Messages in this thread | | | Date | Wed, 30 Jul 2008 10:23:15 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH 0/5] ftrace: do not trace NMI handlers |
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* Mathieu Desnoyers (mathieu.desnoyers@polymtl.ca) wrote: > * Peter Zijlstra (peterz@infradead.org) wrote: > > 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? > > > > By inserting a temporary breakpoint in the code about to be replaced and > by issuing an IPI to every CPU, issuing a cpuid instruction to > synchronize the cores, before reverting the breakpoint to the first byte > of the new instruction. Therefore we are sure that any CPU which could > have seen the old instruction will flush its pipeline cache (cpuid > instruction synchronizes the core) before seeing the new instruction. > Meanwhile, the breakpoint handler executes the original instruction, > dealing with non-relocatable instructions if necessary by emulating them > (e.g. by playing with the return IP). We have some constraints to > follow: every instruction being replaced must stay a valid instruction > to the CPU, because there could be preemption in the middle of the > modified area. Immediate values does not change the size of the > instructions, so we don't run in this problem, but changing > { 0x90, 0x90, 0x90, 0x90, 0x90 } (nops) > into > { 0xe9, 0xXX, 0xXX, 0xXX, 0xXX } (5-bytes jump) > would be problematic because a preempted IP could point right in the > middle of those nops. As a general rule, never try to combine smaller > instructions into a bigger one, except in the case of adding a > lock-prefix to an instruction : this case insures that the non-lock > prefixed instruction is still valid after the change has been done. We > could however run into a nasty non-synchronized atomic instruction use > in SMP mode if a thread happens to be scheduled out right after the lock > prefix. Hopefully the alternative code uses the refrigerator... (hrm, it > doesn't). >
Actually, alternative.c lock-prefix modification is O.K. for spinlocks because they execute with preemption off, but not for other atomic operations which may execute with preemption on.
Mathieu
> All that is documentented thoroughly in arch/x86/kernel/immediate.c > comments. > > Mathieu > > > > > > 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 > > > > > > > -- > Mathieu Desnoyers > OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
-- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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