On Feb 8, 2007, at 11:21 AM, Arnd Bergmann wrote:
On Thursday 08 February 2007 15:18, Milton Miller wrote:
The current patch specifically identifies that only single
elf objects are handled. There is no code to handle dynamic
linked libraries or overlays. Nor is there any method to
present samples that may have been collected during context
switch processing, they must be discarded.
I thought it already did handle overlays, what did I miss here?
It does, see my reply to Maynard. Not sure what I was thinking
when I wrote this, possibly I was thinking of the inaccuracies.
My proposal is to change what is presented to user space. Instead
of trying to translate the SPU address to the backing file
as the samples are recorded, store the samples as the SPU
context and address. The context switch would record tid,
pid, object id as it does now. In addition, if this is a
new object-id, the kernel would read elf headers as it does
today. However, it would then proceed to provide accurate
dcookie information for each loader region and overlay.
Doing the translation in two stages in user space, as you
suggest here, definitely makes sense to me. I think it
can be done a little simpler though:
Why would you need the accurate dcookie information to be
provided by the kernel? The ELF loader is done in user
space, and the kernel only reproduces what it thinks that
came up with. If the kernel only gives the dcookie information
about the SPU ELF binary to the oprofile user space, then
that can easily recreate the same mapping.
Actually, I was trying to cover issues such as anonymous
memory. If the kernel doesn't generate dcookies for
the load segments the information is not recoverable once
the task exits. This would also allow the loader to create
an artifical elf header that covered both the base executable
and a dynamicly linked one.
Other alternatives exist, such as a structure for context-id
that would have its own identifing magic and an array of elf
header pointers.
The kernel still needs to provide the overlay identifiers
though.
Yes, which means it needs to parse the header (or libpse
be enhanced to write the monitor info to a spufs file).
To identify which overlays are active, (instead of the present
read on use and search the list to translate approach) the
kernel would record the location of the overlay identifiers
as it parsed the kernel, but would then read the identification
word and would record the present value as an sample from
a separate but related stream. The kernel could maintain
the last value for each overlay and only send profile events
for the deltas.
right.
This approach trades translation lookup overhead for each
recorded sample for a burst of data on new context activation.
In addition it exposes the sample point of the overlay identifier
vs the address collection. This allows the ambiguity to be
exposed to user space. In addition, with the above proposed
kernel timer vs sample collection, user space could limit the
elapsed time between the address collection and the overlay
id check.
yes, this sounds nice. But tt does not at all help accuracy,
only performance, right?
It allows the user space to know when the sample was taken
and be aware of the ambiguity. If the sample rate is
high enough in relation to the overlay switch rate, user space
could decide to discard the ambiguous samples.
This approach allows multiple objects by its nature. A new
elf header could be constructed in memory that contained
the union of the elf objects load segments, and the tools
will magically work. Alternatively the object id could
point to a new structure, identified via a new header, that
it points to other elf headers (easily differentiated by the
elf magic headers). Other binary formats, including several
objects in a ar archive, could be supported.
Yes, that would be a new feature if the kernel passed dcookie
information for every section, but I doubt that it is worth
it. I have not seen any program that allows loading code
from more than one ELF file. In particular, the ELF format
on the SPU is currently lacking the relocation mechanisms
that you would need for resolving spu-side symbols at load
time
Actually, It could check all load segments, and only report
those where the dcookie changes (as opposed to the offset).
.
If better overlay identification is required, in theory the
overlay switch code could be augmented to record the switches
(DMA reference time from the PowerPC memory and record a
relative decrementer in the SPU), this is obviously a future
item. But it is facilitated by having user space resolve the
SPU to source file translation.
This seems to incur a run-time overhead on the SPU even if not
profiling, I would consider that not acceptable.
It definitely is overhead. Which means it would have to be
optional, like gprof.
milton
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