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Re: Ftrace for MIPS may hang on SMP system

To: Ralf Baechle <ralf@linux-mips.org>, Steven Rostedt <rostedt@goodmis.org>, David Daney <ddaney@caviumnetworks.com>
Subject: Re: Ftrace for MIPS may hang on SMP system
From: wu zhangjin <wuzhangjin@gmail.com>
Date: Sun, 22 Aug 2010 20:20:02 +0800
Cc: linux-mips <linux-mips@linux-mips.org>
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(Add 'another' Steven in this loop)

On 8/22/10, wu zhangjin <wuzhangjin@gmail.com> wrote:
> Hi, all
>
> For I didn't have a SMP machine, I haven't used Ftrace(in 2.6.34) for
> MIPS on SMP system before, Yesterday,  I got a RMI XLS machine and
> found Ftrace for MIPS hanged on it after I issued:
>
> $ echo function > /debug/tracing/current_tracer
>
> I have gotten the root cause, that is:
>
> in kernel/trace/ftrace.c:
>
> stop_machine() disables the irqs of the other cpus and then modify the
> codes via calling the arch specific ftrace_modify_code() in
> __ftrace_modify_code().
>
> As the description about stop_machine() in arch/x86/kernel/ftrace.c shows:
>
> /*
>  * Modifying code must take extra care. On an SMP machine, if
>  * the code being modified is also being executed on another CPU
>  * that CPU will have undefined results and possibly take a GPF.
>  * We use kstop_machine to stop other CPUS from exectuing code.
> [snip]
>
> Then, it is reasonable to use stop_machine() here.
>
> And in arch/mips/kernel/ftrace.c:
>
> flush_icache_range() is called in ftrace_modify_code() to ensure the
> intructions will be executed are what we want.
>
> In UP system, there is no problem for flush_icache_range() simply
> flush the instruction cache, but In SMP system, this may be different,
> for flush_icache_range() may also need to ask the other cpus (via
> sending ipi interrupt) to flush their icaches and will wait for them
> till the other cpus finish their flushing.
>
> But as we know above, the irqs of the other cpus are disabled by
> stop_machine(), they have no opportunity to flush their icache and
> will let the current cpu wait for them all the time, then soft lock
> --> hang.
>
> To fix it, there are two potential solutions:
>
> 1. replace flush_icache_range() by something else, maybe we can use
> the similar method in arch/x86/kernel/ftrace.c, x86 uses sync_core()
> defined in arch/x86/include/asm/processor.h to flush the icache on all
> processors:
>
> /* Stop speculative execution and prefetching of modified code. */
> static inline void sync_core(void)
> {
>         int tmp;
>
> #if defined(CONFIG_M386) || defined(CONFIG_M486)
>         if (boot_cpu_data.x86 < 5)
>                 /* There is no speculative execution.
>                  * jmp is a barrier to prefetching. */
>                 asm volatile("jmp 1f\n1:\n" ::: "memory");
>         else
> #endif
>                 /* cpuid is a barrier to speculative execution.
>                  * Prefetched instructions are automatically
>                  * invalidated when modified. */
>                 asm volatile("cpuid" : "=a" (tmp) : "0" (1)
>                              : "ebx", "ecx", "edx", "memory");
> }
>
> But is there a cpuid like hardware instruction in MIPS SMP? As I know,
> in UP, we may be possible to use prefetch instruction to push the
> instruction to the cache, but in SMP, is there a instruction to force
> the other cpus to flush their cache too?
>
> 2. Replace the stop_machine() by something else
>
> I have written such a patch:
>
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 2404b59..e4d058f 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -1129,13 +1129,18 @@ static int __ftrace_modify_code(void *data)
>  static void ftrace_run_update_code(int command)
>  {
>         int ret;
> +       unsigned long flags;
>
>         ret = ftrace_arch_code_modify_prepare();
>         FTRACE_WARN_ON(ret);
>         if (ret)
>                 return;
>
> -       stop_machine(__ftrace_modify_code, &command, NULL);
> +       preempt_disable();
> +       local_irq_save(flags);
> +       __ftrace_modify_code(&command);
> +       local_irq_restore(flags);
> +       preempt_enable();
>
>         ret = ftrace_arch_code_modify_post_process();
>         FTRACE_WARN_ON(ret);
>
> It works without any hang but I'm not sure whether it will guarantee
> the "undefined results" problem mentioned above. Here we may need to
> prevent the other cpus from executing the source code for we are
> modifying the source code but also need to allow them to get the ipi
> interrupt and flush their icaches.
>
> And I have took a look at the part of code modification in kgdb
> system, seems it doesn't use stop_machine().
>
> What's your ideas?
>
> Thanks & Regards,
> Wu Zhangjin
>


-- 
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