source: src/libcfa/concurrency/preemption.c@ 42be3c3

//
// Cforall Version 1.0.0 Copyright (C) 2016 University of Waterloo
//
// The contents of this file are covered under the licence agreement in the
// file "LICENCE" distributed with Cforall.
//
// signal.c --
//
// Author           : Thierry Delisle
// Created On       : Mon Jun 5 14:20:42 2017
// Last Modified By : Peter A. Buhr
// Last Modified On : Tue Jan 23 17:59:30 2018
// Update Count     : 7
//

#include "preemption.h"

#define ftype `ftype`
extern "C" {
#include <errno.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
}
#undef ftype

#include "bits/signal.h"

//TODO move to defaults
#define __CFA_DEFAULT_PREEMPTION__ 10000

//TODO move to defaults
__attribute__((weak)) unsigned int default_preemption() {
        return __CFA_DEFAULT_PREEMPTION__;
}

// FwdDeclarations : timeout handlers
static void preempt( processor   * this );
static void timeout( thread_desc * this );

// FwdDeclarations : Signal handlers
void sigHandler_ctxSwitch( __CFA_SIGPARMS__ );
void sigHandler_segv     ( __CFA_SIGPARMS__ );
void sigHandler_abort    ( __CFA_SIGPARMS__ );

// FwdDeclarations : alarm thread main
void * alarm_loop( __attribute__((unused)) void * args );

// Machine specific register name
#if   defined(__x86_64__)
#define CFA_REG_IP REG_RIP
#elif defined(__i386__)
#define CFA_REG_IP REG_EIP
#elif defined(__ARM_ARCH__)
#define CFA_REG_IP REG_R15
#endif

KERNEL_STORAGE(event_kernel_t, event_kernel);         // private storage for event kernel
event_kernel_t * event_kernel;                        // kernel public handle to even kernel
static pthread_t alarm_thread;                        // pthread handle to alarm thread

void ?{}(event_kernel_t & this) {
        (this.alarms){};
        (this.lock){};
}

//=============================================================================================
// Kernel Preemption logic
//=============================================================================================

// Get next expired node
static inline alarm_node_t * get_expired( alarm_list_t * alarms, __cfa_time_t currtime ) {
        if( !alarms->head ) return NULL;                          // If no alarms return null
        if( alarms->head->alarm >= currtime ) return NULL;        // If alarms head not expired return null
        return pop(alarms);                                       // Otherwise just pop head
}

// Tick one frame of the Discrete Event Simulation for alarms
void tick_preemption() {
        alarm_node_t * node = NULL;                     // Used in the while loop but cannot be declared in the while condition
        alarm_list_t * alarms = &event_kernel->alarms;  // Local copy for ease of reading
        __cfa_time_t currtime = __kernel_get_time();    // Check current time once so we everything "happens at once"

        //Loop throught every thing expired
        while( node = get_expired( alarms, currtime ) ) {

                // Check if this is a kernel
                if( node->kernel_alarm ) {
                        preempt( node->proc );
                }
                else {
                        timeout( node->thrd );
                }

                // Check if this is a periodic alarm
                __cfa_time_t period = node->period;
                if( period > 0 ) {
                        node->alarm = currtime + period;    // Alarm is periodic, add currtime to it (used cached current time)
                        insert( alarms, node );             // Reinsert the node for the next time it triggers
                }
                else {
                        node->set = false;                  // Node is one-shot, just mark it as not pending
                }
        }

        // If there are still alarms pending, reset the timer
        if( alarms->head ) { __kernel_set_timer( alarms->head->alarm - currtime ); }
}

// Update the preemption of a processor and notify interested parties
void update_preemption( processor * this, __cfa_time_t duration ) {
        alarm_node_t * alarm = this->preemption_alarm;

        // Alarms need to be enabled
        if ( duration > 0 && !alarm->set ) {
                alarm->alarm = __kernel_get_time() + duration;
                alarm->period = duration;
                register_self( alarm );
        }
        // Zero duraction but alarm is set
        else if ( duration == 0 && alarm->set ) {
                unregister_self( alarm );
                alarm->alarm = 0;
                alarm->period = 0;
        }
        // If alarm is different from previous, change it
        else if ( duration > 0 && alarm->period != duration ) {
                unregister_self( alarm );
                alarm->alarm = __kernel_get_time() + duration;
                alarm->period = duration;
                register_self( alarm );
        }
}

//=============================================================================================
// Kernel Signal Tools
//=============================================================================================

__cfaabi_dbg_debug_do( static thread_local void * last_interrupt = 0; )

extern "C" {
        // Disable interrupts by incrementing the counter
        void disable_interrupts() {
                __attribute__((unused)) unsigned short new_val = __atomic_add_fetch_2( &disable_preempt_count, 1, __ATOMIC_SEQ_CST );
                verify( new_val < 65_000u );              // If this triggers someone is disabling interrupts without enabling them
        }

        // Enable interrupts by decrementing the counter
        // If counter reaches 0, execute any pending CtxSwitch
        void enable_interrupts( __cfaabi_dbg_ctx_param ) {
                processor * proc   = this_processor;      // Cache the processor now since interrupts can start happening after the atomic add
                thread_desc * thrd = this_thread;         // Cache the thread now since interrupts can start happening after the atomic add

                unsigned short prev = __atomic_fetch_add_2( &disable_preempt_count, -1, __ATOMIC_SEQ_CST );
                verify( prev != 0u );                     // If this triggers someone is enabled already enabled interruptsverify( prev != 0u );

                // Check if we need to prempt the thread because an interrupt was missed
                if( prev == 1 && proc->pending_preemption ) {
                        proc->pending_preemption = false;
                        BlockInternal( thrd );
                }

                // For debugging purposes : keep track of the last person to enable the interrupts
                __cfaabi_dbg_debug_do( proc->last_enable = caller; )
        }

        // Disable interrupts by incrementint the counter
        // Don't execute any pending CtxSwitch even if counter reaches 0
        void enable_interrupts_noPoll() {
                __attribute__((unused)) unsigned short prev = __atomic_fetch_add_2( &disable_preempt_count, -1, __ATOMIC_SEQ_CST );
                verifyf( prev != 0u, "Incremented from %u\n", prev );                     // If this triggers someone is enabled already enabled interrupts
        }
}

// sigprocmask wrapper : unblock a single signal
static inline void signal_unblock( int sig ) {
        sigset_t mask;
        sigemptyset( &mask );
        sigaddset( &mask, sig );

        if ( pthread_sigmask( SIG_UNBLOCK, &mask, NULL ) == -1 ) {
            abortf( "internal error, pthread_sigmask" );
        }
}

// sigprocmask wrapper : block a single signal
static inline void signal_block( int sig ) {
        sigset_t mask;
        sigemptyset( &mask );
        sigaddset( &mask, sig );

        if ( pthread_sigmask( SIG_BLOCK, &mask, NULL ) == -1 ) {
            abortf( "internal error, pthread_sigmask" );
        }
}

// kill wrapper : signal a processor
static void preempt( processor * this ) {
        pthread_kill( this->kernel_thread, SIGUSR1 );
}

// reserved for future use
static void timeout( thread_desc * this ) {
        //TODO : implement waking threads
}


// Check if a CtxSwitch signal handler shoud defer
// If true  : preemption is safe
// If false : preemption is unsafe and marked as pending
static inline bool preemption_ready() {
        bool ready = disable_preempt_count == 0 && !preemption_in_progress; // Check if preemption is safe
        this_processor->pending_preemption = !ready;                        // Adjust the pending flag accordingly
        return ready;
}

//=============================================================================================
// Kernel Signal Startup/Shutdown logic
//=============================================================================================

// Startup routine to activate preemption
// Called from kernel_startup
void kernel_start_preemption() {
        __cfaabi_dbg_print_safe("Kernel : Starting preemption\n");

        // Start with preemption disabled until ready
        disable_preempt_count = 1;

        // Initialize the event kernel
        event_kernel = (event_kernel_t *)&storage_event_kernel;
        (*event_kernel){};

        // Setup proper signal handlers
        __kernel_sigaction( SIGUSR1, sigHandler_ctxSwitch, SA_SIGINFO | SA_RESTART );         // CtxSwitch handler

        signal_block( SIGALRM );

        pthread_create( &alarm_thread, NULL, alarm_loop, NULL );
}

// Shutdown routine to deactivate preemption
// Called from kernel_shutdown
void kernel_stop_preemption() {
        __cfaabi_dbg_print_safe("Kernel : Preemption stopping\n");

        // Block all signals since we are already shutting down
        sigset_t mask;
        sigfillset( &mask );
        sigprocmask( SIG_BLOCK, &mask, NULL );

        // Notify the alarm thread of the shutdown
        sigval val = { 1 };
        pthread_sigqueue( alarm_thread, SIGALRM, val );

        // Wait for the preemption thread to finish
        pthread_join( alarm_thread, NULL );

        // Preemption is now fully stopped

        __cfaabi_dbg_print_safe("Kernel : Preemption stopped\n");
}

// Raii ctor/dtor for the preemption_scope
// Used by thread to control when they want to receive preemption signals
void ?{}( preemption_scope & this, processor * proc ) {
        (this.alarm){ proc, zero_time, zero_time };
        this.proc = proc;
        this.proc->preemption_alarm = &this.alarm;

        update_preemption( this.proc, from_us(this.proc->cltr->preemption) );
}

void ^?{}( preemption_scope & this ) {
        disable_interrupts();

        update_preemption( this.proc, zero_time );
}

//=============================================================================================
// Kernel Signal Handlers
//=============================================================================================

// Context switch signal handler
// Receives SIGUSR1 signal and causes the current thread to yield
void sigHandler_ctxSwitch( __CFA_SIGPARMS__ ) {
#if defined( __ARM_ARCH )
        __cfaabi_dbg_debug_do( last_interrupt = (void *)(cxt->uc_mcontext.arm_pc); )
#else
        __cfaabi_dbg_debug_do( last_interrupt = (void *)(cxt->uc_mcontext.gregs[CFA_REG_IP]); )
#endif

                // Check if it is safe to preempt here
        if( !preemption_ready() ) { return; }

        __cfaabi_dbg_print_buffer_decl(" KERNEL: preempting core %p (%p).\n", this_processor, this_thread);

        preemption_in_progress = true;                      // Sync flag : prevent recursive calls to the signal handler
        signal_unblock( SIGUSR1 );                          // We are about to CtxSwitch out of the signal handler, let other handlers in
        preemption_in_progress = false;                     // Clear the in progress flag

        // Preemption can occur here

        BlockInternal( (thread_desc*)this_thread );         // Do the actual CtxSwitch
}

// Main of the alarm thread
// Waits on SIGALRM and send SIGUSR1 to whom ever needs it
void * alarm_loop( __attribute__((unused)) void * args ) {
        // Block sigalrms to control when they arrive
        sigset_t mask;
        sigemptyset( &mask );
        sigaddset( &mask, SIGALRM );

        if ( pthread_sigmask( SIG_BLOCK, &mask, NULL ) == -1 ) {
            abortf( "internal error, pthread_sigmask" );
        }

        // Main loop
        while( true ) {
                // Wait for a sigalrm
                siginfo_t info;
                int sig = sigwaitinfo( &mask, &info );

                if( sig < 0 ) {
                        //Error!
                        int err = errno;
                        switch( err ) {
                                case EAGAIN :
                                case EINTR :
                                        continue;
                        case EINVAL :
                                        abortf("Timeout was invalid.");
                                default:
                                        abortf("Unhandled error %d", err);
                        }
                }

                // If another signal arrived something went wrong
                assertf(sig == SIGALRM, "Kernel Internal Error, sigwait: Unexpected signal %d (%d : %d)\n", sig, info.si_code, info.si_value.sival_int);

                // __cfaabi_dbg_print_safe("Kernel : Caught alarm from %d with %d\n", info.si_code, info.si_value.sival_int );
                // Switch on the code (a.k.a. the sender) to
                switch( info.si_code )
                {
                // Timers can apparently be marked as sent for the kernel
                // In either case, tick preemption
                case SI_TIMER:
                case SI_KERNEL:
                        // __cfaabi_dbg_print_safe("Kernel : Preemption thread tick\n");
                        lock( event_kernel->lock __cfaabi_dbg_ctx2 );
                        tick_preemption();
                        unlock( event_kernel->lock );
                        break;
                // Signal was not sent by the kernel but by an other thread
                case SI_QUEUE:
                        // For now, other thread only signal the alarm thread to shut it down
                        // If this needs to change use info.si_value and handle the case here
                        goto EXIT;
                }
        }

EXIT:
        __cfaabi_dbg_print_safe("Kernel : Preemption thread stopping\n");
        return NULL;
}

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