Re: [PATCH v2 1/2] cpuidle : auto-promotion for cpuidle states
From: Abhishek
Date: Tue Apr 09 2019 - 05:29:08 EST
Hi Daniel,
Thanks for such a descriptive review. I will include all the suggestions
made in my next iteration.
--Abhishek
On 04/08/2019 07:42 PM, Daniel Axtens wrote:
Hi Abhishek,
Currently, the cpuidle governors (menu /ladder) determine what idle state
an idling CPU should enter into based on heuristics that depend on the
idle history on that CPU. Given that no predictive heuristic is perfect,
there are cases where the governor predicts a shallow idle state, hoping
that the CPU will be busy soon. However, if no new workload is scheduled
on that CPU in the near future, the CPU will end up in the shallow state.
In case of POWER, this is problematic, when the predicted state in the
aforementioned scenario is a lite stop state, as such lite states will
inhibit SMT folding, thereby depriving the other threads in the core from
using the core resources.
To address this, such lite states need to be autopromoted. The cpuidle-
core can queue timer to correspond with the residency value of the next
available state. Thus leading to auto-promotion to a deeper idle state as
soon as possible.
This sounds sensible to me, although I'm not really qualified to offer a
full power-management opinion on it. I have some general code questions
and comments, however, which are below:
Signed-off-by: Abhishek Goel <huntbag@xxxxxxxxxxxxxxxxxx>
---
v1->v2 : Removed timeout_needed and rebased to current upstream kernel
drivers/cpuidle/cpuidle.c | 68 +++++++++++++++++++++++++++++-
drivers/cpuidle/governors/ladder.c | 3 +-
drivers/cpuidle/governors/menu.c | 22 +++++++++-
include/linux/cpuidle.h | 10 ++++-
4 files changed, 99 insertions(+), 4 deletions(-)
diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
index 7f108309e..11ce43f19 100644
--- a/drivers/cpuidle/cpuidle.c
+++ b/drivers/cpuidle/cpuidle.c
@@ -36,6 +36,11 @@ static int enabled_devices;
static int off __read_mostly;
static int initialized __read_mostly;
+struct auto_promotion {
+ struct hrtimer hrtimer;
+ unsigned long timeout_us;
+};
+
int cpuidle_disabled(void)
{
return off;
@@ -188,6 +193,54 @@ int cpuidle_enter_s2idle(struct cpuidle_driver *drv, struct cpuidle_device *dev)
}
#endif /* CONFIG_SUSPEND */
+enum hrtimer_restart auto_promotion_hrtimer_callback(struct hrtimer *hrtimer)
+{
+ return HRTIMER_NORESTART;
+}
+
+#ifdef CONFIG_CPU_IDLE_AUTO_PROMOTION
As far as I can tell, this config flag isn't defined until the next
patch, making this dead code for now. Is this intentional?
+DEFINE_PER_CPU(struct auto_promotion, ap);
A quick grep suggests that most per-cpu variable have more descriptive
names, perhaps this one should too.
+
+static void cpuidle_auto_promotion_start(int cpu, struct cpuidle_state *state)
+{
+ struct auto_promotion *this_ap = &per_cpu(ap, cpu);
+
+ if (state->flags & CPUIDLE_FLAG_AUTO_PROMOTION)
+ hrtimer_start(&this_ap->hrtimer, ns_to_ktime(this_ap->timeout_us
+ * 1000), HRTIMER_MODE_REL_PINNED);
Would it be clearer to have both sides of the multiplication on the same
line? i.e.
+ hrtimer_start(&this_ap->hrtimer,
+ ns_to_ktime(this_ap->timeout_us * 1000),
+ HRTIMER_MODE_REL_PINNED);
+}
+
+static void cpuidle_auto_promotion_cancel(int cpu)
+{
+ struct hrtimer *hrtimer;
+
+ hrtimer = &per_cpu(ap, cpu).hrtimer;
+ if (hrtimer_is_queued(hrtimer))
+ hrtimer_cancel(hrtimer);
+}
+
+static void cpuidle_auto_promotion_update(int cpu, unsigned long timeout)
+{
+ per_cpu(ap, cpu).timeout_us = timeout;
+}
+
+static void cpuidle_auto_promotion_init(int cpu, struct cpuidle_driver *drv)
+{
+ struct auto_promotion *this_ap = &per_cpu(ap, cpu);
+
+ hrtimer_init(&this_ap->hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
+ this_ap->hrtimer.function = auto_promotion_hrtimer_callback;
+}
+#else
+static inline void cpuidle_auto_promotion_start(int cpu, struct cpuidle_state
+ *state) { }
+static inline void cpuidle_auto_promotion_cancel(int cpu) { }
+static inline void cpuidle_auto_promotion_update(int cpu, unsigned long
+ timeout) { }
+static inline void cpuidle_auto_promotion_init(int cpu, struct cpuidle_driver
+ *drv) { }
Several of these have the type, then a line break, and then the name
(unsigned long\n timeout). This is a bit harder to read, they should
probably all be on the same line.
+#endif
+
/**
* cpuidle_enter_state - enter the state and update stats
* @dev: cpuidle device for this cpu
@@ -225,12 +278,17 @@ int cpuidle_enter_state(struct cpuidle_device *dev, struct cpuidle_driver *drv,
trace_cpu_idle_rcuidle(index, dev->cpu);
time_start = ns_to_ktime(local_clock());
+ cpuidle_auto_promotion_start(dev->cpu, target_state);
+
stop_critical_timings();
entered_state = target_state->enter(dev, drv, index);
start_critical_timings();
sched_clock_idle_wakeup_event();
time_end = ns_to_ktime(local_clock());
+
+ cpuidle_auto_promotion_cancel(dev->cpu);
+
trace_cpu_idle_rcuidle(PWR_EVENT_EXIT, dev->cpu);
/* The cpu is no longer idle or about to enter idle. */
@@ -312,7 +370,13 @@ int cpuidle_enter_state(struct cpuidle_device *dev, struct cpuidle_driver *drv,
int cpuidle_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
bool *stop_tick)
{
- return cpuidle_curr_governor->select(drv, dev, stop_tick);
+ unsigned long timeout_us, ret;
+
+ timeout_us = UINT_MAX;
+ ret = cpuidle_curr_governor->select(drv, dev, stop_tick, &timeout_us);
+ cpuidle_auto_promotion_update(dev->cpu, timeout_us);
+
+ return ret;
}
/**
@@ -658,6 +722,8 @@ int cpuidle_register(struct cpuidle_driver *drv,
device = &per_cpu(cpuidle_dev, cpu);
device->cpu = cpu;
+ cpuidle_auto_promotion_init(cpu, drv);
+
#ifdef CONFIG_ARCH_NEEDS_CPU_IDLE_COUPLED
/*
* On multiplatform for ARM, the coupled idle states could be
diff --git a/drivers/cpuidle/governors/ladder.c b/drivers/cpuidle/governors/ladder.c
index f0dddc66a..65b518dd7 100644
--- a/drivers/cpuidle/governors/ladder.c
+++ b/drivers/cpuidle/governors/ladder.c
@@ -64,7 +64,8 @@ static inline void ladder_do_selection(struct ladder_device *ldev,
* @dummy: not used
I think you need an addition to the docstring for your new variable.
*/
static int ladder_select_state(struct cpuidle_driver *drv,
- struct cpuidle_device *dev, bool *dummy)
+ struct cpuidle_device *dev, bool *dummy,
+ unsigned long *unused)
{
struct ladder_device *ldev = this_cpu_ptr(&ladder_devices);
struct ladder_device_state *last_state;
diff --git a/drivers/cpuidle/governors/menu.c b/drivers/cpuidle/governors/menu.c
index 5951604e7..835e337de 100644
--- a/drivers/cpuidle/governors/menu.c
+++ b/drivers/cpuidle/governors/menu.c
@@ -276,7 +276,7 @@ static unsigned int get_typical_interval(struct menu_device *data,
* @stop_tick: indication on whether or not to stop the tick
Likewise here.
*/
static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
- bool *stop_tick)
+ bool *stop_tick, unsigned long *timeout)
{
struct menu_device *data = this_cpu_ptr(&menu_devices);
int latency_req = cpuidle_governor_latency_req(dev->cpu);
@@ -442,6 +442,26 @@ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
}
}
+#ifdef CPUIDLE_FLAG_AUTO_PROMOTION
+ if (drv->states[idx].flags & CPUIDLE_FLAG_AUTO_PROMOTION) {
+ /*
+ * Timeout is intended to be defined as sum of target residency
+ * of next available state, entry latency and exit latency. If
+ * time interval equal to timeout is spent in current state,
+ * and if it is a shallow lite state, we may want to auto-
+ * promote from such state.
This comment makes sense if you already understand auto-promotion. That's
fair enough - you wrote it and you presumably understand what your code
does :) But for me it's a bit confusing! I think you want to start with
a sentence about what autopromotion is (preferably not using
power-specific terminology) and then explain the calculation of the
timeouts.
+ */
+ for (i = idx + 1; i < drv->state_count; i++) {
+ if (drv->states[i].disabled ||
+ dev->states_usage[i].disable)
+ continue;
+ *timeout = drv->states[i].target_residency +
+ 2 * drv->states[i].exit_latency;
+ break;
+ }
+ }
+#endif
+
return idx;
}
diff --git a/include/linux/cpuidle.h b/include/linux/cpuidle.h
index 3b3947232..84d76d1ec 100644
--- a/include/linux/cpuidle.h
+++ b/include/linux/cpuidle.h
@@ -72,6 +72,13 @@ struct cpuidle_state {
#define CPUIDLE_FLAG_POLLING BIT(0) /* polling state */
#define CPUIDLE_FLAG_COUPLED BIT(1) /* state applies to multiple cpus */
#define CPUIDLE_FLAG_TIMER_STOP BIT(2) /* timer is stopped on this state */
+/*
+ * State with only and only fast state bit set don't even lose user context.
"only and only"?
+ * But such states prevent other sibling threads from thread folding benefits.
+ * And hence we don't want to stay for too long in such states and want to
+ * auto-promote from it.
I think this comment mixes Power-specific and generic concepts. (But I'm
not a PM expert so tell me if I'm wrong here.) I think, if I've
understood correctly: in the generic code, the bit represents a state
that we do not want to linger in, which we want to definitely leave
after some time. On Power, we have a state that doesn't lose user
context but which prevents thread folding, so this is an example of a
state where we want to auto-promote.
+ */
+#define CPUIDLE_FLAG_AUTO_PROMOTION BIT(3)
struct cpuidle_device_kobj;
struct cpuidle_state_kobj;
@@ -243,7 +250,8 @@ struct cpuidle_governor {
int (*select) (struct cpuidle_driver *drv,
struct cpuidle_device *dev,
- bool *stop_tick);
+ bool *stop_tick, unsigned long
+ *timeout);
void (*reflect) (struct cpuidle_device *dev, int index);
};
--
2.17.1