Re: [patch 4/7] tick: Handle broadcast wakeup of multiple cpus
On Wed, 13 Mar 2013, Lorenzo Pieralisi wrote:
> > + /* Take care of enforced broadcast requests */
> > + cpumask_or(tmpmask, tmpmask, tick_broadcast_force_mask);
> > + cpumask_clear(tick_broadcast_force_mask);
>
> I tested the set and it works fine on a dual cluster big.LITTLE testchip
> using broadcast timer to manage deep idle cluster states.
>
> Just asking a question: the force mask is cleared before sending the
> timer IPI. Would not be better to clear it after the IPI is sent in
>
> tick_do_broadcast(...) ?
>
> Can you spot a regression if we do this ? The idle thread checks that
> mask with irqs disabled, so it is possible that we clear the mask before
> the CPU has a chance to get the IPI. If we clear the mask after sending
> the IPI, we are increasing the chances for the idle thread to get it.
>
> It is just a further optimization, just asking, thanks.
Need to think about that.
> > @@ -524,7 +530,16 @@ void tick_broadcast_oneshot_control(unsi
> > WARN_ON_ONCE(cpumask_test_cpu(cpu,
> > tick_broadcast_pending_mask));
> > if (!cpumask_test_and_set_cpu(cpu,
> > tick_broadcast_oneshot_mask)) {
> > clockevents_set_mode(dev, CLOCK_EVT_MODE_SHUTDOWN);
> > - if (dev->next_event.tv64 < bc->next_event.tv64)
> > + /*
> > +* We only reprogram the broadcast timer if we
> > +* did not mark ourself in the force mask and
> > +* if the cpu local event is earlier than the
> > +* broadcast event. If the current CPU is in
> > +* the force mask, then we are going to be
> > +* woken by the IPI right away.
> > +*/
> > + if (!cpumask_test_cpu(cpu, tick_broadcast_force_mask) &&
> Is the test against tick_broadcast_force_mask necessary if we add the check
> in the idle thread before entering idle ? It does not hurt, agreed, and we'd
> better leave it there, it is just for my own understanding, thanks a lot.
Well, it's necessary for all archs which do not have the check (yet).
> Having said that, on the series:
>
> Tested-by: Lorenzo Pieralisi
Thanks,
tglx
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Re: [patch 4/7] tick: Handle broadcast wakeup of multiple cpus
Hi Thomas,
On Wed, Mar 06, 2013 at 11:18:35AM +, Thomas Gleixner wrote:
> Some brilliant hardware implementations wake multiple cores when the
> broadcast timer fires. This leads to the following interesting
> problem:
>
> CPU0 CPU1
> wakeup from idle wakeup from idle
>
> leave broadcast mode leave broadcast mode
> restart per cpu timer restart per cpu timer
> go back to idle
> handle broadcast
> (empty mask)
> enter broadcast mode
> programm broadcast device
> enter broadcast mode
> programm broadcast device
>
> So what happens is that due to the forced reprogramming of the cpu
> local timer, we need to set a event in the future. Now if we manage to
> go back to idle before the timer fires, we switch off the timer and
> arm the broadcast device with an already expired time (covered by
> forced mode). So in the worst case we repeat the above ping pong
> forever.
>
> Unfortunately we have no information about what caused the wakeup, but
> we can check current time against the expiry time of the local cpu. If
> the local event is already in the past, we know that the broadcast
> timer is about to fire and send an IPI. So we mark ourself as an IPI
> target even if we left broadcast mode and avoid the reprogramming of
> the local cpu timer.
>
> This still leaves the possibility that a CPU which is not handling the
> broadcast interrupt is going to reach idle again before the IPI
> arrives. This can't be solved in the core code and will be handled in
> follow up patches.
>
> Reported-by: Jason Liu
> Signed-off-by: Thomas Gleixner
> ---
> kernel/time/tick-broadcast.c | 59
> ++-
> 1 file changed, 58 insertions(+), 1 deletion(-)
>
> Index: tip/kernel/time/tick-broadcast.c
> ===
> --- tip.orig/kernel/time/tick-broadcast.c
> +++ tip/kernel/time/tick-broadcast.c
> @@ -393,6 +393,7 @@ int tick_resume_broadcast(void)
>
> static cpumask_var_t tick_broadcast_oneshot_mask;
> static cpumask_var_t tick_broadcast_pending_mask;
> +static cpumask_var_t tick_broadcast_force_mask;
>
> /*
> * Exposed for debugging: see timer_list.c
> @@ -462,6 +463,10 @@ again:
> }
> }
>
> + /* Take care of enforced broadcast requests */
> + cpumask_or(tmpmask, tmpmask, tick_broadcast_force_mask);
> + cpumask_clear(tick_broadcast_force_mask);
I tested the set and it works fine on a dual cluster big.LITTLE testchip
using broadcast timer to manage deep idle cluster states.
Just asking a question: the force mask is cleared before sending the
timer IPI. Would not be better to clear it after the IPI is sent in
tick_do_broadcast(...) ?
Can you spot a regression if we do this ? The idle thread checks that
mask with irqs disabled, so it is possible that we clear the mask before
the CPU has a chance to get the IPI. If we clear the mask after sending
the IPI, we are increasing the chances for the idle thread to get it.
It is just a further optimization, just asking, thanks.
> +
> /*
>* Wakeup the cpus which have an expired event.
>*/
> @@ -497,6 +502,7 @@ void tick_broadcast_oneshot_control(unsi
> struct clock_event_device *bc, *dev;
> struct tick_device *td;
> unsigned long flags;
> + ktime_t now;
> int cpu;
>
> /*
> @@ -524,7 +530,16 @@ void tick_broadcast_oneshot_control(unsi
> WARN_ON_ONCE(cpumask_test_cpu(cpu,
> tick_broadcast_pending_mask));
> if (!cpumask_test_and_set_cpu(cpu,
> tick_broadcast_oneshot_mask)) {
> clockevents_set_mode(dev, CLOCK_EVT_MODE_SHUTDOWN);
> - if (dev->next_event.tv64 < bc->next_event.tv64)
> + /*
> + * We only reprogram the broadcast timer if we
> + * did not mark ourself in the force mask and
> + * if the cpu local event is earlier than the
> + * broadcast event. If the current CPU is in
> + * the force mask, then we are going to be
> + * woken by the IPI right away.
> + */
> + if (!cpumask_test_cpu(cpu, tick_broadcast_force_mask) &&
Is the test against tick_broadcast_force_mask necessary if we add the check
in the idle thread before entering idle ? It does not hurt, agreed, and we'd
better leave it there, it is just for my own understanding, thanks a lot.
Having said that, on the series:
Tested-by: Lorenzo Pieralisi
> + dev->next_event.tv64 < bc->next_event.tv64)
> tick_broadcast_set_event(dev->next_event, 1);
> }
> } else {
> @@ -545,6 +560,47 @@ void
Re: [patch 4/7] tick: Handle broadcast wakeup of multiple cpus
On Wed, 13 Mar 2013, Lorenzo Pieralisi wrote:
+ /* Take care of enforced broadcast requests */
+ cpumask_or(tmpmask, tmpmask, tick_broadcast_force_mask);
+ cpumask_clear(tick_broadcast_force_mask);
I tested the set and it works fine on a dual cluster big.LITTLE testchip
using broadcast timer to manage deep idle cluster states.
Just asking a question: the force mask is cleared before sending the
timer IPI. Would not be better to clear it after the IPI is sent in
tick_do_broadcast(...) ?
Can you spot a regression if we do this ? The idle thread checks that
mask with irqs disabled, so it is possible that we clear the mask before
the CPU has a chance to get the IPI. If we clear the mask after sending
the IPI, we are increasing the chances for the idle thread to get it.
It is just a further optimization, just asking, thanks.
Need to think about that.
@@ -524,7 +530,16 @@ void tick_broadcast_oneshot_control(unsi
WARN_ON_ONCE(cpumask_test_cpu(cpu,
tick_broadcast_pending_mask));
if (!cpumask_test_and_set_cpu(cpu,
tick_broadcast_oneshot_mask)) {
clockevents_set_mode(dev, CLOCK_EVT_MODE_SHUTDOWN);
- if (dev-next_event.tv64 bc-next_event.tv64)
+ /*
+* We only reprogram the broadcast timer if we
+* did not mark ourself in the force mask and
+* if the cpu local event is earlier than the
+* broadcast event. If the current CPU is in
+* the force mask, then we are going to be
+* woken by the IPI right away.
+*/
+ if (!cpumask_test_cpu(cpu, tick_broadcast_force_mask)
Is the test against tick_broadcast_force_mask necessary if we add the check
in the idle thread before entering idle ? It does not hurt, agreed, and we'd
better leave it there, it is just for my own understanding, thanks a lot.
Well, it's necessary for all archs which do not have the check (yet).
Having said that, on the series:
Tested-by: Lorenzo Pieralisi lorenzo.pieral...@arm.com
Thanks,
tglx
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To unsubscribe from this list: send the line unsubscribe linux-kernel in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
Re: [patch 4/7] tick: Handle broadcast wakeup of multiple cpus
Hi Thomas,
On Wed, Mar 06, 2013 at 11:18:35AM +, Thomas Gleixner wrote:
Some brilliant hardware implementations wake multiple cores when the
broadcast timer fires. This leads to the following interesting
problem:
CPU0 CPU1
wakeup from idle wakeup from idle
leave broadcast mode leave broadcast mode
restart per cpu timer restart per cpu timer
go back to idle
handle broadcast
(empty mask)
enter broadcast mode
programm broadcast device
enter broadcast mode
programm broadcast device
So what happens is that due to the forced reprogramming of the cpu
local timer, we need to set a event in the future. Now if we manage to
go back to idle before the timer fires, we switch off the timer and
arm the broadcast device with an already expired time (covered by
forced mode). So in the worst case we repeat the above ping pong
forever.
Unfortunately we have no information about what caused the wakeup, but
we can check current time against the expiry time of the local cpu. If
the local event is already in the past, we know that the broadcast
timer is about to fire and send an IPI. So we mark ourself as an IPI
target even if we left broadcast mode and avoid the reprogramming of
the local cpu timer.
This still leaves the possibility that a CPU which is not handling the
broadcast interrupt is going to reach idle again before the IPI
arrives. This can't be solved in the core code and will be handled in
follow up patches.
Reported-by: Jason Liu liu.h.ja...@gmail.com
Signed-off-by: Thomas Gleixner t...@linutronix.de
---
kernel/time/tick-broadcast.c | 59
++-
1 file changed, 58 insertions(+), 1 deletion(-)
Index: tip/kernel/time/tick-broadcast.c
===
--- tip.orig/kernel/time/tick-broadcast.c
+++ tip/kernel/time/tick-broadcast.c
@@ -393,6 +393,7 @@ int tick_resume_broadcast(void)
static cpumask_var_t tick_broadcast_oneshot_mask;
static cpumask_var_t tick_broadcast_pending_mask;
+static cpumask_var_t tick_broadcast_force_mask;
/*
* Exposed for debugging: see timer_list.c
@@ -462,6 +463,10 @@ again:
}
}
+ /* Take care of enforced broadcast requests */
+ cpumask_or(tmpmask, tmpmask, tick_broadcast_force_mask);
+ cpumask_clear(tick_broadcast_force_mask);
I tested the set and it works fine on a dual cluster big.LITTLE testchip
using broadcast timer to manage deep idle cluster states.
Just asking a question: the force mask is cleared before sending the
timer IPI. Would not be better to clear it after the IPI is sent in
tick_do_broadcast(...) ?
Can you spot a regression if we do this ? The idle thread checks that
mask with irqs disabled, so it is possible that we clear the mask before
the CPU has a chance to get the IPI. If we clear the mask after sending
the IPI, we are increasing the chances for the idle thread to get it.
It is just a further optimization, just asking, thanks.
+
/*
* Wakeup the cpus which have an expired event.
*/
@@ -497,6 +502,7 @@ void tick_broadcast_oneshot_control(unsi
struct clock_event_device *bc, *dev;
struct tick_device *td;
unsigned long flags;
+ ktime_t now;
int cpu;
/*
@@ -524,7 +530,16 @@ void tick_broadcast_oneshot_control(unsi
WARN_ON_ONCE(cpumask_test_cpu(cpu,
tick_broadcast_pending_mask));
if (!cpumask_test_and_set_cpu(cpu,
tick_broadcast_oneshot_mask)) {
clockevents_set_mode(dev, CLOCK_EVT_MODE_SHUTDOWN);
- if (dev-next_event.tv64 bc-next_event.tv64)
+ /*
+ * We only reprogram the broadcast timer if we
+ * did not mark ourself in the force mask and
+ * if the cpu local event is earlier than the
+ * broadcast event. If the current CPU is in
+ * the force mask, then we are going to be
+ * woken by the IPI right away.
+ */
+ if (!cpumask_test_cpu(cpu, tick_broadcast_force_mask)
Is the test against tick_broadcast_force_mask necessary if we add the check
in the idle thread before entering idle ? It does not hurt, agreed, and we'd
better leave it there, it is just for my own understanding, thanks a lot.
Having said that, on the series:
Tested-by: Lorenzo Pieralisi lorenzo.pieral...@arm.com
+ dev-next_event.tv64 bc-next_event.tv64)
tick_broadcast_set_event(dev-next_event, 1);
}
} else {
@@ -545,6 +560,47 @@ void tick_broadcast_oneshot_control(unsi
