Re: [PATCH 0/5] ftrace: do not trace NMI handlers

[Peter Zijlstra]

On Tue, 2008-07-29 at 21:29 -0400, Steven Rostedt wrote:
> The dynamic ftrace code modifies code text at run time. Arjan informed
> me that there is no safe way to modify code text on an SMP system when
> the other CPUs might execute that same code. The reason has to do with
> pipeline caches and CPUs might do funny things if the code being pushed
> in the pipeline also happens to be modified at that same time. (Arjan
> correct me if I'm wrong here).

How does the immediate value stuff get around this issue?

> We use kstop_machine to put the system into a UP like mode. This prevents
> other CPUs from executing code while we modify it. Under stress testing
> Ingo discovered that NMIs can cause the system to crash. This was due
> to NMIs calling code that is being modified. Some boxes are more prone to
> failure than others.
> 
> This series of patches performs two tasks:
> 
>  1) Add notrace to functions called by NMI, or simply remove the tracing
>     completely from files that are primarily used by NMI.
> 
>  2) Add a warning when code that will be modified is called by an NMI.
>     This also disables ftraced when it is detected, to prevent the
>     race with the NMI and code modification from happneing.
> 
> The warning looks something like this:
> 
> --------------- cut here ---------------
> WARNING: ftraced code called from NMI context lapic_wd_event+0xd/0x65
>          Please report this to the ftrace maintainer.
>          Disabling ftraced. Boot with ftrace_keep_on_nmi to not disable.
> Pid: 0, comm: swapper Not tainted 2.6.26-tip #96
> 
> Call Trace:
>  <NMI>  [<ffffffff8021c6d0>] ? lapic_wd_event+0xd/0x65
>  [<ffffffff8027b9c1>] ftrace_record_ip+0xa3/0x357
>  [<ffffffff8020c0f4>] mcount_call+0x5/0x31
>  [<ffffffff8021c6d5>] ? lapic_wd_event+0x12/0x65
>  [<ffffffff804b90d4>] nmi_watchdog_tick+0x21b/0x230
>  [<ffffffff804b8487>] default_do_nmi+0x73/0x1e0
>  [<ffffffff804b8a04>] do_nmi+0x64/0x91
>  [<ffffffff804b80bf>] nmi+0x7f/0x80
>  [<ffffffff80212c14>] ? default_idle+0x35/0x4f
>  <<EOE>>  [<ffffffff8020ae42>] cpu_idle+0x8a/0xc9
>  [<ffffffff804b15a6>] start_secondary+0x172/0x177
> 
> --------------- end cut here ---------------
> 
> 
> This appears once when it is caught. We are hoping that this will not
> appear often, and are running code to catch it as it does.
> 
> -- Steve
> 

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