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| <h1 id="schedulercontext-switching">Scheduler/Context Switching</h1> |
| <p>Scheduler's job is to maintain the list of tasks and decide which one should be running next.</p> |
| <h2 id="description">Description</h2> |
| <p>Task states can be <em>running</em>, <em>ready to run</em> or <em>sleeping</em>.</p> |
| <p>When task is <em>running</em>, CPU is executing in that task's context. The program counter (PC) is pointing to instructions task wants to execute and stack pointer (SP) is pointing to task's stack.</p> |
| <p>Task which is <em>ready to run</em> wants to get on the CPU to do its work.</p> |
| <p>Task which is <em>sleeping</em> has no more work to do. It's waiting for someone else to wake it up.</p> |
| <p>Scheduler algorithm is simple: from among the tasks which are ready to run, pick the the one with highest priority (lowest numeric value in task's t_prio field), and make its state <em>running</em>.</p> |
| <p>Tasks which are either <em>running</em> or <em>ready to run</em> are kept in linked list <code>g_os_run_list</code>. This list is ordered by priority.</p> |
| <p>Tasks which are <em>sleeping</em> are kept in linked list <code>g_os_sleep_list</code>.</p> |
| <p>Scheduler has a CPU architecture specific component; this code is responsible for swapping in the task which should be <em>running</em>. This process is called context switch. During context switching the state of the CPU (e.g. registers) for the currently <em>running</em> task is stored and the new task is swapped in.</p> |
| <h2 id="list-of-functions">List of Functions</h2> |
| <p>The functions available in context_switch are:</p> |
| <table> |
| <thead> |
| <tr> |
| <th><strong>Function</strong></th> |
| <th><strong>Description</strong></th> |
| </tr> |
| </thead> |
| <tbody> |
| <tr> |
| <td><a href="../os_sched/">os_sched</a></td> |
| <td>Performs context switch if needed.</td> |
| </tr> |
| <tr> |
| <td><a href="../os_arch_ctx_sw/">os_arch_ctx_sw</a></td> |
| <td>Change the state of task given task to <em>running</em>.</td> |
| </tr> |
| <tr> |
| <td><a href="../os_sched_ctx_sw_hook/">os_sched_ctx_sw_hook</a></td> |
| <td>Performs task accounting when context switching.</td> |
| </tr> |
| <tr> |
| <td><a href="../os_sched_get_current_task/">os_sched_get_current_task</a></td> |
| <td>Returns the pointer to task which is currently <em>running</em>.</td> |
| </tr> |
| <tr> |
| <td><a href="../os_sched_insert/">os_sched_insert</a></td> |
| <td>Insert task into scheduler's ready to run list.</td> |
| </tr> |
| <tr> |
| <td><a href="../os_sched_next_task/">os_sched_next_task</a></td> |
| <td>Returns the pointer to highest priority task from the list which are <em>ready to run</em>.</td> |
| </tr> |
| <tr> |
| <td><a href="../os_sched_os_timer_exp/">os_sched_os_timer_exp</a></td> |
| <td>Inform scheduler that OS time has moved forward.</td> |
| </tr> |
| <tr> |
| <td><a href="../os_sched_remove/">os_sched_remove</a></td> |
| <td>Stops a task and removes it from all the OS task lists.</td> |
| </tr> |
| <tr> |
| <td><a href="../os_sched_resort/">os_sched_resort</a></td> |
| <td>Inform scheduler that the priority of the given task has changed and the <em>ready to run</em> list should be re-sorted.</td> |
| </tr> |
| <tr> |
| <td><a href="../os_sched_set_current_task/">os_sched_set_current_task</a></td> |
| <td>Sets the given task to <em>running</em>.</td> |
| </tr> |
| <tr> |
| <td><a href="../os_sched_sleep/">os_sched_sleep</a></td> |
| <td>The given task's state is changed from <em>ready to run</em> to <em>sleeping</em>.</td> |
| </tr> |
| <tr> |
| <td><a href="../os_sched_wakeup/">os_sched_wakeup</a></td> |
| <td>Called to make task <em>ready to run</em>. If task is <em>sleeping</em>, it is woken up.</td> |
| </tr> |
| </tbody> |
| </table> |
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