Review Board 2.0.15


proto, probe: Add elastic trace probe to o3 cpu

Review Request #3027 - Created Aug. 11, 2015 and submitted

Information
Curtis Dunham
gem5
default
Reviewers
Default 
The elastic trace is a type of probe listener and listens to probe points

in multiple stages of the O3CPU. The notify method is called on a probe

point typically when an instruction successfully progresses through that

stage.


As different listener methods mapped to the different probe points execute,

relevant information about the instruction, e.g. timestamps and register

accesses, are captured and stored in temporary InstExecInfo class objects.

When the instruction progresses through the commit stage, the timing and the

dependency information about the instruction is finalised and encapsulated in

a struct called TraceInfo. TraceInfo objects are collected in a list instead

of writing them out to the trace file one a time. This is required as the

trace is processed in chunks to evaluate order dependencies and computational

delay in case an instruction does not have any register dependencies. By this

we achieve a simpler algorithm during replay because every record in the

trace can be hooked onto a record in its past. The instruction dependency

trace is written out as a protobuf format file. A second trace containing

fetch requests at absolute timestamps is written to a separate protobuf

format file.


If the instruction is not executed then it is not added to the trace.

The code checks if the instruction had a fault, if it predicated

false and thus previous register values were restored or if it was a

load/store that did not have a request (e.g. when the size of the

request is zero). In all these cases the instruction is set as

executed by the Execute stage and is picked up by the commit probe

listener. But a request is not issued and registers are not written.

So practically, skipping these should not hurt the dependency modelling.


If squashing results in squashing younger instructions, it may happen that

the squash probe discards the inst and removes it from the temporary

store but execute stage deals with the instruction in the next cycle which

results in the execute probe seeing this inst as 'new' inst. A sequence

number of the last processed trace record is used to trap these cases and

not add to the temporary store.


The elastic instruction trace and fetch request trace can be read in and

played back by the TraceCPU.

   

  


 


 

  

  






 
 
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Review request changed

   
Updated  (Dec. 7, 2015, 3:19 p.m.)

  

  


   

    Status:
    

    Closed (submitted)