* Metzger, Markus T <[email protected]> wrote:
> Andi suggested to make this a sysctl.
that's just as arbitrary ...
> Would it be safe to drop the artificial limit and let the limit be the
> available memory?
no, that would be a DoS :-/
mlock() is rlimit controlled and is available to unprivileged users - up
to a small amount of memory can be locked down. But i agree that mlock()
can be problematic - see below.
> > There's also no real mechanism that i can see to create a guaranteed
> > flow of this information between the debugger and debuggee (unless i
> > missed something), the code appears to overflow the array, and
> > destroy earlier entries, right? That's "by design" for debugging,
> > but quite a limitation for instrumentation which might want to have
> > a reliable stream of the data (and would like the originating task
> > to block until the debugger had an opportunity to siphoon out the
> > data).
>
> That's correct as well. My focus is on debugging. And that's actually
> the most useful behavior in that case. I'm not sure what you mean with
> 'instrumentation'.
the branch trace can be used to generate a very finegrained
profile/histogram of code execution - even of rarely executed codepaths
which cannot be captured via timer/perf-counter based profiling.
another potential use would be for call graph coverage testing. (which
currently requires compiler-inserted calls - would be much nicer if we
could do this via the hardware.)
etc. Branch tracing isnt just for debugging i think - as long as the
framework is flexible enough.
> The actual physical memory consumption will be worse (or at best
> equal) compared to kalloc()ed memory, since we need to pin down entire
> pages, whereas kalloc() would allocate just the memory that is
> actually needed.
i agree that mlock() has problems. A different model would be: no mlock
and no get_user_pages() - something quite close to what you have
already. Data is streamed out of the internal (hardware-filled,
kmalloc()-ed, user-inaccessible) buffer, we stop task execution until it
is copied to the larger, user-provided buffer. The debugging feature you
are interested in could be enabled as a special-case of this mechanism:
if the user-space buffer is not larger than the hardware buffer then no
streaming is needed, you can just capture into the kernel buffer.
User-space would have to do a PTRACE_BTS_DRAIN_BUFFER call (or something
like that) to get the "final portion" of the branch trace out into the
user-space buffer. [which, in your debugging special-case, would the
full internal buffer.]
that way the kmalloc()-ed buffer becomes an internal detail of buffering
that you rarely have to be aware of. (it could still be queried - like
your patch does it now.)
or something else that is intelligent. Basically, what we'd like to have
is a future-proof, extensible approach that does not necessarily stop at
debugging and integrates this hardware capability into Linux
intelligently.
Ingo
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