing -rt to run on POWER7 / Powerpc.

From: Milton Miller <[email-masked]>
Date: Sun, 11 Jul 2010 02:49:54 -0500

On Fri, 09 Jul 2010 about 08:55:01 -0000, Will Schmidt wrote:
> We've been seeing some issues with userspace randomly SIGSEGV'ing while
> running the -RT kernels on POWER7 based systems.   After lots of
> debugging, head scratching, and experimental changes to the code, the
> problem has been narrowed down such that we can avoid the problems by
> disabling the TLB batching.
> 
> After some input from Ben and further debug, we've found that the
> restoration of the batch->active value near the end of __switch_to()
> seems to be the key.    ( The -RT related  changes within
> arch/powerpc/kernel/processor.c __switch_to()  do the equivalent of a
> arch_leave_lazy_mmu_mode() before calling _switch, use a hadbatch flag
> to indicate if batching was active, and then restore that batch->active
> value on the way out after the call to _switch_to.    That particular
> code is in the -RT branch, and not found in mainline  )
> 
> Deferring to Ben (or others in the know) for whether this is the proper
> solution or if there is something deeper, but.. 

I looked at the patch and noticed 2 changes:
	1) the batch is checked and cleared after local_irq_save
	2) enabling the batch is skipped

I talked to Will and had him try moving the local_irq_save above the
check for the active batch.  That alone did not seem to be enough.
However, he confirmed that we are setting batch to active when it is
already active in lazy_mmu_enter, meaning that batching is being turned
on recursively.  I suggested debug to check that irqs are off after the
restore when re-enabling when our debug session timed out.

milton

> 
> diff -aurp linux-2.6.33.5-rt23.orig/arch/powerpc/kernel/process.c linux-2.6.33.5-rt23.exp/arch/powerpc/kernel/process.c
> --- linux-2.6.33.5-rt23.orig/arch/powerpc/kernel/process.c	2010-06-21 11:41:34.402513904 -0500
> +++ linux-2.6.33.5-rt23.exp/arch/powerpc/kernel/process.c	2010-07-09 13:15:13.533269904 -0500
> @@ -304,10 +304,6 @@ struct task_struct *__switch_to(struct t
>  	struct thread_struct *new_thread, *old_thread;
>  	unsigned long flags;
>  	struct task_struct *last;
> -#if defined(CONFIG_PPC64) && defined (CONFIG_PREEMPT_RT)
> -	struct ppc64_tlb_batch *batch;
> -	int hadbatch;
> -#endif
>  
>  #ifdef CONFIG_SMP
>  	/* avoid complexity of lazy save/restore of fpu
> @@ -401,16 +397,6 @@ struct task_struct *__switch_to(struct t
>  		new_thread->start_tb = current_tb;
>  	}
>  
> -#ifdef CONFIG_PREEMPT_RT
> -	batch = &__get_cpu_var(ppc64_tlb_batch);
> -	if (batch->active) {
> -		hadbatch = 1;
> -		if (batch->index) {
> -			__flush_tlb_pending(batch);
> -		}
> -		batch->active = 0;
> -	}
> -#endif /* #ifdef CONFIG_PREEMPT_RT */
>  #endif
>  
>  	local_irq_save(flags);
> @@ -425,16 +411,13 @@ struct task_struct *__switch_to(struct t
>  	 * of sync. Hard disable here.
>  	 */
>  	hard_irq_disable();
> -	last = _switch(old_thread, new_thread);
> -
> -	local_irq_restore(flags);
>  
>  #if defined(CONFIG_PPC64) && defined(CONFIG_PREEMPT_RT)
> -	if (hadbatch) {
> -		batch = &__get_cpu_var(ppc64_tlb_batch);
> -		batch->active = 1;
> -	}
> +	arch_leave_lazy_mmu_mode();
>  #endif
> +	last = _switch(old_thread, new_thread);
> +
> +	local_irq_restore(flags);
>  
>  	return last;
>  }