Process and thread creation/destruction

The process is the internal représentation of an user application. A process can be running as a single thread (called main thread), or can be multi-threaded. ALMOS-MKH supports the POSIX thread API. For a multi-threaded application, the number of threads can be very large, and the threads of a given process can be distributed on all cores available in the shared memory architecture, for maximal parallelism. Therefore, a single process can spread on all clusters. To avoid contention, the process descriptor of a P process, and the associated structures, such as the list of registered vsegs (VSL), the generic page table (GPT), or the file descriptors table (FDT) are (partially) replicated in all clusters containing at least one thread of P.

1) Process definition

The PID (Process Identifier) is coded on 32 bits. It is unique in the system, and has a fixed format: The 16 MSB (CXY) contain the owner cluster identifier. The 16 LSB bits (LPID) contain the local process index in owner cluster. The owner cluster is therefore defined by the 16 MSB bits of PID.

As it exists several copies of the process descriptors, ALMOS-MKH defines a reference process descriptor, located in the reference cluster. The other copies are used as local caches, and ALMOS-MKH must guaranty the coherence between the reference and the copies.

As ALMOS-MKH supports process migration, the reference cluster can be different from the owner cluster. The owner cluster cannot change (because the PID is fixed), but the reference cluster can change in case of process migration.

In each cluster K, the local cluster manager ( cluster_t type in ALMOS-MKH ) contains a process manager ( pmgr_t type in ALMOS-MKH ) that maintains three structures for all process owned by K :

• The PREF_TBL[lpid] is an array indexed by the local process index. Each entry contains an extended pointer on the reference process descriptor.
• The COPIES_ROOT[lpid] array is also indexed by the local process index. Each entry contains the root of the global list of copies for each process owned by cluster K.
• The LOCAL_ROOT is the local list of all process descriptors in cluster K. A process descriptor copy of P is present in K, as soon as P has a thread in cluster K.

A process can be in four states:

• RUNNING : the process is normally executing.
• STOPPED : the process received a SIGSTOP signal. It can return to RUNNING state by a SIGCONT signal.
• EXITED : the process terminated by an exit() syscall. It will be destroyed by the parent process executing a wait() syscall.
• KILLED : the process received a SIGKILL signal. It will be destroyed by the parent process executing a wait() sys call.

There is a partial list of informations stored in a process descriptor ( process_t in ALMOS-MKH ):

• PID : proces identifier.
• PPID : parent process identifier,
• PREF : extended pointer on the reference process descriptor.
• STATE: current process state.
• VSL : root of the local list of virtual segments defining the memory image.
• GPT : generic page table defining the physical memory mapping.
• FDT : open file descriptors table.
• TH_TBL : local table of threads owned by this process in this cluster.
• LOCAL_LIST : member of local list of all process descriptors in same cluster.
• COPIES_LIST : member of global list of all descriptors of same process.
• CHILDREN_LIST : member of global list of all children of same parent process.
• CHILDREN_ROOT : root of global list of children process.

All elements of a local list are in the same cluster, and ALMOS-MKH uses local pointers. Elements of a global list can be distributed on all clusters, and ALMOS-MKH uses extended pointers.

2) Thread definition

ALMOS-MKH defines four types of threads :

• one USR thread is created by a pthread_create() system call.
• one DEV thread is created by the kernel to execute all I/O operations for a given channel device.
• one RPC thread is activated by the kernel to execute pending RPC requests in the local RPC fifo.
• the IDL thread is executed when there is no other thread to execute on a core.

From the point of view of scheduling, a thread can be in three states : RUNNING, RUNNABLE or BLOCKED.

In a given process, a thread is identified by a fixed format TRDID kernel identifier, coded on 32 bits : The 16 MSB bits (CXY) define the cluster K where the thread has been created. The 16 LSB bits (LTID) define the thread local index in the local TH_TBL[K,P] of a process descriptor P in a cluster K. This LTID index is allocated by the local process descriptor when the thread is created.

Therefore, the TH_TBL(K,P) thread table for a given process in a given clusters contains only the threads of P placed in cluster K. The set of all threads of a given process is defined by the union of all TH_TBL(K,P) for all active clusters K. To scan the set off all threads of a process P, ALMOS-MKH traverse the COPIES_LIST of all process_descriptors associated to P process.

This implementation of ALMOS-MKH does not support thread migration: a thread is pinned on a given core in a given cluster. In the future process migration mechanism, all threads of given process in a given cluster can migrate to another cluster for load balancing. This mechanism is not implemented yet (february 2018), and will require to distinguish the kernel thread identifier (TRDID, that will be modified by a migration), and the user thread identifier (THREAD, that cannot be modified by a migration). In the current implementation, the user identifier (returned by the pthread_create() sys call, is identical to the kernel identifier.

There is a partial list of informations stored in a thread descriptor (thread_t in ALMOS-MKH):

• TRDID : thread identifier
• TYPE : KERNEL / USER / IDLE / RPC
• FLAGS : bit_vector of thread attributes.
• BLOCKED : bit_vector of blocking causes.
• SIGNALS : bit vector permettant d’enregistrer les signaux reçus par le thread.
• PROCESS : pointer on the local process descriptor
• SCHED : pointer on the scheduler in charge of this thread.
• CORE : pointer on the owner processor core.
• LOCKS_COUNT : current number of locks taken by this thread
• CPU_CONTEXT : save the CPU registers when descheduled.
• FPU_CONTEXT : save the FPU registers when descheduled.
• XLIST : member of the global list of threads waiting on the same resource.
• CHILDREN_ROOT : root of the global list of children threads.
• CHILDREN_LIST : member of the global list of all children of same parent.

• etc.

3) Process creation

The process creation in a remote cluster implement the POSIX fork() / exec() mechanism. When a parent process P executes the fork() system call, a new child process C is created. The new C process inherit from the parent process P the open files (FDT), and the memory image (VSL and GPT). These structures must be replicated in the new process descriptor. After a fork(), the C process can execute an exec() system call, that allocate a new memory image to the C process, but the new process can also continue to execute with the inherited memory image. For load balancing, ALMOS-MKH uses the DQDT to create the child process C on a different cluster from the parent cluster P, but the user application can also use the non-standard fork_place() system call to specify the target cluster.

3.1) fork()

The fork() system call is the only method to create a new process. A thread of parent process P, running in a cluster X, executes the fork() system call to create a child process C on a remote cluster Y, that will become both the owner and the reference cluster for the C process. A new process descriptor, and a new thread descriptor are created and initialized in target cluster Y for the child process. The calling thread can run in any cluster. If the target cluster Y is different from the calling thread cluster X, the calling thread uses a RPC to ask the target cluster Y to do the work, because only the target cluster Y can allocate memory for the new process and thread descriptor.

Regarding the process descriptor, a new PID is allocated in cluster Y. The child process C inherit the vsegs registered in the parent process reference VSL, but the ALMOS-MKH replication policy depends on the vseg type:

• for the DATA, MMAP, REMOTE vsegs (containing shared, non replicated data), all vsegs registered in the parent reference VSL(Z,P) are registered in the child reference VSL(Y,C), and all valid GPT entries in the reference parent GPT(Z,P) are copied in the child reference GPT(Y,C). For all pages, the WRITABLE flag is reset and the COW flag is set, in both (parent and child) GPTs. This require to update all corresponding entries in the parent GPT copies (in clusters other than the reference).
• for the STACK vsegs (that are private), only one vseg is registered in the child reference VSL(Y,C). This vseg contains the user stack of the user thread requesting the fork, running in cluster X. All valid GPT entries in the parent GPT(X,P) are copied in the child GPT(Y,C). For all pages, the WRITABLE flag is reset and the COW flag is set, in both (client and child) GPTs.
• for the CODE vsegs (that must be replicated in all clusters containing a thread), all vsegs registered in the reference parent VSL(Z,P) are registered in the child reference VSL(Y,C), but the reference child GPT(Y,C) is not updated by the fork: It will be dynamically updated on demand in case of page fault.
• for the FILE vsegs (containing shared memory mapped files), all vsegs registered in the reference parent VSL(Z,P) are registered in the child reference VSL(Y,C), and all valid entries registered in the reference parent GPT(Z,P) are copied in the reference child GPT(Y,C). The COW flag is not set for these shared data.

Regarding the thread descriptor, a new TRDID is allocated in cluster Y, and the calling parent thread context (current values stored in the CPU and FPU registers) is saved in the child thread CPU and FPU contexts, to be restored when the child thread will be selected for execution. Three CPU context slots are not simple copies of the parent value:

• the thread pointer register contains the current thread descriptor address. This thread pointer register cannot have the same value for parent and child.
• the stack pointer register contains the current pointer on the kernel stack. ALMOS-MKH uses a specific kernel stack when an user thread enters the kernel, and this kernel stack is implemented in the thread descriptor. As parent and child cannot use the same kernel stack, the parent kernel stack content is copied to the child kernel stack, and the stack pointer register cannot have the same value for parent and child.
• the page table pointer register contains the physical base address of the current generic page table. As the child GPT is a copy of the parent GPT in the child cluster, this page table register cannot have the same value for parent and child.

At the end of the fork(), cluster Y is both the owner cluster and the reference cluster for the new C process, that contains one single thread running in the Y cluster. All pages of DATA, REMOTE, and MMAP vsegs are marked Copy On Write in the child C process GPT (clusters Y), and in all copies of the parent P process GPT (all clusters containing a copy of P).

3.2) exec()

After a fork() system call, any thread of a process P can execute an exec() system call. This system call forces the P process to execute a new application, while keeping the same PID, the same parent process, the same open file descriptors, and the same environment variables. The existing P process descriptors (both the reference and the copies) and all associated threads will be destroyed. A new process descriptor and a new main thread descriptor are created in the reference cluster, and initialized from values found in the existing process descriptor, and from values contained in the .elf file defining the new application. The calling thread can run in any cluster. If the reference cluster Z for process P is different from the calling thread cluster X, the calling thread must use a RPC to ask the reference cluster Z to do the work.

At the end of the exec() system call, the cluster Z is both the owner and the reference cluster for process C, that contains one single thread in cluster Z.

4) Thread creation

Any user thread T of any process P, running in any cluster K, can create a new thread NT in any cluster K'. This creation is initiated by the pthread_create system call.

• The target cluster K' can be specified by the user application, using the CXY field of the pthread_attr_t argument. If the CXY is not defined by the user, the target cluster K' is selected by the kernel K, using the DQDT.
• The target core in cluster K' can be specified by the user application, using the CORE_LID field of the pthread_attr_t argument. If the CORE_LID is not defined by the user, the target core is selected by the target kernel K'.

If the target cluster K' is different from the client cluster K, the cluster K send a RPC_THREAD_USER_CREATE request to cluster K'. The argument is a complete structure pthread_attr_t (defined in the thread.h file in ALMOS-MK), containing the PID, the function to execute and its arguments, and optionally, the target cluster and target core. This RPC should return the thread TRDID.

• If the target cluster K' does not contain a copy of the P process descriptor, the kernel K' creates a process descriptor copy from the reference P process descriptor, using a remote_memcpy(), and using the cluster_get_reference_process_from_pid() to get the extended pointer on reference cluster. It allocates memory for the associated structures GPT(M,P), VSL(M,P), FDT(M,P). These structures being used as read-only caches will be dynamically filled by the page faults. This new process descriptor is registered in the COPIES_LIST and in the LOCAL_LIST.
• When the local process descriptor is set, the kernel K' select the core that will execute the new thread, allocates a TRDID to this thread, creates the thread descritor, and registers it in the local process descriptor, and in the selected core scheduler.

5) Thread destruction

The destruction of an user thread T can be caused by another thread K, executing the pthread_cancel() sys call requesting the target thread T to stop execution. It can be caused by the thread T itself, executing the pthread_exit() sys call to suicide. Finally, it can be caused by the exit() or kill() syscalls requesting the destruction of all threads of a given process.

The unique method to destroy a thread is to call the thread_kill() function, that set the THREAD_FLAG_REQ_DELETE bit in the flags field of the target thread descriptor. The thread will be asynchronously deleted by the scheduler at the next scheduling point, that calls the thread_destroy() function. This function detach the thread from the scheduler, detach the thread from the local process descriptor, and releases the memory allocated to the thread descriptor. The thread_kill() function can be called by the target thread itself (for an exit), or it can be done by another killer thread (for a kill).

The main thread (i.e. the thread 0 in the process owner cluster) is a special case, because the main thread of a given process can only be deleted (i.e. marked for delete) by the parent process main thread executing the sys_wait() syscall (see section [6] below).

5.1) thread running in DETACHED mode

The scenario is rather simple when the target thread T is not running in ATTACHED mode. The killer thread (that can be the target thread itself for an exit) call the thread_kill() function that does the following actions:

• the killer thread sets the THREAD_BLOCKED_GLOBAL bit in the target thread blocked field,
• the killer thread sets the THREAD_FLAG_REQ_DELETE bit in the target thread flags field,
• the killer thread returns without waiting the actual deletion.

5.2) thread running in ATTACHED mode

The thread destruction is more complex if the target thread T is running in ATTACHED mode, because another joining thread J, executing the pthread_join() system call, must be informed of the termination of thread T. As the thread_kill() function, executed by the killer thread K, and the sys_thread_join(), executed by the joining thread J, can be executed in any order, this requires a "rendez-vous": The first arrived thread blocks and deschedules. It will be unblocked by the other thread.

This destruction mechanism can involve three threads: the target thread T, the killer thread K, and the joining thread J:

It uses three specific fields in the thread descriptor: the join_lock field is a remote_spin_lock; the join_value field contains the termination status returned by the T thread; the join_xp field contains an extended pointer on the first arrived thread. It uses also two specific THREAD_FLAG_JOIN_DONE and THREAD_FLAG_KILL_DONE flags in the thread descriptor flags field, and one specific blocking bit THREAD_BLOCKED_JOIN, in the blocked field.

• Both the killer thread K, executing the thread_kill() function), and the joining thread J, executing the sys_thread_join() function, try to take the join_lock implemented in the T thread descriptor (the join_lock in the J thread is not used).
• The K thread test the FLAG_JOIN_DONE in the T thread descriptor:
  • If the FLAG_JOIN_DONE is set, the J thread arrived first and is blocked: the K thread reset the BLOCKED_JOIN bit in the J thread (using the extended pointer stored in the join_xp field), reset the JOIN_DONE flag, releases the join_lock in T thread, and completes the T thread destruction as described in the detached case and return.
  • If the FLAG_JOIN_DONE is not set, the K thread arrived first: the K thread set the BLOCKED_JOIN bit in the K thread, set the FLAG_KILL_DONE in the T thread, register its extended pointer in the T thread join_xp field, releases the join_lock in the T thread, and deschedules. It will complete the T thread destruction as described in the detached case and return when it resume.
• The J thread test the FLAG_KILL_DONE in the T thread descriptor:s
  • If the FLAG_KILL_DONE is set, the K thread arrived first and is blocked: the J thread reset the BLOCKED_JOIN bit in the K thread, get the exit value from the T thread "join_value" field, releases the "join_lock" in T thread, and return.
  • If the FLAG_KILL_DONE is not set, the J thread arrived first: the J thread register its extended pointer in the T thread "join_xp" field, set the FLAG_JOIN_DONE in the T thread, sets the BLOCKED_EXIT bit in the J thread, releases the "join_lock" in the T thread, and deschedules. It will get the exit value from the T thread "join_value" field and return when it resumes.

6) Process destruction

The destruction of a process P can be caused by a sys_exit() system call executed by the main thread of process P, or by another process executing the sys_kill(SIGKILL) system call. It can also be caused by a CtrlC signal typed on the process terminal. In all cases, the work must be done for all process copies in all clusters, using the list of copies rooted in the owner cluster.

6.1) parent / children synchronization

The process descriptors copies (other than the process descriptor in owner cluster) are simply deleted by the scheduler when the last thread of a given process in a given cluster is deleted. The process descriptor copy is removed from the list of copies in the owner process cluster descriptor, and the process copy disappears.

But the process destruction in the owner cluster is more complex, because the child process destruction must be reported to the parent process when the parent process executes the blocking sys_wait() system call. Therefore, the child process destruction cannot be done before the parent calls the sys_wait() function. As the sys_wait() function, and the sys_kill() / sys_exit() function are executed by different threads running in different clusters, this requires a parent/child synchronization: After a sys-kill() or sys_exit(), the process descriptor in owner cluster is kept in zombi state : the main thread (i.e. thread 0 in process owner cluster) is not deleted until the sys_wait() syscall is executed by the parent process. The synchronization uses the term_state field in the child process descriptor, that contains the following informations :

• the PROCESS_FLAG_KILL indicates that a KILL request has been received by the child;
• the PROCESS_FLAG_EXIT indicates that an EXIT request has been made by the child;
• the PROCESS_FLAG_BLOCK indicates that a SIGSTOP signal has been received by the child;
• the PROCESS_FLAG_WAIT flag indicates that a WAIT request from parent has been received by the child and reported to parent.
• moreover, for an exit(), the exit() argument is registered in this term_state field.

The actual deletion of the process descriptor in owner cluster is done by the sys_wait() function, that must be executed by the parent main thread (i.e. thread 0 in parent owner cluster). This sys_wait() function executes an infinite loop. At each iteration the parent scan all children owner descriptors. When it detects that one child terminated, it set the PROCESS_FLAG_WAIT in child descriptor, set the THREAD_FLAG_DELETE in the child main thread, and exit to report the child termination state to parent process. It is the responsibility of the parent process to re-enter the sys_wait() syscall for the other children. When the parent process does not detect a terminated child at the end of an iteration, it deschedules without blocking.

6.2) detailed exit scenario

This section describes the termination of a process caused by a sys_exit().

1. The sys_exit() syscall must be executed by the main thread of the calling process( i.e. thread 0 in owner cluster),
2. The sys_exit() function calls the process_sigaction() function that send a multicast, parallel and non blocking RPC to all clusters containing at least one thread of the calling process, to block all process threads, but the main (calling) thread. This function returns only when all threads (but the main) are blocked and descheduled.
3. The sys_exit() function calls again the process_sigaction() function that send another multicast, parallel and non blocking RPC to the same clusters, to mark for delete all process threads, but the main (calling) thread. The marked threads will be actually destroyed by the scheduler at the next scheduling point. The remote process descriptor copies are actually destroyed by the scheduler when the last thread in remote cluster is destroyed.
4. The sys_exit() function set the PROCESS_TERM_EXIT flag in owner process descriptor to synchronize with the parent process, and deschedules.
5. This main thread, and the process descriptor in owner cluster will be actually destroyed by the scheduler when the parent process sys_wait() function marks this main thread for delete.

6.3) detailed kill scenario

This section describes the termination of a target process caused by a sys_kill( SIGKILL ).

1. The sys_kill() syscali must be executed by the main thread of the target process, OR by any thread of another process than the target process.
2. The sys_kill() function calls the process_sigaction() function that send a multicast, parallel and non blocking RPC to all clusters containing at least one thread of the target process, to block all process threads, but the main thread. This function returns only when all threads (but the main) are blocked and descheduled.
3. The sys_kill() function calls again the process_sigaction() function that send another multicast, parallel and non blocking RPC to the same clusters, to mark for delete all process threads, but the main thread. The marked threads will be actually destroyed by the scheduler at the next scheduling point. The target process descriptor copies are actually destroyed by the scheduler when the last thread in remote cluster is destroyed.
4. The sys_kill() function set the PROCESS_TERM_KILL flag in the target process descriptor in owner cluster to synchronize with its parent process, and returns.
5. The target process main thread, and the target process descriptor in owner cluster will be actually destroyed by the scheduler when the parent process sys_wait() function marks this main thread for delete.