Solaris User Contexts (and some other stuff)

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Solaris User Contexts(and some other stuff)

• Cse 380
• Tuesday, october 10th
• Robert spier

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User Contexts

• A user context (ucontext) defines the context of
  a thread of control within an executing process.
• These are not the same as threads.
• Threads are built on top of ucontexts.

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Saving State

• The idea of a context is to save the state of the
  machine/program/process in such a way that it can
  be restored later.

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Saved State

• Signal Mask
• Stack
• Machine Context
• (link)

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Signal Mask

• The signal mask is stored to allow different
  components to be interrupted by different
  signals.
• Only one is live at a time.
• Automatic Restoration

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Stack

• Imagine the confusion if multiple processes
  were using the same stack.
• Saving the stack (and having an independent one
  for each ucontext) allows for function calls from
  the start function of a context.

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Machine State (Context)

• CPU Stores State in Registers
• Integer Registers
• Floating Point
• mcontext_t stores them.

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Using Contexts

• getcontext( uc ) / initialize with current
  state of machine /
• stack (ulong) malloc(SS) / create new stack
  /
• / configure fields /
• / stacks work backwards on SPARC /
• uc-gtuc_stack.ss_sp (void )(stack SS - 32)
• uc-gtuc_stack.ss_size SS
• uc-gtuc_stack.ss_flags 0
• sigemptyset( uc-gtuc_sigmask ) /configure
  sigmask/
• makecontext( uc, function, 1 ) / set initial
  function, and pass arguments /

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Passing Arguments

• void f_add (int x, int y) output (xy)
• makecontext( uc, f_add, 3, 7, 20 )

Result? Output -gt 27
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Functions

• int getcontext(ucontext_t ucp)
• initializes the structure pointed to by ucp to
  the current user context of the calling process
• int setcontext(ucontext_t ucp)
• restores the user context pointed to by ucp.
  (does not return)

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Functions

• void makecontext(ucontext_t ucp, void(func)(),
  int argc, ...)
• when this context (ucp) is resumed (swap or set)
  execution continues by calling the function func,
  passing it the arguments that follow argc.
• swapcontext(ucontext_t oucp, ucontext_t ucp)
• saves the current context in the context
  structure pointed to by oucp and sets the
  context to the context structure pointed to by
  ucp.

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Signals

• include ltsignal.hgt
• include ltstdio.hgt
• include ltstdlib.hgt
• void signal_handler( int s)
• printf("I just received signal d\n",s)
• void main()
• sigset(SIGUSR1, signal_handler)
• sigset(SIGALRM, signal_handler)
• printf("My pid is d\n", getpid())
• while(1) pause()

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Signals (2)
kill -USR1 4437 kill -USR1 4437 kill
-USR1 4437 kill -ALRM 4437

• ./a.out
• My pid is 4437
• I just received signal 16
• I just received signal 16
• I just received signal 16
• I just received signal 14

(You dont need to implement generic signals for
the project.)
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Context Switching
Round Robin
Time
Contexts
C1
C1
C2
C2
C3
C3
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Context Demo (detail)
Quantas
C1
Scheduler
C2
Signal Context
C3
Time
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Timing the Switch

• In our system, each of these dots represents a
  timer tick.
• Our program receives a SIGALRM.

• We use an Interval Alarm (itimer) for this.
• Itimers are created with setitimer(2)
• The scheduler is called from the signal handler
  routine.

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References

• ucontext(5)
• Portable Multithreading The Signal Stack Trick
  For User Space Thread Creation, Ralf S.
  Engelschall
• Modern Operating Systems, Andrew S. Tannenbaum