Message-ID: <49A82851.5080707@redhat.com>
Date:	Fri, 27 Feb 2009 12:52:17 -0500
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
CC:	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"H. Peter Anvin" <hpa@...or.com>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH 4/6] ftrace, x86: make kernel text writable only for conversions

Steven Rostedt 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.

Hi,

With my patch, text_poke() never allocate pages any more :)

BTW, IMHO, both of your methods are useful and have trade-off.

ftrace wants to change massive amount of code at once. If we do
that with text_poke(), we have to map/unmap pages each time and
it will take a long time -- might be longer than one stop_machine_run().

On the other hand, text_poke() user like as kprobes and tracepoints,
just want to change a few amount of code at once, and it will be
added/removed incrementally. If we do that with stop_machine_run(),
we'll be annoyed by frequent machine stops.(Moreover, kprobes uses
breakpoint, so it doesn't need stop_machine_run())


Thank you,

>> 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?  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.
> 
> 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.
> 
> 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).
> 
> 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.
> 
> 
> 
>>>> 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
> 
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-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division

e-mail: mhiramat@...hat.com


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