RE: [tip:tracing/core] x86, bts: reenable ptrace branch tracesupport

[Metzger, Markus T]

>-----Original Message-----
>From: Peter Zijlstra [mailto:peterz@xxxxxxxxxxxxx]
>Sent: Wednesday, June 10, 2009 3:44 PM
>To: Metzger, Markus T
>Cc: Ingo Molnar; mingo@xxxxxxxxxx; hpa@xxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; tglx@xxxxxxxxxxxxx;
>oleg@xxxxxxxxxx; linux-tip-commits@xxxxxxxxxxxxxxx
>Subject: RE: [tip:tracing/core] x86, bts: reenable ptrace branch trace support
>
>On Wed, 2009-06-10 at 14:32 +0100, Metzger, Markus T wrote:
>> >-----Original Message-----
>> >From: Peter Zijlstra [mailto:peterz@xxxxxxxxxxxxx]
>> >Sent: Wednesday, June 10, 2009 3:29 PM
>> >To: Metzger, Markus T
>> >Cc: Ingo Molnar; mingo@xxxxxxxxxx; hpa@xxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxxx;
>tglx@xxxxxxxxxxxxx;
>> >oleg@xxxxxxxxxx; linux-tip-commits@xxxxxxxxxxxxxxx
>> >Subject: RE: [tip:tracing/core] x86, bts: reenable ptrace branch trace support
>> >
>> >On Wed, 2009-06-10 at 14:22 +0100, Metzger, Markus T wrote:
>> >>
>> >> The Debug Store interface is completely in-kernel. It does not expose
>> >> anything to the outside world.
>> >>
>> >> What we expose is a ptrace interface for branch tracing.
>> >> That this is built on top of Debug Store is completely hidden.
>> >> The Debug Store interface may be changed without impacting the
>> >> user-visible part at any time.
>> >>
>> >> I do think that a ptrace interface makes sense since debuggers are the
>> >> targeted users for branch tracing.
>> >>
>> >> I don't see why we should not merge the fixes now and then rework the
>> >> in-kernel parts as needed for supporting PEBS.
>> >
>> >Ok, so what is all that account_locked_memory() for?
>>
>> To allow debuggers (users) to decide how much memory they want to spend
>> on branch tracing.
>
>But that is the debug store, right? User visible through some mlock
>accounting and limit.

Debug Store is the underlying implementation, that's all.

In the current implementation, we lock the entire buffer. In a future
implementation we might not need to lock the entire buffer, any more.
The only user-visible effect is that he will get -ENOMEM for bigger
buffers than today.


>Furthermore, it appears there is an interface for setting the size,
>that's also user visible and not fixable after the fact.

I don't see how that differs from setting the initial size.


>Once you want to multiplex cpu-wide and per task BTS/PEBS contexts,
>there is no choice but to view the DS as a cpu resource, not a task
>resource, therefore you cannot specify a size, nor attribute it to
>specific tasks mm accounting.

We can still attach a buffer to a task to hold the branch or pebs trace.
When can copy the trace from the limited cpu resource to that per-task buffer
when the Debug Store buffer overflows.

The accounting is done to have the tracer, i.e. the debugger, pay for the
system resources it uses.
If we later on decide to use a fixed-size statically allocated per-cpu
Debug Store buffer, the tracer would no longer need to pay for the
locked memory. It would still have to pay for the memory to hold the bigger
tracer buffer that the debugger requested.


regards,
markus.

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