Messages in this thread | | | Date | Mon, 23 Feb 2009 13:34:14 -0500 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH 4/6] ftrace, x86: make kernel text writable only for conversions |
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* Steven Rostedt (rostedt@goodmis.org) wrote: > > On Mon, 23 Feb 2009, Mathieu Desnoyers wrote: > > > > > > Hmm, lets see. I simply set a bit in the PTE mappings. There's not many, > > > since a lot are 2M pages, for x86_64. Call stop_machine, and now I can > > > modify 1 or 20,000 locations. Set the PTE bit back. Note, the changing of > > > the bits are only done when CONFIG_DEBUG_RODATA is set. > > > > > > text_poke requires allocating a page. Map the page into memory. Set up a > > > break point. > > > > text_poke does not _require_ a break point. text_poke can work with > > stop_machine. > > It can? Doesn't text_poke require allocating pages? The code called by > stop_machine is all atomic. vmap does not give an option to allocate with > GFP_ATOMIC. > > > > There are two different problems here : > > I agree that they are two different problems. The reason I relate them is > because text_poke can not be called from a stop_machine call. > > > > > - How you deal with concurrency > > - you use stop machine > > - I use breakpoints > > - How you deal with RO page mappings > > - you change the kernel page flags > > - i use text_poke > > > > Please don't mix those separate concerns. > > So you have two different concerns. One is that I use stop_machine, > instead of break points, the other is that I modify all kernel text to > make the change. > > Lets look at them separately. > > The stop_machine vs. break points. > > breakpoints is a cool trick, but is not implemented on all the archs that > dynamic ftrace is. > > break points are performed on a running system. This may be lower in > latency tracing when the tracer is started, but can create a large number > of variables that can not all be understood. > > stop_machine is quite simple. No need to take traps, no need to handle > what to do when another process runs the code being changed. > > When making the hooks, stop_machine can add a bit of a interrupt latency. > But this is only when the hooks are added or removed. Why is this such a > big deal?
On a live system, adding interrupt latency even when tracing is not active yet _is_ a big deal.
> It is much easier to add the hooks with tracing disabled (via > a simple toggle bit). Then start and stop your tracing by using the toggle > bit. After you are all done, then remove the hooks. Or just keep them > on since they are low overhead anyway (only a few hooks right?) > > > CONFIG_DEBUG_RODATA (only an x86 issue at the moment) > > text_poke vs changing all pages: > > You said this is a separate issue than stop_machine. But that is not the > case. text_poke can not be done in an atomic section. This removes it from > being used by stop_machine. >
Hrm, I wonder if we could create a variant of vmap_ram that would be atomic ? That would clearly fix our problems.
> As you said, text_poke only handles the RO/RW issue, not the modifying of > code on the fly. Thus, keeping stop_machine around, we must also not use > text_poke.
Not if we modify vmap_ram...
> > I guess this takes the text_poke vs changing all pages out of the > question. While stop_machine is still being used, we can not use > text_poke (without rewriting it).
Where is the problem ? Let's improve it if needed.
> > Also when we want to trace all functions, is it really necessary to vmap > each one at a time? Andi suggested that we could optimise by mapping > larger pages, and finding the ones that share the page. This too would > require a rewrite of text_poke. >
This is an optimization, we should see the performance penality first before we start optimizing things too early.
Mathieu
> > > > > > > > > > If, in the end, your argument is "the function tracer works as-is now, > > > > and I have no time to change it given it represents too much work" or "I > > > > don't care about your use-cases", I'm OK with that. But please then don't > > > > argue that it's because it's the best technical solution when it isn't. > > > > > > No, I have yet to hear a valuable argument against stop_machine. You are > > > pushing the burden of proof on me, when we have something that does work, > > > on several archs. You want me to redesign the system to be x86 only, and > > > then say, hey, my original code works better. > > > > > > > stop_machine involves high interrupt latency. This is the argument I've > > been repeating for 1-2 emails already. And I have to disagree with you : > > we can do this code generically given the right abstractions > > (BREAKPOINT_INSN* macros I proposed earlier). Is having something that > > "works" your only argument to stop improving it ? > > The high interrupt latency only happens at the time we need to hook the > functions. This does not mean it is the time to start the tracing. That > can be done separately. > > Your only concern is the stop_machine latency? Then you might as well also > prevent modules, since that uses stop machine too. Again, this happens > only when the tracer hooks are added or removed. This is done at a time > the sys-admin will activate it. It is not a random latency that is > occurred by some timer or other asynchronous event. > > > > > > I do not see text_poke being theoretically better. The only reason you > > > given me to use it is because you dislike stop_machine. > > > > > > > There is absolutely no link between stop_machine and text_poke. I argue > > against stop_machine saying that the breakpoint approach is less > > intrusive because it does not involve disabling interrupts for so long, > > and I argue against modifying the kernel page flags because that > > modifies the access rights of the core kernel and modules to RO > > mappings, which is IMO a side-effect that we should eliminate _if we > > can_. Please keep those two concerns separate. > > text_poke can not be executed from stop_machine. There's the link. The two > concerns are not separate. > > Your concern with stop_machine is that it will cause an interrupt latency > when the sysadmin enables or disables the functions. There exists other > interrupt latencies that can be worst that are asynchronous. Run hackbench > with the irqs off tracer and see for yourself. > > -- Steve >
-- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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