schulung_tools/notes add some kernel notes

they should be converted to a 'Kernel API' slideset.

Signed-off-by: Manuel Traut <manut@linutronix.de>

1 files changed, 81 insertions, 0 deletions

diff --git a/schulung_tools/notes/deferred-work.txt b/schulung_tools/notes/deferred-work.txt
new file mode 100644
index 0000000..5ff5846
--- /dev/null
+++ b/schulung_tools/notes/deferred-work.txt
@@ -0,0 +1,81 @@
+(time units)
+- jiffies (1 jiffy = 1 "heartbeat")
+- "jiffies" is a global variable
+
+(delaying/sleeping)
+- ndelay, udelay, mdelay (busy waiting)
+- msleep, ssleep (sleeping)
+
+(deferred work methods)
+wait queues
+- declare wait queue
+- task 1 will wait on the wait queue (i.e. worker kernel thread)
+- task 2 will notify the wait queue to wake up (i.e. interrupt handler)
+- wait_event_timeout(waitqueue, time_in_jiffies)
+  - HZ is defined to 1 second of jiffies
+  - example of 5 seconds:    wait_event_timeout(wq, 5 * HZ)
+  - example of 0.01 seconds: wait_event_timeout(wq, HZ / 100)
+
+scheduling
+- set_current_state(TASK_UNINTERRUPTIBLE);
+- schedule_timeout(time_in_jiffies);
+  - give the cpu to someone else
+  - I want the cpu back at time_in_jiffies (as a maximum)
+
+timers
+- add_timer, mod_timer, del_timer, timer_pending
+- specify a function callback and private data (with add_timer)
+- specify a delay when it should be called
+  (with add_timer, specified in jiffies)
+- softirq context (this is atomic!)
+- run on whatever cpu called add_timer
+- in_softirq()
+- hrtimer: net/can/bcm.c:970 hrtimer_init / hrtimer_start / hrtimer_cancel
+- jiffie:  drivers/pcmcia/soc_common.c:802 timer_setup / mod_timer / del_timer_sync
+- ndelay, udelay, mdelay (busy waiting)
+- msleep, ssleep (sleeping)
+
+tasklets
+- tasklets run atomically in a run-to-complete fashion, but HW interrupts are
+    enabled!
+- tasklet_enable, tasklet_disable, tasklet_schedule, tasklet_kill
+- specify a function callback and private data (with tasklet creation)
+- runs on whatever cpu called tasklet_schedule
+- softirq context (this is atomic!)
+    -> scheduled function isn't allowed to use sleep
+- serialized with itself
+- https://linux-kernel-labs.github.io/master/labs/deferred_work.html#tasklets
+- drivers/firewire/ohci.c:993 tasklet_init / tasklet_kill / tasklet_disable / tasklet_schedule
+
+workqueues
+- schedule a function to be executed in a different context
+- PREPARE_WORK, queue_work, queue_work_delayed (in jiffies),
+  cancel_delayed_work, flush_workqueue
+- specify a function callback and private data (with PREPARE_WORK)
+- runs on whatever cpu called queue_work
+  (unless WQ_UNBOUND is used, then runs on any cpu)
+- execution can be delayed (queue_work_delayed)
+- runs in a kernel thread
+- can use shared/system workqueues (you do not need to create your own)
+- serialized with itself
+- scheduled function can use sleep
+- Documentation/core-api/workqueue.rst
+- include/linux/workqueue.h
+- net/rfkill/core.c:
+  INIT_WORK - schedule_work, cancel_work_sync
+  INIT_DELAYED_WORK - queue_delayed_work, cancel_delayed_work_sync
+      delayed* enables to specify delay while queuing
+      schedule_ vs. queue_ : schedule_* uses a global queue, queue_*
+      requires us to specify a specific queue
+
+
+threaded interrupt handlers
+- useful for handling interrupts in a non-atomic context
+
+completions
+- Documentation/scheduler/completion.txt
+- wait_for_completion() is not usable in atomic, preempt_disabled or irq_off
+    situations, use try_wait_for_completion() there
+- fs/btrfs/qgroup.c:3288:
+    init_completion / wait_for_completion / complete_all done in rescan_worker
+