windows-server-2003/base/ntos/ke/idsched.c

/*++

Copyright (c) 2002  Microsoft Corporation

Module Name:

    idsched.c

Abstract:

    This module implement idle scheduling. Idle scheduling is performed when
    the current thread is entering a wait state or must be rescheduled for
    any other reason (e.g., affinity change), and a suitable thread cannot be
    found after searching a suitable subset of the processor ready queues.

Author:

    David N. Cutler (davec) 25-Jan-2002

Environment:

    Kernel mode only.

--*/

#include "ki.h"

#if !defined(NT_UP)

PKTHREAD
FASTCALL
KiIdleSchedule (
    PKPRCB CurrentPrcb
    )

/*++

Routine Description:

    This function is called when the idle schedule flag is set in the current
    PRCB. This flag is set when the current thread is entering a wait state
    or must be rescheduled for any other reason (e.g., affinity change) and
    a suitable thread cannot be found after searching all the processor ready
    queues outside the dispatcher database lock. A second pass over the ready
    queues is required since the processor was not idle during the first scan,
    and therefore, a potential candidate thread may have been missed.

Arguments:

    CurrentPrcb - Supplies a pointer to the current processor block.

Return Value:

    If a thread is found to run, then a pointer to the thread is returned.
    Otherwise, NULL is returned.

    N.B. If a thread is found, then IRQL is returned at SYNCH_LEVEL. If a
         thread is not found, then IRQL is returned at DISPATCH_LEVEL.

--*/

{

    LONG Index;
    LONG Limit;
    LONG Number;
    PKTHREAD NewThread;
    ULONG Processor;
    PKPRCB TargetPrcb;

    ASSERT (CurrentPrcb == KeGetCurrentPrcb());

    //
    // Raise IRQL to SYNCH_LEVEL, acquire the current PRCB lock, and clear
    // idle schedule.
    //

    KeRaiseIrqlToSynchLevel();
    KiAcquirePrcbLock(CurrentPrcb);
    CurrentPrcb->IdleSchedule = FALSE;

    //
    // If a thread has already been selected to run on the current processor,
    // then select that thread. Otherwise, attempt to select a thread from
    // the current processor dispatcher ready queues.
    //

    if ((NewThread = CurrentPrcb->NextThread) != NULL) {

        //
        // Clear the next thread address, set the current thread address, and
        // set the thread state to running.
        //

        CurrentPrcb->NextThread = NULL;
        CurrentPrcb->CurrentThread = NewThread;
        NewThread->State = Running;

    } else {

        //
        // Attempt to select a thread from the current processor dispatcher
        // ready queues.
        //

        NewThread = KiSelectReadyThread(0, CurrentPrcb);
        if (NewThread != NULL) {
            CurrentPrcb->NextThread = NULL;
            CurrentPrcb->CurrentThread = NewThread;
            NewThread->State = Running;

        } else {

            //
            // Release the current PRCB lock and attempt to select a thread
            // from any processor dispatcher ready queues.
            //
            // If this is a multinode system, then start with the processors
            // on the same node. Otherwise, start with the current processor.
            //

            KiReleasePrcbLock(CurrentPrcb);
            Processor = CurrentPrcb->Number;
            Index = Processor;
            if (KeNumberNodes > 1) {
                KeFindFirstSetLeftAffinity(CurrentPrcb->ParentNode->ProcessorMask,
                                           (PULONG)&Index);
            }
        
            Limit = KeNumberProcessors - 1;
            Number = Limit;
            do {
                TargetPrcb = KiProcessorBlock[Index];
                if (CurrentPrcb != TargetPrcb) {
                    if (TargetPrcb->ReadySummary != 0) {

                        //
                        // Acquire both current and target PRCB locks in
                        // address order to prevent deadlock.
                        //
            
                        if (CurrentPrcb < TargetPrcb) {
                            KiAcquirePrcbLock(CurrentPrcb);
                            KiAcquirePrcbLock(TargetPrcb);
    
                        } else {
                            KiAcquirePrcbLock(TargetPrcb);
                            KiAcquirePrcbLock(CurrentPrcb);
                        }

                        //
                        // If a new thread has not been selected to run on
                        // the current processor, then attempt to select a
                        // thread to run on the current processor.
                        //

                        if ((NewThread = CurrentPrcb->NextThread) == NULL) {
                            if ((TargetPrcb->ReadySummary != 0) &&
                                (NewThread = KiFindReadyThread(Processor,
                                                               TargetPrcb)) != NULL) {
    
                                //
                                // A new thread has been found to run on the
                                // current processor.
                                //
    
                                NewThread->State = Running;
                                KiReleasePrcbLock(TargetPrcb);
                                CurrentPrcb->NextThread = NULL;
                                CurrentPrcb->CurrentThread = NewThread;

                                //
                                // Clear idle on the current processor
                                // and update the idle SMT summary set to
                                // indicate the set is not idle.
                                //

                                KiClearIdleSummary(AFFINITY_MASK(Processor));
                                KiClearSMTSummary(CurrentPrcb->MultiThreadProcessorSet);
                                goto ThreadFound;

                            } else {
                                KiReleasePrcbLock(CurrentPrcb);
                                KiReleasePrcbLock(TargetPrcb);
                            }

                        } else {

                            //
                            // A thread has already been selected to run on
                            // the current processor. It is possible that
                            // the thread is the idle thread due to a state
                            // change that made a scheduled runable thread
                            // unrunable.
                            //
                            // N.B. If the idle thread is selected, then the
                            //      current processor is idle. Otherwise,
                            //      the current processor is not idle.
                            //

                            if (NewThread == CurrentPrcb->IdleThread) {
                                CurrentPrcb->NextThread = NULL;
                                CurrentPrcb->IdleSchedule = FALSE;
                                KiReleasePrcbLock(CurrentPrcb);
                                KiReleasePrcbLock(TargetPrcb);
                                continue;

                            } else {
                                NewThread->State = Running;
                                KiReleasePrcbLock(TargetPrcb);
                                CurrentPrcb->NextThread = NULL;
                                CurrentPrcb->CurrentThread = NewThread;
                                goto ThreadFound;
                            }
                        }
                    }
                }
        
                Index -= 1;
                if (Index < 0) {
                    Index = Limit;
                }
        
                Number -= 1;
            } while (Number >= 0);
        }
    }

    //
    // If a new thread has been selected for execution, then release the
    // PRCB lock and acquire the idle thread lock. Otherwise, lower IRQL
    // to DISPATCH_LEVEL.
    //

ThreadFound:;
    if (NewThread == NULL) {
        KeLowerIrql(DISPATCH_LEVEL);

    } else {
        KiSetContextSwapBusy(CurrentPrcb->IdleThread);
        KiReleasePrcbLock(CurrentPrcb);
    }

    return NewThread;
}

#endif