Messages in this thread | | | Date | Tue, 29 Nov 2011 17:14:20 -0500 | From | Jason Baron <> | Subject | Re: Perhaps a side effect regarding NMI returns |
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On Tue, Nov 29, 2011 at 03:58:21PM -0500, Steven Rostedt wrote: > On Tue, 2011-11-29 at 12:36 -0800, Linus Torvalds wrote: > > On Tue, Nov 29, 2011 at 12:31 PM, Andi Kleen <andi@firstfloor.org> wrote: > > > > > > As a simple fix your proposal of forcing IRET sounds good. > > > > We could of course use iret to return to the regular kernel stack, and > > do the schedule from there. > > > > So instead of doing the manual stack switch, just build a fake iret > > stack on our exception stack. Subtle and somewhat complicated. I'd > > almost rather just do a blind iret, and leave the 'iret to regular > > stack' as a possible future option. > > Note, the reason that I've been looking at this code, is because I'm > looking at implementing your idea to handle irets in NMIs, caused by > faults, exceptions, and the reason I really care about: debugging. > > Your proposal is here: > > https://lkml.org/lkml/2010/7/14/264 > > But to make this work, it would be really nice if the NMI routine wasn't > convoluted with the paranoid_exit code. > > For things like static_branch()/jump_label and modifying ftrace nops to > calls and back, we currently use the big hammer approach stop_machine(). > This keeps another CPU from executing code that is being modified. > There's also tricks to handle NMIs that may be running on the stopped > CPUs. > > But people don't like the overhead that stop_machine() causes, and I > have code that can make the modifications for ftrace with break points. > By adding a break point, syncing, then modifying the code and break
But if there's still has to be some sort of 'syncing' after we add a break point, how much are we going to save? Or I guess your're using an IPI?
Thanks,
-Jason
> point to a new op will greatly reduce the overhead. At least the latency > will be much less. > > The problem is that ftrace affects code in NMIs. We tried to not trace > NMIs, but there's so many functions that NMIs call, it ended up being a > losing battle. But if we can fix the NMI enabled on iret, we can then > use the break point scheme for both static_branch() and ftrace, and > remove the overhead of stop_machine. I think there's a possibility to > use kprobes in NMIs too, with this fix. > > -- Steve > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/
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