REPORT Assignment - 4

This report explains briefly the changes and improvements made to the existing xv6 riscv operating system.

Running the modified xv7 riscv

cd to the directory containing Makefile and run make qemu SCHEDULER=[OPTIONS] CPUS=[1-8]. The SCHEDULER and CPUS flags are optional. If not specified, SCHEDULER=RR and CPUS=8.

The trace system call, when called upon by a process, tracks down the system calls being issued by that process during its execution. The children of the calling processes are also tracked, but any other process executing parallely is not tracked, unless the process itself calls trace.
The trace system call takes one argument, a bitmask of the system call functions that are to be traced. The call is implemented by making modifications to the proc structure. A new integer variable mask is added.
mask keeps track of which system calls to trace by setting the corresponding bit "high", in accordance with the numbering in the file kernel/syscall.h.
In the allocproc structure, the mask is initialized to 0, indicating that by default no system calls need to be tracked. When trace is called, it sets the mask of the current process equal to the argument taken as input.
In the file kernel/syscall.c, whenever a system call is performed, an additional piece of code checks, based off the value of mask, whether the current system call needs to be tracked or not. In the former case, it prints the required information about the system call after it has executed.
The strace function is a command line function and is implemented by simply executing trace at the start of the process and thereafter calling the exec function for the arguments after the first two ones, which denote the name of the call, strace and the second argument contains the bitmask.

The sigalarm system call informs a process as it takes CPU time. It takes two arguments, <interval> and <handler>, and calls the function handler after every interval ticks of the CPU.
This system call is implemented by further modifying the proc structure and adding new variables, alarm, alarmTime, interruptFunction, tickCount.
alarm stores whether the sigalarm function is called or not.
alarmTime stores the duration after which the handler needs to be called.
interruptFunction, as the name suggests, stores the function pointer handler, and is initialized to 0 in allocproc.
tickCount stores the number of ticks that the process has currently used up. When tickCount equals alarmTime, it is set back to 0, and the program counter of the calling process is set to interruptFunction.
The modified code is largely present in kernel/trap.c inside the function usertrap.

We modify the Makefile so that we can take the scheduler argument and use desired scheduler.

OPTION	Scheduler	rtime	wtime
RR	Round Robin	20	182
FCFS	First Come First Serve	41	185
LBS	Lottery Based Scheduler	21	186
PBS	Priority Based Scheduler	20	143
MLFQ	Multi-Level Feedback Queue