Re: debugctl msr

[stephane eranian]

Markus,

I looked at the ds.c source code some more. I think the problem when
trying to use it with perfmon is because you are assuming that by the
time you allocate the buffer you know the task it is going to be attached to.
With perfmon this assumption is always wrong. Perfmon decouples each
step:
    1- allocation of the memory for the buffer
    2- initialization of the buffer
    3- binding of the buffer to its target thread or CPU
    4- saving/restoring of PMU/DS/PEBS registers

The idea is that you can prepare your measurement/buffer and then
attach to a task.
Should you be following across fork/pthread_create, you could prepare in advance
a pool of ready-to-go perfmon sessions, all you'd have to do then is
attach + start.

In ds.c:ds_request_pebs(), you assume that if task == NULL, then this
is a per-cpu session.
If you could decouple that by passing a type parameter for instance
and if you were to add
a ds_attach_pebs() then I think I could use the interface.


On Wed, Nov 12, 2008 at 11:59 AM, Metzger, Markus T
<markus.t.metzger@xxxxxxxxx> wrote:
>>-----Original Message-----
>>From: stephane eranian [mailto:eranian@xxxxxxxxxxxxxx]
>>Sent: Mittwoch, 12. November 2008 11:10
>>To: Markus Metzger
>
>>>> For perfmon, PEBS can be used for both per-thread and per-cpu. With
>>>> perfmon, the allocation/initialization of the buffer is
>>separated from its
>>>> activation. In other words, allocation/initialization may
>>be done before
>>>> you
>>>> actually have to write the DS_AREA MSR.
>>Allocation/initialization may not
>>>> necessary be done on the cpu you want to measure in per-cpu
>>mode. The
>>>> logic of DS is different, ds_request_pebs() allocates and
>>immediately
>>>> writes
>>>> DS_AREA. I cannot use that.
>>>
>>> DS_AREA contains a pointer to the actual configuration struct.
>>> The idea is that once I allocated the configuration struct,
>>I can write
>>> the pointer to it into DS_AREA. It won't be used unless I
>>turn on a feature
>>> that uses DS.
>>> So, the flow is:
>>> 1. allocate configuration struct
>>> 2. write DS_AREA
>>
>>What if you are allocating the buffer for another task. For instance,
>>a tool is attaching
>>to a running process and wants to use PEBS. The tool allocates and
>>initializes the
>>PEBS buffer and then attaches it to the task to monitor (which is
>>stopped). When the
>>task is scheduled again, it picks up the PEBS buffer, and
>>DS_AREA is written to.
>
> That tool should request DS for the task it intends to attach its
> PEBS buffer to. Who knows, maybe PEBS is already in use for that task
> by someone else. Or maybe there is a system wide BTS or PEBS session
> and the per-task request would need to be rejected (until we can
> support this scenario).
>
>
>
>>There is another reason why the DS_AREA is exposed. This is
>>the only way for
>>tools to see the current position in the PEBS buffer. They may
>>want to poll on
>>that position index. The PEBS buffer is not necessarily full. What if
>>the session
>>terminates with a partial buffer. There must be a way for the
>>tool to figure out
>>where the last sample is. By exposing DS read-only the index
>>is always available
>>and always guaranteed current. Without this, the kernel would have to
>>extract the
>>index (pebs_get_index) and store it somewhere so the user can
>>see it. This copy
>>could be tirggered by a PEBS buffer overflow or a stop of
>>monitoring. Sampling
>>buffers formats currently do not have a stop callback but this can be
>>added easily.
>
> With the current interface, you would need to call ds_get_pebs_index().
>
> The rfc patch I sent out some weeks ago in the scope of multiplexing
> uses the approach you described - at least in-kernel.
> Upon ds_request_pebs(), you would get a const struct pebs_tracer* that
> contains a const view of the DS configuration.
>
>
> In any case, I think the DS configuration needs to be allocated by ds.c
> since we have two independent users that need to share a single
> configuration.
>
> Would it help if we changed the DS interface as proposed in that rfc
> patch (without all the multiplexing stuff)?
>
>
> regards,
> markus.
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