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 <an...@samba.org>
Acked-by: Paul E. McKenney <paul...@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)--;
>
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