Messages in this thread | | | Subject | Re: [PATCH 0/5] ftrace: do not trace NMI handlers | From | Peter Zijlstra <> | Date | Wed, 30 Jul 2008 09:28:05 +0200 |
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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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