Re: [PATCH] powerpc: Fix RCU idle and hcall tracing

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Jan 10, 2012 at 11:29:15AM +1100, Anton Blanchard wrote:
> 
> Tracepoints should not be called inside an rcu_idle_enter/rcu_idle_exit
> region. Since pSeries calls H_CEDE in the idle loop, we were violating
> this rule.
> 
> commit a7b152d5342c (powerpc: Tell RCU about idle after hcall tracing)
> tried to work around it by delaying the rcu_idle_enter until after we
> called the hcall tracepoint, but there are a number of issues with it.
> 
> The hcall tracepoint trampoline code is called conditionally when the
> tracepoint is enabled. If the tracepoint is not enabled we never call
> rcu_idle_enter. The idle_uses_rcu check was also done at compile time
> which breaks multiplatform builds.
> 
> The simple fix is to avoid tracing H_CEDE and rely on other tracepoints
> and the hypervisor dispatch trace log to work out if we called H_CEDE.
> 
> This fixes a hang during boot on pSeries.
> 
> Signed-off-by: Anton Blanchard <anton@samba.org>

Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

> ---
> 
> Index: linux-build/arch/powerpc/kernel/idle.c
> ===================================================================
> --- linux-build.orig/arch/powerpc/kernel/idle.c	2012-01-10 11:07:22.091615183 +1100
> +++ linux-build/arch/powerpc/kernel/idle.c	2012-01-10 11:07:57.172264229 +1100
> @@ -50,12 +50,6 @@ static int __init powersave_off(char *ar
>  }
>  __setup("powersave=off", powersave_off);
> 
> -#if defined(CONFIG_PPC_PSERIES) && defined(CONFIG_TRACEPOINTS)
> -static const bool idle_uses_rcu = 1;
> -#else
> -static const bool idle_uses_rcu;
> -#endif
> -
>  /*
>   * The body of the idle task.
>   */
> @@ -67,8 +61,7 @@ void cpu_idle(void)
>  	set_thread_flag(TIF_POLLING_NRFLAG);
>  	while (1) {
>  		tick_nohz_idle_enter();
> -		if (!idle_uses_rcu)
> -			rcu_idle_enter();
> +		rcu_idle_enter();
> 
>  		while (!need_resched() && !cpu_should_die()) {
>  			ppc64_runlatch_off();
> @@ -106,8 +99,7 @@ void cpu_idle(void)
> 
>  		HMT_medium();
>  		ppc64_runlatch_on();
> -		if (!idle_uses_rcu)
> -			rcu_idle_exit();
> +		rcu_idle_exit();
>  		tick_nohz_idle_exit();
>  		preempt_enable_no_resched();
>  		if (cpu_should_die())
> Index: linux-build/arch/powerpc/platforms/pseries/lpar.c
> ===================================================================
> --- linux-build.orig/arch/powerpc/platforms/pseries/lpar.c	2012-01-10 11:07:22.079614961 +1100
> +++ linux-build/arch/powerpc/platforms/pseries/lpar.c	2012-01-10 11:16:55.710226236 +1100
> @@ -546,6 +546,13 @@ void __trace_hcall_entry(unsigned long o
>  	unsigned long flags;
>  	unsigned int *depth;
> 
> +	/*
> +	 * We cannot call tracepoints inside RCU idle regions which
> +	 * means we must not trace H_CEDE.
> +	 */
> +	if (opcode == H_CEDE)
> +		return;
> +
>  	local_irq_save(flags);
> 
>  	depth = &__get_cpu_var(hcall_trace_depth);
> @@ -556,8 +563,6 @@ void __trace_hcall_entry(unsigned long o
>  	(*depth)++;
>  	preempt_disable();
>  	trace_hcall_entry(opcode, args);
> -	if (opcode == H_CEDE)
> -		rcu_idle_enter();
>  	(*depth)--;
> 
>  out:
> @@ -570,6 +575,9 @@ void __trace_hcall_exit(long opcode, uns
>  	unsigned long flags;
>  	unsigned int *depth;
> 
> +	if (opcode == H_CEDE)
> +		return;
> +
>  	local_irq_save(flags);
> 
>  	depth = &__get_cpu_var(hcall_trace_depth);
> @@ -578,8 +586,6 @@ void __trace_hcall_exit(long opcode, uns
>  		goto out;
> 
>  	(*depth)++;
> -	if (opcode == H_CEDE)
> -		rcu_idle_exit();
>  	trace_hcall_exit(opcode, retval, retbuf);
>  	preempt_enable();
>  	(*depth)--;
>