engines: Adding exec engine

engines: Adding exec engine

When performing benchmarks with fio, some need to execute
tasks in parallel to the job execution. A typical use-case would be
observing performance/power metrics.

Several implementations were possible :
 - Adding an exec_run in addition of the existing exec_{pre|post}run
 - Implementating performance/power metrics in fio
 - Adding an exec engine

1°) Adding an exec_run
This was my first intention but quickly noticed that exec_{pre-post}run
are executed for each 'numjob'. In the case of performance/power
monitoring, this doesn't make sense to spawn an instance for each
thread.

2°) Implementing performance/power metrics
This is possible but would require lot of work to maintain this part of
fio while 3rd party tools already take care of that perfectly.

3°) Adding an engine
Adding an engine let users defining when and how many instances of the program they want.
In the provided example, a single monitoring job is spawning at the same
time as the worker thread which could be composed of several worker
threads.
A stonewall barrier is used to define which jobs must run together
(monitoring / benchmark).

The engine has two parameters :
- program: name of the program to run
- arguments: arguments to pass to the program
- grace_time: duration between SIGTERM and SIGKILL
- std_redirect: redirect std{err|out} to dedicated files

Arguments can have special variables to be expanded before the execution:
- %r will be replaced by the job duration in seconds
- %n will be replaced by the job name

During the program execution, the std{out|err} are redirected to files if std_redirect option is set (default).
- stdout: <job_name>.stdout
- stderr: <job_name>.stderr

If the executed program has a nice stdout output, after the fio
execution, the stdout file can be parsed by other tools like CI jobs or graphing tools.

A sample job is provided here to show how this can be used.
It runs twice the CPU engine with two different CPU modes (noop vs qsort).
For each benchmark, the output of turbostat is saved for later analysis.
After the fio run, it is possible to compare the impact of the two modes
on the CPU frequency and power consumption.

This can be easily extended to any other usage that needs to analysis
the behavior of the host during some jobs.

About the implementation, the exec engine forks :
- the child doing an execvp() of the program.
- the parent, fio, will monitor the time passed into the job

Once the time is over, the program is SIGTERM followed by a SIGKILL to
ensure it will not run _after_ the job is completed.
This mechanism is required as :
- not all programs can be controlled properly
- that's last resort protection if the program gets crazy
The delay is controlled by grace_time option, default is 1 sec.

If the program can be limited in its duration, using the %r variable in
the arguments can be used to request the program to stop _before_ the
job finished like :
        program=/usr/bin/mytool.sh
        arguments=--duration %r

Signed-off-by: Erwan Velu <e.velu@criteo.com>