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hw: fix scheduler timing on prio preemption

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Previously, the timer was used to remember the state of the time slices.
This was sufficient before priorities entered the scene as a thread always
received a fresh time slice when he was scheduled away. However, with
priorities this isn't always the case. A thread can be preempted by another
thread due to a higher priority. In this case the low-priority thread must
remember how much time he has consumed from its current time slice because
the timer gets re-programmed. Otherwise, if we have high-priority threads
that block and unblock with high frequency, the head of the next lower
priority would start with a fresh time slice all the time and is never
superseded.

fix #1287
This commit is contained in:
Martin Stein 2014-06-20 17:44:09 +02:00 committed by Christian Helmuth
parent dda8044183
commit 8dad54c914
7 changed files with 166 additions and 105 deletions
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68
repos/base-hw/src/core/include/kernel/processor.h
View File

@ -96,6 +96,8 @@ class Kernel::Processor_client : public Processor_scheduler::Item
Processor * _processor;
Cpu_lazy_state _lazy_state;
unsigned _tics_consumed;
/**
* Handle an interrupt exception that occured during execution
*
@ -143,7 +145,8 @@ class Kernel::Processor_client : public Processor_scheduler::Item
Processor_client(Processor * const processor, Priority const priority)
:
Processor_scheduler::Item(priority),
_processor(processor)
_processor(processor),
_tics_consumed(0)
{ }
/**
@ -155,12 +158,30 @@ class Kernel::Processor_client : public Processor_scheduler::Item
_unschedule();
}
/**
* Update how many tics the client consumed from its current time slice
*
* \param tics_left tics that aren't consumed yet at the slice
* \param tics_per_slice tics that the slice provides
*/
void update_tics_consumed(unsigned const tics_left,
unsigned const tics_per_slice)
{
unsigned const old_tics_left = tics_per_slice - _tics_consumed;
_tics_consumed += old_tics_left - tics_left;
}
/**
* Reset how many tics the client consumed from its current time slice
*/
void reset_tics_consumed() { _tics_consumed = 0; }
/***************
** Accessors **
***************/
Cpu_lazy_state * lazy_state() { return &_lazy_state; }
unsigned tics_consumed() { return _tics_consumed; }
};
class Kernel::Processor : public Cpu
@ -172,13 +193,18 @@ class Kernel::Processor : public Cpu
bool _ip_interrupt_pending;
Timer * const _timer;
/**
* Reset the scheduling timer for a new scheduling interval
*/
void _reset_timer()
void _start_timer(unsigned const tics) {
_timer->start_one_shot(tics, _id); }
unsigned _tics_per_slice() {
return _timer->ms_to_tics(USER_LAP_TIME_MS); }
void _update_timer(unsigned const tics_consumed,
unsigned const tics_per_slice)
{
unsigned const tics = _timer->ms_to_tics(USER_LAP_TIME_MS);
_timer->start_one_shot(tics, _id);
assert(tics_consumed <= tics_per_slice);
if (tics_consumed >= tics_per_slice) { _start_timer(1); }
else { _start_timer(tics_per_slice - tics_consumed); }
}
public:
@ -200,7 +226,7 @@ class Kernel::Processor : public Cpu
/**
* Initializate on the processor that this object corresponds to
*/
void init_processor_local() { _reset_timer(); }
void init_processor_local() { _update_timer(0, _tics_per_slice()); }
/**
* Check for a scheduling timeout and handle it in case
@ -213,7 +239,6 @@ class Kernel::Processor : public Cpu
{
if (_timer->interrupt_id(_id) != interrupt_id) { return false; }
_scheduler.yield_occupation();
_timer->clear_interrupt(_id);
return true;
}
@ -237,30 +262,7 @@ class Kernel::Processor : public Cpu
/**
* Handle exception of the processor and proceed its user execution
*/
void exception()
{
/*
* Request the current occupant without any update. While the
* processor was outside the kernel, another processor may have changed the
* scheduling of the local activities in a way that an update would return
* an occupant other than that whose exception caused the kernel entry.
*/
Processor_client * const old_client = _scheduler.occupant();
Cpu_lazy_state * const old_state = old_client->lazy_state();
old_client->exception(_id);
/*
* The processor local as well as remote exception-handling may have
* changed the scheduling of the local activities. Hence we must update the
* occupant.
*/
bool update;
Processor_client * const new_client = _scheduler.update_occupant(update);
if (update) { _reset_timer(); }
Cpu_lazy_state * const new_state = new_client->lazy_state();
prepare_proceeding(old_state, new_state);
new_client->proceed(_id);
}
void exception();
/***************
** Accessors **

29
repos/base-hw/src/core/include/kernel/scheduler.h
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@ -116,7 +116,7 @@ class Kernel::Scheduler
Double_list<T> _items[Priority::MAX + 1];
bool _yield;
bool _check_update(T * const occupant)
bool _does_update(T * const occupant)
{
if (_yield) {
_yield = false;
@ -138,18 +138,29 @@ class Kernel::Scheduler
/**
* Adjust occupant reference to the current scheduling plan
*
* \return updated occupant reference
* \param updated true on return if the occupant has changed/yielded
* \param refreshed true on return if the occupant got a new timeslice
*
* \return updated occupant
*/
T * update_occupant(bool & update)
T * update_occupant(bool & updated, bool & refreshed)
{
for (int i = Priority::MAX; i >= 0 ; i--) {
T * const head = _items[i].head();
if (!head) { continue; }
update = _check_update(head);
_occupant = head;
return head;
T * const new_occupant = _items[i].head();
if (!new_occupant) { continue; }
updated = _does_update(new_occupant);
T * const old_occupant = _occupant;
if (!old_occupant) { refreshed = true; }
else {
unsigned const new_prio = new_occupant->priority();
unsigned const old_prio = old_occupant->priority();
refreshed = new_prio <= old_prio;
}
_occupant = new_occupant;
return new_occupant;
}
update = _check_update(_idle);
updated = _does_update(_idle);
refreshed = true;
_occupant = 0;
return _idle;
}

16
repos/base-hw/src/core/include/spec/cortex_a9/timer.h
View File

@ -34,6 +34,11 @@ namespace Genode
*/
struct Load : Register<0x0, 32> { };
/**
* Counter value register
*/
struct Counter : Register<0x4, 32> { };
/**
* Timer control register
*/
@ -51,17 +56,14 @@ namespace Genode
struct Event : Bitfield<0,1> { }; /* if counter hit zero */
};
void _clear_interrupt() { write<Interrupt_status::Event>(1); }
public:
/**
* Constructor, clears the interrupt output
* Constructor
*/
Timer() : Mmio(Cpu::PRIVATE_TIMER_MMIO_BASE)
{
write<Control::Timer_enable>(0);
_clear_interrupt();
}
/**
@ -80,7 +82,7 @@ namespace Genode
inline void start_one_shot(unsigned const tics, unsigned)
{
/* reset timer */
_clear_interrupt();
write<Interrupt_status::Event>(1);
Control::access_t control = 0;
Control::Irq_enable::set(control, 1);
write<Control>(control);
@ -99,9 +101,9 @@ namespace Genode
}
/**
* Clear interrupt output line
* Return current native timer value
*/
void clear_interrupt(unsigned) { _clear_interrupt(); }
unsigned value(unsigned const) { return read<Counter>(); }
};
}

27
repos/base-hw/src/core/include/spec/exynos5/timer.h
View File

@ -228,11 +228,13 @@ class Genode::Timer : public Mmio
{
switch (processor_id) {
case 0:
write<L0_int_cstat::Frcnt>(1);
_run_0(0);
_acked_write<L0_frcntb, L0_wstat::Frcntb>(tics);
_run_0(1);
return;
case 1:
write<L1_int_cstat::Frcnt>(1);
_run_1(0);
_acked_write<L1_frcntb, L1_wstat::Frcntb>(tics);
_run_1(1);
@ -246,22 +248,6 @@ class Genode::Timer : public Mmio
*/
unsigned ms_to_tics(unsigned const ms) { return ms * _tics_per_ms; }
/**
* Clear interrupt output line
*/
void clear_interrupt(unsigned const processor_id)
{
switch (processor_id) {
case 0:
write<L0_int_cstat::Frcnt>(1);
return;
case 1:
write<L1_int_cstat::Frcnt>(1);
return;
default: return;
}
}
unsigned value(unsigned const processor_id)
{
switch (processor_id) {
@ -270,15 +256,6 @@ class Genode::Timer : public Mmio
default: return 0;
}
}
unsigned irq_state(unsigned const processor_id)
{
switch (processor_id) {
case 0: return read<L0_int_cstat::Frcnt>();
case 1: return read<L1_int_cstat::Frcnt>();
default: return 0;
}
}
};
namespace Kernel { class Timer : public Genode::Timer { }; }

35
repos/base-hw/src/core/include/spec/rpi/timer.h
View File

@ -24,6 +24,8 @@ namespace Genode
{
/**
* Timer driver for core
*
* Timer channel 0 apparently doesn't work on the RPI, so we use channel 1
*/
class Timer;
}
@ -32,16 +34,7 @@ class Genode::Timer : public Mmio
{
private:
/*
* The timer channel 0 apparently does not work on the Raspberry Pi.
* So we use channel 1.
*/
struct Cs : Register<0x0, 32>
{
struct Status : Bitfield<1, 1> { };
};
struct Cs : Register<0x0, 32> { struct M1 : Bitfield<1, 1> { }; };
struct Clo : Register<0x4, 32> { };
struct Cmp : Register<0x10, 32> { };
@ -49,28 +42,30 @@ class Genode::Timer : public Mmio
Timer() : Mmio(Board::SYSTEM_TIMER_MMIO_BASE) { }
static unsigned interrupt_id(unsigned)
{
return Board::SYSTEM_TIMER_IRQ;
}
static unsigned interrupt_id(int) { return Board::SYSTEM_TIMER_IRQ; }
inline void start_one_shot(uint32_t const tics, unsigned)
{
write<Clo>(0);
write<Cmp>(read<Clo>() + tics);
write<Cs::Status>(1);
write<Cs::M1>(1);
}
static uint32_t ms_to_tics(unsigned const ms)
{
return (Board::SYSTEM_TIMER_CLOCK / 1000) * ms;
}
static uint32_t ms_to_tics(unsigned const ms) {
return (Board::SYSTEM_TIMER_CLOCK / 1000) * ms; }
void clear_interrupt(unsigned)
{
write<Cs::Status>(1);
write<Cs::M1>(1);
read<Cs>();
}
unsigned value(unsigned)
{
Cmp::access_t const cmp = read<Cmp>();
Clo::access_t const clo = read<Clo>();
return cmp > clo ? cmp - clo : 0;
}
};
namespace Kernel { class Timer : public Genode::Timer { }; }

61
repos/base-hw/src/core/kernel/processor.cc
View File

@ -190,3 +190,64 @@ bool Kernel::Processor_domain_update::_perform(unsigned const domain_id)
}
return true;
}
void Kernel::Processor::exception()
{
/*
* Request the current occupant without any update. While the
* processor was outside the kernel, another processor may have changed the
* scheduling of the local activities in a way that an update would return
* an occupant other than that whose exception caused the kernel entry.
*/
Processor_client * const old_client = _scheduler.occupant();
Cpu_lazy_state * const old_state = old_client->lazy_state();
old_client->exception(_id);
/*
* The processor local as well as remote exception-handling may have
* changed the scheduling of the local activities. Hence we must update the
* occupant.
*/
bool updated, refreshed;
Processor_client * const new_client =
_scheduler.update_occupant(updated, refreshed);
/**
* There are three scheduling situations we have to deal with:
*
* The client has not changed and didn't yield:
*
* The client must not update its time-slice state as the timer
* can continue as is and hence keeps providing the information.
*
* The client has changed or did yield and the previous client
* received a fresh timeslice:
*
* The previous client can reset his time-slice state.
* The timer must be re-programmed according to the time-slice
* state of the new client.
*
* The client has changed and the previous client did not receive
* a fresh timeslice:
*
* The previous client must update its time-slice state. The timer
* must be re-programmed according to the time-slice state of the
* new client.
*/
if (updated) {
unsigned const tics_per_slice = _tics_per_slice();
if (refreshed) { old_client->reset_tics_consumed(); }
else {
unsigned const tics_left = _timer->value(_id);
old_client->update_tics_consumed(tics_left, tics_per_slice);
}
_update_timer(new_client->tics_consumed(), tics_per_slice);
}
/**
* Apply the CPU state of the new client and continue his execution
*/
Cpu_lazy_state * const new_state = new_client->lazy_state();
prepare_proceeding(old_state, new_state);
new_client->proceed(_id);
}

35
repos/base/include/drivers/timer/epit_base.h
View File

@ -114,7 +114,9 @@ namespace Genode
/* disable timer */
write<Cr::En>(0);
clear_interrupt(0);
/* clear interrupt */
write<Sr::Ocif>(1);
}
void _start_one_shot(unsigned const tics)
@ -157,21 +159,32 @@ namespace Genode
{
/* disable timer */
write<Cr::En>(0);
/* if the timer has hit zero already return 0 */
return read<Sr::Ocif>() ? 0 : read<Cnt>();
return value(0);
}
/**
* Clear interrupt output line
*/
void clear_interrupt(unsigned) { write<Sr::Ocif>(1); }
/**
* Translate milliseconds to a native timer value
*/
static unsigned ms_to_tics(unsigned const ms) {
return TICS_PER_MS * ms; }
unsigned ms_to_tics(unsigned const ms)
{
return TICS_PER_MS * ms;
}
/**
* Translate native timer value to milliseconds
*/
unsigned tics_to_ms(unsigned const tics)
{
return tics / TICS_PER_MS;
}
/**
* Return current native timer value
*/
unsigned value(unsigned const)
{
return read<Sr::Ocif>() ? 0 : read<Cnt>();
}
};
}