Re: 回复: [PATCH 01/10] tick/nohz: Prevent tick_nohz_get_sleep_length() from returning negative value

Re: 回复: [PATCH 01/10] tick/nohz: Prevent tick_nohz_get_sleep_length() from returning negative value

[Rafael J. Wysocki]

On Thursday, March 25, 2021 2:14:00 PM CET Frederic Weisbecker wrote:
> On Tue, Mar 16, 2021 at 04:08:08PM +0000, Zhou Ti (x2019cwm) wrote:
> > But I don't think it's a good idea to handle this in callers, because logically the function shouldn't return negative values. Returning 0 directly would allow idle governors to get another chance to select again.
> 
> Hmm, I'm going to leave the last word to Rafael since cpuidle are the only
> callers of this. In any case we need to fix it.

Yes, we do.

So I said that I preferred to address this in the callers and the reason why
is because, for example, for the teo governor it would be a matter of using
a different data type to store the tick_nohz_get_sleep_length() return value,
like in the (untested) patch below.

So at least in this case there is no need to add any new branches anywhere.

I'm still not sure about menu, because it is more complicated, but even if
that one needs an extra branch, that is a win already.

---
 drivers/cpuidle/governors/teo.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

Index: linux-pm/drivers/cpuidle/governors/teo.c
===================================================================
--- linux-pm.orig/drivers/cpuidle/governors/teo.c
+++ linux-pm/drivers/cpuidle/governors/teo.c
@@ -100,8 +100,8 @@ struct teo_idle_state {
  * @intervals: Saved idle duration values.
  */
 struct teo_cpu {
-	u64 time_span_ns;
-	u64 sleep_length_ns;
+	s64 time_span_ns;
+	s64 sleep_length_ns;
 	struct teo_idle_state states[CPUIDLE_STATE_MAX];
 	int interval_idx;
 	u64 intervals[INTERVALS];
@@ -216,7 +216,7 @@ static bool teo_time_ok(u64 interval_ns)
  */
 static int teo_find_shallower_state(struct cpuidle_driver *drv,
 				    struct cpuidle_device *dev, int state_idx,
-				    u64 duration_ns)
+				    s64 duration_ns)
 {
 	int i;
 
@@ -242,7 +242,7 @@ static int teo_select(struct cpuidle_dri
 {
 	struct teo_cpu *cpu_data = per_cpu_ptr(&teo_cpus, dev->cpu);
 	s64 latency_req = cpuidle_governor_latency_req(dev->cpu);
-	u64 duration_ns;
+	s64 duration_ns;
 	unsigned int hits, misses, early_hits;
 	int max_early_idx, prev_max_early_idx, constraint_idx, idx, i;
 	ktime_t delta_tick;

Re: 回复: [PATCH 01/10] tick/nohz: Prevent tick_nohz_get_sleep_length() from returning negative value

[Zhou Ti (x2019cwm)]

On March 25, 2021 14:56, Rafael J. Wysocki wrote:
> On Thursday, March 25, 2021 2:14:00 PM CET Frederic Weisbecker wrote:
> > On Tue, Mar 16, 2021 at 04:08:08PM +0000, Zhou Ti (x2019cwm) wrote:
> > > But I don't think it's a good idea to handle this in callers, because logically the function shouldn't return negative values. Returning 0 directly would allow idle governors to get another chance to select again.
> >
> > Hmm, I'm going to leave the last word to Rafael since cpuidle are the only
> > callers of this. In any case we need to fix it.
> 
> Yes, we do.
> 
> So I said that I preferred to address this in the callers and the reason why
> is because, for example, for the teo governor it would be a matter of using
> a different data type to store the tick_nohz_get_sleep_length() return value,
> like in the (untested) patch below.
> 
> So at least in this case there is no need to add any new branches anywhere.
> 
> I'm still not sure about menu, because it is more complicated, but even if
> that one needs an extra branch, that is a win already.

I would like to point out the potential trouble that fixing this issue in the 
callers could cause.

1. This function is called multiple times in menu governor and TEO 
governor. I'm not sure that receiving results using signed integers is enough 
to solve all the problems, in the worst case it may require increasing 
the logical complexity of the code.

2. This function is important for developing idle governor. 
If the problem is not fixed in the function itself, then this potential 
pitfall should be explicitly stated in the documentation. This is because 
it is difficult to predict from the definition and naming of the function 
that it might return a negative number. I actually discovered this anomaly 
when I was doing data analysis on my own idle governor. For some idle control 
algorithms, this exception return could lead to serious consequences, 
because negative return logically won't happen.

> 
> ---
>  drivers/cpuidle/governors/teo.c |    8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> Index: linux-pm/drivers/cpuidle/governors/teo.c
> ===================================================================
> --- linux-pm.orig/drivers/cpuidle/governors/teo.c
> +++ linux-pm/drivers/cpuidle/governors/teo.c
> @@ -100,8 +100,8 @@ struct teo_idle_state {
>   * @intervals: Saved idle duration values.
>   */
>  struct teo_cpu {
> -       u64 time_span_ns;
> -       u64 sleep_length_ns;
> +       s64 time_span_ns;
> +       s64 sleep_length_ns;
>         struct teo_idle_state states[CPUIDLE_STATE_MAX];
>         int interval_idx;
>         u64 intervals[INTERVALS];
> @@ -216,7 +216,7 @@ static bool teo_time_ok(u64 interval_ns)
>   */
>  static int teo_find_shallower_state(struct cpuidle_driver *drv,
>                                     struct cpuidle_device *dev, int state_idx,
> -                                   u64 duration_ns)
> +                                   s64 duration_ns)
>  {
>         int i;
> 
> @@ -242,7 +242,7 @@ static int teo_select(struct cpuidle_dri
>  {
>         struct teo_cpu *cpu_data = per_cpu_ptr(&teo_cpus, dev->cpu);
>         s64 latency_req = cpuidle_governor_latency_req(dev->cpu);
> -       u64 duration_ns;
> +       s64 duration_ns;
>         unsigned int hits, misses, early_hits;
>         int max_early_idx, prev_max_early_idx, constraint_idx, idx, i;
>         ktime_t delta_tick;

Re: 回复: [PATCH 01/10] tick/nohz: Prevent tick_nohz_get_sleep_length() from returning negative value

[Rafael J. Wysocki]

On Thu, Mar 25, 2021 at 8:18 PM Zhou Ti (x2019cwm) <x2019cwm@stfx.ca> wrote:
>
> On March 25, 2021 14:56, Rafael J. Wysocki wrote:
> > On Thursday, March 25, 2021 2:14:00 PM CET Frederic Weisbecker wrote:
> > > On Tue, Mar 16, 2021 at 04:08:08PM +0000, Zhou Ti (x2019cwm) wrote:
> > > > But I don't think it's a good idea to handle this in callers, because logically the function shouldn't return negative values. Returning 0 directly would allow idle governors to get another chance to select again.
> > >
> > > Hmm, I'm going to leave the last word to Rafael since cpuidle are the only
> > > callers of this. In any case we need to fix it.
> >
> > Yes, we do.
> >
> > So I said that I preferred to address this in the callers and the reason why
> > is because, for example, for the teo governor it would be a matter of using
> > a different data type to store the tick_nohz_get_sleep_length() return value,
> > like in the (untested) patch below.
> >
> > So at least in this case there is no need to add any new branches anywhere.
> >
> > I'm still not sure about menu, because it is more complicated, but even if
> > that one needs an extra branch, that is a win already.
>
> I would like to point out the potential trouble that fixing this issue in the
> callers could cause.
>
> 1. This function is called multiple times in menu governor and TEO
> governor.

What do you mean by "multiple times"?

Each of the governors calls it once per cycle and its previous return
value is not used in the next cycle at least in teo.

> I'm not sure that receiving results using signed integers is enough
> to solve all the problems, in the worst case it may require increasing
> the logical complexity of the code.

That is a valid concern, so it is a tradeoff between increasing the
logical complexity of the code and adding branches to it.

> 2. This function is important for developing idle governor.
> If the problem is not fixed in the function itself, then this potential
> pitfall should be explicitly stated in the documentation.

That I can agree with.

> This is because
> it is difficult to predict from the definition and naming of the function
> that it might return a negative number. I actually discovered this anomaly
> when I was doing data analysis on my own idle governor. For some idle control
> algorithms, this exception return could lead to serious consequences,
> because negative return logically won't happen.

Well, it's a matter of how to take the possible negative return value
into account so it does not affect the result of the computations.

> >
> > ---
> >  drivers/cpuidle/governors/teo.c |    8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> >
> > Index: linux-pm/drivers/cpuidle/governors/teo.c
> > ===================================================================
> > --- linux-pm.orig/drivers/cpuidle/governors/teo.c
> > +++ linux-pm/drivers/cpuidle/governors/teo.c
> > @@ -100,8 +100,8 @@ struct teo_idle_state {
> >   * @intervals: Saved idle duration values.
> >   */
> >  struct teo_cpu {
> > -       u64 time_span_ns;
> > -       u64 sleep_length_ns;
> > +       s64 time_span_ns;
> > +       s64 sleep_length_ns;
> >         struct teo_idle_state states[CPUIDLE_STATE_MAX];
> >         int interval_idx;
> >         u64 intervals[INTERVALS];
> > @@ -216,7 +216,7 @@ static bool teo_time_ok(u64 interval_ns)
> >   */
> >  static int teo_find_shallower_state(struct cpuidle_driver *drv,
> >                                     struct cpuidle_device *dev, int state_idx,
> > -                                   u64 duration_ns)
> > +                                   s64 duration_ns)
> >  {
> >         int i;
> >
> > @@ -242,7 +242,7 @@ static int teo_select(struct cpuidle_dri
> >  {
> >         struct teo_cpu *cpu_data = per_cpu_ptr(&teo_cpus, dev->cpu);
> >         s64 latency_req = cpuidle_governor_latency_req(dev->cpu);
> > -       u64 duration_ns;
> > +       s64 duration_ns;
> >         unsigned int hits, misses, early_hits;
> >         int max_early_idx, prev_max_early_idx, constraint_idx, idx, i;
> >         ktime_t delta_tick;
>

Re: 回复: [PATCH 01/10] tick/nohz: Prevent tick_nohz_get_sleep_length() from returning negative value

[Zhou Ti (x2019cwm)]

On March 25, 2021 15:50, Rafael J. Wysocki wrote:
> On Thu, Mar 25, 2021 at 8:18 PM Zhou Ti (x2019cwm) <x2019cwm@stfx.ca> wrote:
> >
> > On March 25, 2021 14:56, Rafael J. Wysocki wrote:
> > > On Thursday, March 25, 2021 2:14:00 PM CET Frederic Weisbecker wrote:
> > > > On Tue, Mar 16, 2021 at 04:08:08PM +0000, Zhou Ti (x2019cwm) wrote:
> > > > > But I don't think it's a good idea to handle this in callers, because logically the function shouldn't return negative values. Returning 0 directly would allow idle governors to get another chance to select again.
> > > >
> > > > Hmm, I'm going to leave the last word to Rafael since cpuidle are the only
> > > > callers of this. In any case we need to fix it.
> > >
> > > Yes, we do.
> > >
> > > So I said that I preferred to address this in the callers and the reason why
> > > is because, for example, for the teo governor it would be a matter of using
> > > a different data type to store the tick_nohz_get_sleep_length() return value,
> > > like in the (untested) patch below.
> > >
> > > So at least in this case there is no need to add any new branches anywhere.
> > >
> > > I'm still not sure about menu, because it is more complicated, but even if
> > > that one needs an extra branch, that is a win already.
> >
> > I would like to point out the potential trouble that fixing this issue in the
> > callers could cause.
> >
> > 1. This function is called multiple times in menu governor and TEO
> > governor.
> 
> What do you mean by "multiple times"?
> 
> Each of the governors calls it once per cycle and its previous return
> value is not used in the next cycle at least in teo.

I remember a governor called this function twice in a cycle, I guess I remember 
wrong.

> 
> > I'm not sure that receiving results using signed integers is enough
> > to solve all the problems, in the worst case it may require increasing
> > the logical complexity of the code.
> 
> That is a valid concern, so it is a tradeoff between increasing the
> logical complexity of the code and adding branches to it.
> 
> > 2. This function is important for developing idle governor.
> > If the problem is not fixed in the function itself, then this potential
> > pitfall should be explicitly stated in the documentation.
> 
> That I can agree with.
> 
> > This is because
> > it is difficult to predict from the definition and naming of the function
> > that it might return a negative number. I actually discovered this anomaly
> > when I was doing data analysis on my own idle governor. For some idle control
> > algorithms, this exception return could lead to serious consequences,
> > because negative return logically won't happen.
> 
> Well, it's a matter of how to take the possible negative return value
> into account so it does not affect the result of the computations.

I think it is challenging for some algorithms to take negative return values 
into account properly. For TEO (and even menu), it is possible to 
solve the problem by just changing the way the data is received is because the 
learning mechanism for both algorithms is simple. 

One of the interesting things about the CPUIdle subsystem is that it is well 
suited to introduce machine learning and probabilistic statistical methods.
This means that many of the more complex and data-sensitive algorithms can 
potentially be explored. In the best case we will still need to add additional 
code complexity to a new algorithm.

It would reduce a lot of unnecessary considerations (for example, highlight 
this shortcoming in the documentation) if we could ensure that this function 
would work as it is logically defined. But I don't really understand 
how much of a burden adding an extra branch would impose, so I don't know if 
this tradeoff is worth it.

Re: 回复: [PATCH 01/10] tick/nohz: Prevent tick_nohz_get_sleep_length() from returning negative value

[Rafael J. Wysocki]

On Thu, Mar 25, 2021 at 9:37 PM Zhou Ti (x2019cwm) <x2019cwm@stfx.ca> wrote:
>
> On March 25, 2021 15:50, Rafael J. Wysocki wrote:
> > On Thu, Mar 25, 2021 at 8:18 PM Zhou Ti (x2019cwm) <x2019cwm@stfx.ca> wrote:
> > >
> > > On March 25, 2021 14:56, Rafael J. Wysocki wrote:
> > > > On Thursday, March 25, 2021 2:14:00 PM CET Frederic Weisbecker wrote:
> > > > > On Tue, Mar 16, 2021 at 04:08:08PM +0000, Zhou Ti (x2019cwm) wrote:
> > > > > > But I don't think it's a good idea to handle this in callers, because logically the function shouldn't return negative values. Returning 0 directly would allow idle governors to get another chance to select again.
> > > > >
> > > > > Hmm, I'm going to leave the last word to Rafael since cpuidle are the only
> > > > > callers of this. In any case we need to fix it.
> > > >
> > > > Yes, we do.
> > > >
> > > > So I said that I preferred to address this in the callers and the reason why
> > > > is because, for example, for the teo governor it would be a matter of using
> > > > a different data type to store the tick_nohz_get_sleep_length() return value,
> > > > like in the (untested) patch below.
> > > >
> > > > So at least in this case there is no need to add any new branches anywhere.
> > > >
> > > > I'm still not sure about menu, because it is more complicated, but even if
> > > > that one needs an extra branch, that is a win already.
> > >
> > > I would like to point out the potential trouble that fixing this issue in the
> > > callers could cause.
> > >
> > > 1. This function is called multiple times in menu governor and TEO
> > > governor.
> >
> > What do you mean by "multiple times"?
> >
> > Each of the governors calls it once per cycle and its previous return
> > value is not used in the next cycle at least in teo.
>
> I remember a governor called this function twice in a cycle, I guess I remember
> wrong.

That obviously depends on the governor, but both teo and menu call it
once per cycle.

> > > I'm not sure that receiving results using signed integers is enough
> > > to solve all the problems, in the worst case it may require increasing
> > > the logical complexity of the code.
> >
> > That is a valid concern, so it is a tradeoff between increasing the
> > logical complexity of the code and adding branches to it.
> >
> > > 2. This function is important for developing idle governor.
> > > If the problem is not fixed in the function itself, then this potential
> > > pitfall should be explicitly stated in the documentation.
> >
> > That I can agree with.
> >
> > > This is because
> > > it is difficult to predict from the definition and naming of the function
> > > that it might return a negative number. I actually discovered this anomaly
> > > when I was doing data analysis on my own idle governor. For some idle control
> > > algorithms, this exception return could lead to serious consequences,
> > > because negative return logically won't happen.
> >
> > Well, it's a matter of how to take the possible negative return value
> > into account so it does not affect the result of the computations.
>
> I think it is challenging for some algorithms to take negative return values
> into account properly. For TEO (and even menu), it is possible to
> solve the problem by just changing the way the data is received is because the
> learning mechanism for both algorithms is simple.

Of course this depends on the governor.

> One of the interesting things about the CPUIdle subsystem is that it is well
> suited to introduce machine learning and probabilistic statistical methods.

You need to remember that the governor code runs in the idle loop
context which is expected to be reasonably fast.

That's why we are worrying about individual branches here.

> This means that many of the more complex and data-sensitive algorithms can
> potentially be explored. In the best case we will still need to add additional
> code complexity to a new algorithm.

So I'm not sure what the problem with adding an upfront negative value
check to the governor is.

> It would reduce a lot of unnecessary considerations (for example, highlight
> this shortcoming in the documentation) if we could ensure that this function
> would work as it is logically defined. But I don't really understand
> how much of a burden adding an extra branch would impose, so I don't know if
> this tradeoff is worth it.

It ultimately depends on the governor, which is why I think that the
negative value check should be done by the governor, if needed, and
not by the function called by it, because in the latter case the check
may be redundant and we end up with an extra branch (or two branches
in this particular case) for no good reason whatsoever.

Yes, there are governors which simply can do the negative value check
upfront right after calling that function and ensure that they will
not deal with negative values going forward.  This is probably what
I'll do in the menu case.

However, if the governor is simple enough and it can avoid doing the
explicit negative value check, I don't see a reason to do that check
elsewhere "just in case".

Re: 回复: [PATCH 01/10] tick/nohz: Prevent tick_nohz_get_sleep_length() from returning negative value

[Zhou Ti (x2019cwm)]

On Fri, 26 Mar 2021 18:01:47 +0100, Rafael J. Wysocki wrote:
> On Thu, Mar 25, 2021 at 9:37 PM Zhou Ti (x2019cwm) <x2019cwm@stfx.ca> wrote:
> >
> > On March 25, 2021 15:50, Rafael J. Wysocki wrote:
> > > On Thu, Mar 25, 2021 at 8:18 PM Zhou Ti (x2019cwm) <x2019cwm@stfx.ca> wrote:
> > > >
> > > > On March 25, 2021 14:56, Rafael J. Wysocki wrote:
> > > > > On Thursday, March 25, 2021 2:14:00 PM CET Frederic Weisbecker wrote:
> > > > > > On Tue, Mar 16, 2021 at 04:08:08PM +0000, Zhou Ti (x2019cwm) wrote:
> > > > > > > But I don't think it's a good idea to handle this in callers, because logically the function shouldn't return negative values. Returning 0 directly would allow idle governors to get another chance to select again.
> > > > > >
> > > > > > Hmm, I'm going to leave the last word to Rafael since cpuidle are the only
> > > > > > callers of this. In any case we need to fix it.
> > > > >
> > > > > Yes, we do.
> > > > >
> > > > > So I said that I preferred to address this in the callers and the reason why
> > > > > is because, for example, for the teo governor it would be a matter of using
> > > > > a different data type to store the tick_nohz_get_sleep_length() return value,
> > > > > like in the (untested) patch below.
> > > > >
> > > > > So at least in this case there is no need to add any new branches anywhere.
> > > > >
> > > > > I'm still not sure about menu, because it is more complicated, but even if
> > > > > that one needs an extra branch, that is a win already.
> > > >
> > > > I would like to point out the potential trouble that fixing this issue in the
> > > > callers could cause.
> > > >
> > > > 1. This function is called multiple times in menu governor and TEO
> > > > governor.
> > >
> > > What do you mean by "multiple times"?
> > >
> > > Each of the governors calls it once per cycle and its previous return
> > > value is not used in the next cycle at least in teo.
> >
> > I remember a governor called this function twice in a cycle, I guess I remember
> > wrong.
>
> That obviously depends on the governor, but both teo and menu call it
> once per cycle.
>
> > > > I'm not sure that receiving results using signed integers is enough
> > > > to solve all the problems, in the worst case it may require increasing
> > > > the logical complexity of the code.
> > >
> > > That is a valid concern, so it is a tradeoff between increasing the
> > > logical complexity of the code and adding branches to it.
> > >
> > > > 2. This function is important for developing idle governor.
> > > > If the problem is not fixed in the function itself, then this potential
> > > > pitfall should be explicitly stated in the documentation.
> > >
> > > That I can agree with.
> > >
> > > > This is because
> > > > it is difficult to predict from the definition and naming of the function
> > > > that it might return a negative number. I actually discovered this anomaly
> > > > when I was doing data analysis on my own idle governor. For some idle control
> > > > algorithms, this exception return could lead to serious consequences,
> > > > because negative return logically won't happen.
> > >
> > > Well, it's a matter of how to take the possible negative return value
> > > into account so it does not affect the result of the computations.
> >
> > I think it is challenging for some algorithms to take negative return values
> > into account properly. For TEO (and even menu), it is possible to
> > solve the problem by just changing the way the data is received is because the
> > learning mechanism for both algorithms is simple.
>
> Of course this depends on the governor.
>
> > One of the interesting things about the CPUIdle subsystem is that it is well
> > suited to introduce machine learning and probabilistic statistical methods.
>
> You need to remember that the governor code runs in the idle loop
> context which is expected to be reasonably fast.
>
> That's why we are worrying about individual branches here.
>
> > This means that many of the more complex and data-sensitive algorithms can
> > potentially be explored. In the best case we will still need to add additional
> > code complexity to a new algorithm.
>
> So I'm not sure what the problem with adding an upfront negative value
> check to the governor is.
>
> > It would reduce a lot of unnecessary considerations (for example, highlight
> > this shortcoming in the documentation) if we could ensure that this function
> > would work as it is logically defined. But I don't really understand
> > how much of a burden adding an extra branch would impose, so I don't know if
> > this tradeoff is worth it.
>
> It ultimately depends on the governor, which is why I think that the
> negative value check should be done by the governor, if needed, and
> not by the function called by it, because in the latter case the check
> may be redundant and we end up with an extra branch (or two branches
> in this particular case) for no good reason whatsoever.
>
> Yes, there are governors which simply can do the negative value check
> upfront right after calling that function and ensure that they will
> not deal with negative values going forward.  This is probably what
> I'll do in the menu case.
>
> However, if the governor is simple enough and it can avoid doing the
> explicit negative value check, I don't see a reason to do that check
> elsewhere "just in case".

Makes sense. I will submit my patch to fix this issue in menu and TEO.

________________________________________
From: Rafael J. Wysocki <rafael@kernel.org>
Sent: March 26, 2021 13:01
To: Zhou Ti (x2019cwm)
Cc: Rafael J. Wysocki; Rafael J. Wysocki; Frederic Weisbecker; LKML; Rafael J. Wysocki; Peter Zijlstra; Thomas Gleixner; Yunfeng Ye; Paul E . McKenney; Marcelo Tosatti; Ingo Molnar; Linux PM
Subject: Re: 回复: [PATCH 01/10] tick/nohz: Prevent tick_nohz_get_sleep_length() from returning negative value

On Thu, Mar 25, 2021 at 9:37 PM Zhou Ti (x2019cwm) <x2019cwm@stfx.ca> wrote:
>
> On March 25, 2021 15:50, Rafael J. Wysocki wrote:
> > On Thu, Mar 25, 2021 at 8:18 PM Zhou Ti (x2019cwm) <x2019cwm@stfx.ca> wrote:
> > >
> > > On March 25, 2021 14:56, Rafael J. Wysocki wrote:
> > > > On Thursday, March 25, 2021 2:14:00 PM CET Frederic Weisbecker wrote:
> > > > > On Tue, Mar 16, 2021 at 04:08:08PM +0000, Zhou Ti (x2019cwm) wrote:
> > > > > > But I don't think it's a good idea to handle this in callers, because logically the function shouldn't return negative values. Returning 0 directly would allow idle governors to get another chance to select again.
> > > > >
> > > > > Hmm, I'm going to leave the last word to Rafael since cpuidle are the only
> > > > > callers of this. In any case we need to fix it.
> > > >
> > > > Yes, we do.
> > > >
> > > > So I said that I preferred to address this in the callers and the reason why
> > > > is because, for example, for the teo governor it would be a matter of using
> > > > a different data type to store the tick_nohz_get_sleep_length() return value,
> > > > like in the (untested) patch below.
> > > >
> > > > So at least in this case there is no need to add any new branches anywhere.
> > > >
> > > > I'm still not sure about menu, because it is more complicated, but even if
> > > > that one needs an extra branch, that is a win already.
> > >
> > > I would like to point out the potential trouble that fixing this issue in the
> > > callers could cause.
> > >
> > > 1. This function is called multiple times in menu governor and TEO
> > > governor.
> >
> > What do you mean by "multiple times"?
> >
> > Each of the governors calls it once per cycle and its previous return
> > value is not used in the next cycle at least in teo.
>
> I remember a governor called this function twice in a cycle, I guess I remember
> wrong.

That obviously depends on the governor, but both teo and menu call it
once per cycle.

> > > I'm not sure that receiving results using signed integers is enough
> > > to solve all the problems, in the worst case it may require increasing
> > > the logical complexity of the code.
> >
> > That is a valid concern, so it is a tradeoff between increasing the
> > logical complexity of the code and adding branches to it.
> >
> > > 2. This function is important for developing idle governor.
> > > If the problem is not fixed in the function itself, then this potential
> > > pitfall should be explicitly stated in the documentation.
> >
> > That I can agree with.
> >
> > > This is because
> > > it is difficult to predict from the definition and naming of the function
> > > that it might return a negative number. I actually discovered this anomaly
> > > when I was doing data analysis on my own idle governor. For some idle control
> > > algorithms, this exception return could lead to serious consequences,
> > > because negative return logically won't happen.
> >
> > Well, it's a matter of how to take the possible negative return value
> > into account so it does not affect the result of the computations.
>
> I think it is challenging for some algorithms to take negative return values
> into account properly. For TEO (and even menu), it is possible to
> solve the problem by just changing the way the data is received is because the
> learning mechanism for both algorithms is simple.

Of course this depends on the governor.

> One of the interesting things about the CPUIdle subsystem is that it is well
> suited to introduce machine learning and probabilistic statistical methods.

You need to remember that the governor code runs in the idle loop
context which is expected to be reasonably fast.

That's why we are worrying about individual branches here.

> This means that many of the more complex and data-sensitive algorithms can
> potentially be explored. In the best case we will still need to add additional
> code complexity to a new algorithm.

So I'm not sure what the problem with adding an upfront negative value
check to the governor is.

> It would reduce a lot of unnecessary considerations (for example, highlight
> this shortcoming in the documentation) if we could ensure that this function
> would work as it is logically defined. But I don't really understand
> how much of a burden adding an extra branch would impose, so I don't know if
> this tradeoff is worth it.

It ultimately depends on the governor, which is why I think that the
negative value check should be done by the governor, if needed, and
not by the function called by it, because in the latter case the check
may be redundant and we end up with an extra branch (or two branches
in this particular case) for no good reason whatsoever.

Yes, there are governors which simply can do the negative value check
upfront right after calling that function and ensure that they will
not deal with negative values going forward.  This is probably what
I'll do in the menu case.

However, if the governor is simple enough and it can avoid doing the
explicit negative value check, I don't see a reason to do that check
elsewhere "just in case".

Re: 回复: [PATCH 01/10] tick/nohz: Prevent tick_nohz_get_sleep_length() from returning negative value

[Rafael J. Wysocki]

On Fri, Mar 26, 2021 at 6:53 PM Zhou Ti (x2019cwm) <x2019cwm@stfx.ca> wrote:
>
> On Fri, 26 Mar 2021 18:01:47 +0100, Rafael J. Wysocki wrote:
> > On Thu, Mar 25, 2021 at 9:37 PM Zhou Ti (x2019cwm) <x2019cwm@stfx.ca> wrote:
> > >
> > > On March 25, 2021 15:50, Rafael J. Wysocki wrote:
> > > > On Thu, Mar 25, 2021 at 8:18 PM Zhou Ti (x2019cwm) <x2019cwm@stfx.ca> wrote:
> > > > >
> > > > > On March 25, 2021 14:56, Rafael J. Wysocki wrote:
> > > > > > On Thursday, March 25, 2021 2:14:00 PM CET Frederic Weisbecker wrote:
> > > > > > > On Tue, Mar 16, 2021 at 04:08:08PM +0000, Zhou Ti (x2019cwm) wrote:
> > > > > > > > But I don't think it's a good idea to handle this in callers, because logically the function shouldn't return negative values. Returning 0 directly would allow idle governors to get another chance to select again.
> > > > > > >
> > > > > > > Hmm, I'm going to leave the last word to Rafael since cpuidle are the only
> > > > > > > callers of this. In any case we need to fix it.
> > > > > >
> > > > > > Yes, we do.
> > > > > >
> > > > > > So I said that I preferred to address this in the callers and the reason why
> > > > > > is because, for example, for the teo governor it would be a matter of using
> > > > > > a different data type to store the tick_nohz_get_sleep_length() return value,
> > > > > > like in the (untested) patch below.
> > > > > >
> > > > > > So at least in this case there is no need to add any new branches anywhere.
> > > > > >
> > > > > > I'm still not sure about menu, because it is more complicated, but even if
> > > > > > that one needs an extra branch, that is a win already.
> > > > >
> > > > > I would like to point out the potential trouble that fixing this issue in the
> > > > > callers could cause.
> > > > >
> > > > > 1. This function is called multiple times in menu governor and TEO
> > > > > governor.
> > > >
> > > > What do you mean by "multiple times"?
> > > >
> > > > Each of the governors calls it once per cycle and its previous return
> > > > value is not used in the next cycle at least in teo.
> > >
> > > I remember a governor called this function twice in a cycle, I guess I remember
> > > wrong.
> >
> > That obviously depends on the governor, but both teo and menu call it
> > once per cycle.
> >
> > > > > I'm not sure that receiving results using signed integers is enough
> > > > > to solve all the problems, in the worst case it may require increasing
> > > > > the logical complexity of the code.
> > > >
> > > > That is a valid concern, so it is a tradeoff between increasing the
> > > > logical complexity of the code and adding branches to it.
> > > >
> > > > > 2. This function is important for developing idle governor.
> > > > > If the problem is not fixed in the function itself, then this potential
> > > > > pitfall should be explicitly stated in the documentation.
> > > >
> > > > That I can agree with.
> > > >
> > > > > This is because
> > > > > it is difficult to predict from the definition and naming of the function
> > > > > that it might return a negative number. I actually discovered this anomaly
> > > > > when I was doing data analysis on my own idle governor. For some idle control
> > > > > algorithms, this exception return could lead to serious consequences,
> > > > > because negative return logically won't happen.
> > > >
> > > > Well, it's a matter of how to take the possible negative return value
> > > > into account so it does not affect the result of the computations.
> > >
> > > I think it is challenging for some algorithms to take negative return values
> > > into account properly. For TEO (and even menu), it is possible to
> > > solve the problem by just changing the way the data is received is because the
> > > learning mechanism for both algorithms is simple.
> >
> > Of course this depends on the governor.
> >
> > > One of the interesting things about the CPUIdle subsystem is that it is well
> > > suited to introduce machine learning and probabilistic statistical methods.
> >
> > You need to remember that the governor code runs in the idle loop
> > context which is expected to be reasonably fast.
> >
> > That's why we are worrying about individual branches here.
> >
> > > This means that many of the more complex and data-sensitive algorithms can
> > > potentially be explored. In the best case we will still need to add additional
> > > code complexity to a new algorithm.
> >
> > So I'm not sure what the problem with adding an upfront negative value
> > check to the governor is.
> >
> > > It would reduce a lot of unnecessary considerations (for example, highlight
> > > this shortcoming in the documentation) if we could ensure that this function
> > > would work as it is logically defined. But I don't really understand
> > > how much of a burden adding an extra branch would impose, so I don't know if
> > > this tradeoff is worth it.
> >
> > It ultimately depends on the governor, which is why I think that the
> > negative value check should be done by the governor, if needed, and
> > not by the function called by it, because in the latter case the check
> > may be redundant and we end up with an extra branch (or two branches
> > in this particular case) for no good reason whatsoever.
> >
> > Yes, there are governors which simply can do the negative value check
> > upfront right after calling that function and ensure that they will
> > not deal with negative values going forward.  This is probably what
> > I'll do in the menu case.
> >
> > However, if the governor is simple enough and it can avoid doing the
> > explicit negative value check, I don't see a reason to do that check
> > elsewhere "just in case".
>
> Makes sense. I will submit my patch to fix this issue in menu and TEO.

Well, I have patches for that already and they are not
super-straightforward.  Though If you want to try to fix this
yourself, I'll wait for your submission.

Re: 回复: [PATCH 01/10] tick/nohz: Prevent tick_nohz_get_sleep_length() from returning negative value

[Zhou Ti (x2019cwm)]

On Fri, 26 Mar 2021 19:54:26 +0100, Rafael J. Wysocki wrote:
> On Fri, Mar 26, 2021 at 6:53 PM Zhou Ti (x2019cwm) <x2019cwm@stfx.ca> wrote:
> >
> > On Fri, 26 Mar 2021 18:01:47 +0100, Rafael J. Wysocki wrote:
> > > On Thu, Mar 25, 2021 at 9:37 PM Zhou Ti (x2019cwm) <x2019cwm@stfx.ca> wrote:
> > > >
> > > > On March 25, 2021 15:50, Rafael J. Wysocki wrote:
> > > > > On Thu, Mar 25, 2021 at 8:18 PM Zhou Ti (x2019cwm) <x2019cwm@stfx.ca> wrote:
> > > > > >
> > > > > > On March 25, 2021 14:56, Rafael J. Wysocki wrote:
> > > > > > > On Thursday, March 25, 2021 2:14:00 PM CET Frederic Weisbecker wrote:
> > > > > > > > On Tue, Mar 16, 2021 at 04:08:08PM +0000, Zhou Ti (x2019cwm) wrote:
> > > > > > > > > But I don't think it's a good idea to handle this in callers, because logically the function shouldn't return negative values. Returning 0 directly would allow idle governors to get another chance to select again.
> > > > > > > >
> > > > > > > > Hmm, I'm going to leave the last word to Rafael since cpuidle are the only
> > > > > > > > callers of this. In any case we need to fix it.
> > > > > > >
> > > > > > > Yes, we do.
> > > > > > >
> > > > > > > So I said that I preferred to address this in the callers and the reason why
> > > > > > > is because, for example, for the teo governor it would be a matter of using
> > > > > > > a different data type to store the tick_nohz_get_sleep_length() return value,
> > > > > > > like in the (untested) patch below.
> > > > > > >
> > > > > > > So at least in this case there is no need to add any new branches anywhere.
> > > > > > >
> > > > > > > I'm still not sure about menu, because it is more complicated, but even if
> > > > > > > that one needs an extra branch, that is a win already.
> > > > > >
> > > > > > I would like to point out the potential trouble that fixing this issue in the
> > > > > > callers could cause.
> > > > > >
> > > > > > 1. This function is called multiple times in menu governor and TEO
> > > > > > governor.
> > > > >
> > > > > What do you mean by "multiple times"?
> > > > >
> > > > > Each of the governors calls it once per cycle and its previous return
> > > > > value is not used in the next cycle at least in teo.
> > > >
> > > > I remember a governor called this function twice in a cycle, I guess I remember
> > > > wrong.
> > >
> > > That obviously depends on the governor, but both teo and menu call it
> > > once per cycle.
> > >
> > > > > > I'm not sure that receiving results using signed integers is enough
> > > > > > to solve all the problems, in the worst case it may require increasing
> > > > > > the logical complexity of the code.
> > > > >
> > > > > That is a valid concern, so it is a tradeoff between increasing the
> > > > > logical complexity of the code and adding branches to it.
> > > > >
> > > > > > 2. This function is important for developing idle governor.
> > > > > > If the problem is not fixed in the function itself, then this potential
> > > > > > pitfall should be explicitly stated in the documentation.
> > > > >
> > > > > That I can agree with.
> > > > >
> > > > > > This is because
> > > > > > it is difficult to predict from the definition and naming of the function
> > > > > > that it might return a negative number. I actually discovered this anomaly
> > > > > > when I was doing data analysis on my own idle governor. For some idle control
> > > > > > algorithms, this exception return could lead to serious consequences,
> > > > > > because negative return logically won't happen.
> > > > >
> > > > > Well, it's a matter of how to take the possible negative return value
> > > > > into account so it does not affect the result of the computations.
> > > >
> > > > I think it is challenging for some algorithms to take negative return values
> > > > into account properly. For TEO (and even menu), it is possible to
> > > > solve the problem by just changing the way the data is received is because the
> > > > learning mechanism for both algorithms is simple.
> > >
> > > Of course this depends on the governor.
> > >
> > > > One of the interesting things about the CPUIdle subsystem is that it is well
> > > > suited to introduce machine learning and probabilistic statistical methods.
> > >
> > > You need to remember that the governor code runs in the idle loop
> > > context which is expected to be reasonably fast.
> > >
> > > That's why we are worrying about individual branches here.
> > >
> > > > This means that many of the more complex and data-sensitive algorithms can
> > > > potentially be explored. In the best case we will still need to add additional
> > > > code complexity to a new algorithm.
> > >
> > > So I'm not sure what the problem with adding an upfront negative value
> > > check to the governor is.
> > >
> > > > It would reduce a lot of unnecessary considerations (for example, highlight
> > > > this shortcoming in the documentation) if we could ensure that this function
> > > > would work as it is logically defined. But I don't really understand
> > > > how much of a burden adding an extra branch would impose, so I don't know if
> > > > this tradeoff is worth it.
> > >
> > > It ultimately depends on the governor, which is why I think that the
> > > negative value check should be done by the governor, if needed, and
> > > not by the function called by it, because in the latter case the check
> > > may be redundant and we end up with an extra branch (or two branches
> > > in this particular case) for no good reason whatsoever.
> > >
> > > Yes, there are governors which simply can do the negative value check
> > > upfront right after calling that function and ensure that they will
> > > not deal with negative values going forward.  This is probably what
> > > I'll do in the menu case.
> > >
> > > However, if the governor is simple enough and it can avoid doing the
> > > explicit negative value check, I don't see a reason to do that check
> > > elsewhere "just in case".
> >
> > Makes sense. I will submit my patch to fix this issue in menu and TEO.
> 
> Well, I have patches for that already and they are not
> super-straightforward.  Though If you want to try to fix this
> yourself, I'll wait for your submission.

Thanks! I really like this subsystem, so I hope to contribute a little.
I still have some questions.

For TEO governor:
    Even if we change some datatypes as your patch did before, some explicit 
    type conversions still need to be added to prevent wrong results.

    For example:
        line 276 (because 1u > -1 will be false)
        line 329
        line 422
    
    Since some of them are in a loop, does the overhead caused by the
    type conversions worth it? or do I need to do some pre-processing to avoid 
    duplicate conversions (which may cause additional space overhead) ?
    
For menu governor:
    If we simply change the datatypes, the conversions required are even more.

    For example:
        lines 133-142
        line 202
        line 289
        lines 302-309
        line 320
        lines 327-328
        line 347

    If we just do exception handling on next_timer_ns and delta_next,
    that still requires two additional branches.

Re: 回复: [PATCH 01/10] tick/nohz: Prevent tick_nohz_get_sleep_length() from returning negative value

[Rafael J. Wysocki]

On Fri, Mar 26, 2021 at 11:53 PM Zhou Ti (x2019cwm) <x2019cwm@stfx.ca> wrote:
>
> On Fri, 26 Mar 2021 19:54:26 +0100, Rafael J. Wysocki wrote:
> > On Fri, Mar 26, 2021 at 6:53 PM Zhou Ti (x2019cwm) <x2019cwm@stfx.ca> wrote:
> > >
> > > On Fri, 26 Mar 2021 18:01:47 +0100, Rafael J. Wysocki wrote:
> > > > On Thu, Mar 25, 2021 at 9:37 PM Zhou Ti (x2019cwm) <x2019cwm@stfx.ca> wrote:
> > > > >
> > > > > On March 25, 2021 15:50, Rafael J. Wysocki wrote:
> > > > > > On Thu, Mar 25, 2021 at 8:18 PM Zhou Ti (x2019cwm) <x2019cwm@stfx.ca> wrote:
> > > > > > >
> > > > > > > On March 25, 2021 14:56, Rafael J. Wysocki wrote:
> > > > > > > > On Thursday, March 25, 2021 2:14:00 PM CET Frederic Weisbecker wrote:
> > > > > > > > > On Tue, Mar 16, 2021 at 04:08:08PM +0000, Zhou Ti (x2019cwm) wrote:
> > > > > > > > > > But I don't think it's a good idea to handle this in callers, because logically the function shouldn't return negative values. Returning 0 directly would allow idle governors to get another chance to select again.
> > > > > > > > >
> > > > > > > > > Hmm, I'm going to leave the last word to Rafael since cpuidle are the only
> > > > > > > > > callers of this. In any case we need to fix it.
> > > > > > > >
> > > > > > > > Yes, we do.
> > > > > > > >
> > > > > > > > So I said that I preferred to address this in the callers and the reason why
> > > > > > > > is because, for example, for the teo governor it would be a matter of using
> > > > > > > > a different data type to store the tick_nohz_get_sleep_length() return value,
> > > > > > > > like in the (untested) patch below.
> > > > > > > >
> > > > > > > > So at least in this case there is no need to add any new branches anywhere.
> > > > > > > >
> > > > > > > > I'm still not sure about menu, because it is more complicated, but even if
> > > > > > > > that one needs an extra branch, that is a win already.
> > > > > > >
> > > > > > > I would like to point out the potential trouble that fixing this issue in the
> > > > > > > callers could cause.
> > > > > > >
> > > > > > > 1. This function is called multiple times in menu governor and TEO
> > > > > > > governor.
> > > > > >
> > > > > > What do you mean by "multiple times"?
> > > > > >
> > > > > > Each of the governors calls it once per cycle and its previous return
> > > > > > value is not used in the next cycle at least in teo.
> > > > >
> > > > > I remember a governor called this function twice in a cycle, I guess I remember
> > > > > wrong.
> > > >
> > > > That obviously depends on the governor, but both teo and menu call it
> > > > once per cycle.
> > > >
> > > > > > > I'm not sure that receiving results using signed integers is enough
> > > > > > > to solve all the problems, in the worst case it may require increasing
> > > > > > > the logical complexity of the code.
> > > > > >
> > > > > > That is a valid concern, so it is a tradeoff between increasing the
> > > > > > logical complexity of the code and adding branches to it.
> > > > > >
> > > > > > > 2. This function is important for developing idle governor.
> > > > > > > If the problem is not fixed in the function itself, then this potential
> > > > > > > pitfall should be explicitly stated in the documentation.
> > > > > >
> > > > > > That I can agree with.
> > > > > >
> > > > > > > This is because
> > > > > > > it is difficult to predict from the definition and naming of the function
> > > > > > > that it might return a negative number. I actually discovered this anomaly
> > > > > > > when I was doing data analysis on my own idle governor. For some idle control
> > > > > > > algorithms, this exception return could lead to serious consequences,
> > > > > > > because negative return logically won't happen.
> > > > > >
> > > > > > Well, it's a matter of how to take the possible negative return value
> > > > > > into account so it does not affect the result of the computations.
> > > > >
> > > > > I think it is challenging for some algorithms to take negative return values
> > > > > into account properly. For TEO (and even menu), it is possible to
> > > > > solve the problem by just changing the way the data is received is because the
> > > > > learning mechanism for both algorithms is simple.
> > > >
> > > > Of course this depends on the governor.
> > > >
> > > > > One of the interesting things about the CPUIdle subsystem is that it is well
> > > > > suited to introduce machine learning and probabilistic statistical methods.
> > > >
> > > > You need to remember that the governor code runs in the idle loop
> > > > context which is expected to be reasonably fast.
> > > >
> > > > That's why we are worrying about individual branches here.
> > > >
> > > > > This means that many of the more complex and data-sensitive algorithms can
> > > > > potentially be explored. In the best case we will still need to add additional
> > > > > code complexity to a new algorithm.
> > > >
> > > > So I'm not sure what the problem with adding an upfront negative value
> > > > check to the governor is.
> > > >
> > > > > It would reduce a lot of unnecessary considerations (for example, highlight
> > > > > this shortcoming in the documentation) if we could ensure that this function
> > > > > would work as it is logically defined. But I don't really understand
> > > > > how much of a burden adding an extra branch would impose, so I don't know if
> > > > > this tradeoff is worth it.
> > > >
> > > > It ultimately depends on the governor, which is why I think that the
> > > > negative value check should be done by the governor, if needed, and
> > > > not by the function called by it, because in the latter case the check
> > > > may be redundant and we end up with an extra branch (or two branches
> > > > in this particular case) for no good reason whatsoever.
> > > >
> > > > Yes, there are governors which simply can do the negative value check
> > > > upfront right after calling that function and ensure that they will
> > > > not deal with negative values going forward.  This is probably what
> > > > I'll do in the menu case.
> > > >
> > > > However, if the governor is simple enough and it can avoid doing the
> > > > explicit negative value check, I don't see a reason to do that check
> > > > elsewhere "just in case".
> > >
> > > Makes sense. I will submit my patch to fix this issue in menu and TEO.
> >
> > Well, I have patches for that already and they are not
> > super-straightforward.  Though If you want to try to fix this
> > yourself, I'll wait for your submission.
>
> Thanks! I really like this subsystem, so I hope to contribute a little.
> I still have some questions.
>
> For TEO governor:
>     Even if we change some datatypes as your patch did before, some explicit
>     type conversions still need to be added to prevent wrong results.

That's true.  That patch was way incomplete and I would start with
changing the data type of exit_latency_ns and target_residency_ns in
struct cpuidle_state to s64 for this reason.

>     For example:
>         line 276 (because 1u > -1 will be false)
>         line 329
>         line 422
>
>     Since some of them are in a loop, does the overhead caused by the
>     type conversions worth it? or do I need to do some pre-processing to avoid
>     duplicate conversions (which may cause additional space overhead) ?

Well, it looks like it may be better if I send my patches after all,
because it will take less time overall than me explaining here what I
would do.  Let me do that.

> For menu governor:
>     If we simply change the datatypes, the conversions required are even more.

I would add a negative return value check to menu as I said before.

>     For example:
>         lines 133-142
>         line 202
>         line 289
>         lines 302-309
>         line 320
>         lines 327-328
>         line 347
>
>     If we just do exception handling on next_timer_ns and delta_next,
>     that still requires two additional branches.

Not really AFAICS, but again let me send a patch to illustrate my point.

Re: 回复: [PATCH 01/10] tick/nohz: Prevent tick_nohz_get_sleep_length() from returning negative value

[Zhou Ti (x2019cwm)]

On Mon 2021-03-29 8:45, Rafael J. Wysocki wrote:
> On Fri, Mar 26, 2021 at 11:53 PM Zhou Ti (x2019cwm) <x2019cwm@stfx.ca> wrote:
> >
> > On Fri, 26 Mar 2021 19:54:26 +0100, Rafael J. Wysocki wrote:
> > > On Fri, Mar 26, 2021 at 6:53 PM Zhou Ti (x2019cwm) <x2019cwm@stfx.ca> wrote:
> > > >
> > > > On Fri, 26 Mar 2021 18:01:47 +0100, Rafael J. Wysocki wrote:
> > > > > On Thu, Mar 25, 2021 at 9:37 PM Zhou Ti (x2019cwm) <x2019cwm@stfx.ca> wrote:
> > > > > >
> > > > > > On March 25, 2021 15:50, Rafael J. Wysocki wrote:
> > > > > > > On Thu, Mar 25, 2021 at 8:18 PM Zhou Ti (x2019cwm) <x2019cwm@stfx.ca> wrote:
> > > > > > > >
> > > > > > > > On March 25, 2021 14:56, Rafael J. Wysocki wrote:
> > > > > > > > > On Thursday, March 25, 2021 2:14:00 PM CET Frederic Weisbecker wrote:
> > > > > > > > > > On Tue, Mar 16, 2021 at 04:08:08PM +0000, Zhou Ti (x2019cwm) wrote:
> > > > > > > > > > > But I don't think it's a good idea to handle this in callers, because logically the function shouldn't return negative values. Returning 0 directly would allow idle governors to get another chance to select again.
> > > > > > > > > >
> > > > > > > > > > Hmm, I'm going to leave the last word to Rafael since cpuidle are the only
> > > > > > > > > > callers of this. In any case we need to fix it.
> > > > > > > > >
> > > > > > > > > Yes, we do.
> > > > > > > > >
> > > > > > > > > So I said that I preferred to address this in the callers and the reason why
> > > > > > > > > is because, for example, for the teo governor it would be a matter of using
> > > > > > > > > a different data type to store the tick_nohz_get_sleep_length() return value,
> > > > > > > > > like in the (untested) patch below.
> > > > > > > > >
> > > > > > > > > So at least in this case there is no need to add any new branches anywhere.
> > > > > > > > >
> > > > > > > > > I'm still not sure about menu, because it is more complicated, but even if
> > > > > > > > > that one needs an extra branch, that is a win already.
> > > > > > > >
> > > > > > > > I would like to point out the potential trouble that fixing this issue in the
> > > > > > > > callers could cause.
> > > > > > > >
> > > > > > > > 1. This function is called multiple times in menu governor and TEO
> > > > > > > > governor.
> > > > > > >
> > > > > > > What do you mean by "multiple times"?
> > > > > > >
> > > > > > > Each of the governors calls it once per cycle and its previous return
> > > > > > > value is not used in the next cycle at least in teo.
> > > > > >
> > > > > > I remember a governor called this function twice in a cycle, I guess I remember
> > > > > > wrong.
> > > > >
> > > > > That obviously depends on the governor, but both teo and menu call it
> > > > > once per cycle.
> > > > >
> > > > > > > > I'm not sure that receiving results using signed integers is enough
> > > > > > > > to solve all the problems, in the worst case it may require increasing
> > > > > > > > the logical complexity of the code.
> > > > > > >
> > > > > > > That is a valid concern, so it is a tradeoff between increasing the
> > > > > > > logical complexity of the code and adding branches to it.
> > > > > > >
> > > > > > > > 2. This function is important for developing idle governor.
> > > > > > > > If the problem is not fixed in the function itself, then this potential
> > > > > > > > pitfall should be explicitly stated in the documentation.
> > > > > > >
> > > > > > > That I can agree with.
> > > > > > >
> > > > > > > > This is because
> > > > > > > > it is difficult to predict from the definition and naming of the function
> > > > > > > > that it might return a negative number. I actually discovered this anomaly
> > > > > > > > when I was doing data analysis on my own idle governor. For some idle control
> > > > > > > > algorithms, this exception return could lead to serious consequences,
> > > > > > > > because negative return logically won't happen.
> > > > > > >
> > > > > > > Well, it's a matter of how to take the possible negative return value
> > > > > > > into account so it does not affect the result of the computations.
> > > > > >
> > > > > > I think it is challenging for some algorithms to take negative return values
> > > > > > into account properly. For TEO (and even menu), it is possible to
> > > > > > solve the problem by just changing the way the data is received is because the
> > > > > > learning mechanism for both algorithms is simple.
> > > > >
> > > > > Of course this depends on the governor.
> > > > >
> > > > > > One of the interesting things about the CPUIdle subsystem is that it is well
> > > > > > suited to introduce machine learning and probabilistic statistical methods.
> > > > >
> > > > > You need to remember that the governor code runs in the idle loop
> > > > > context which is expected to be reasonably fast.
> > > > >
> > > > > That's why we are worrying about individual branches here.
> > > > >
> > > > > > This means that many of the more complex and data-sensitive algorithms can
> > > > > > potentially be explored. In the best case we will still need to add additional
> > > > > > code complexity to a new algorithm.
> > > > >
> > > > > So I'm not sure what the problem with adding an upfront negative value
> > > > > check to the governor is.
> > > > >
> > > > > > It would reduce a lot of unnecessary considerations (for example, highlight
> > > > > > this shortcoming in the documentation) if we could ensure that this function
> > > > > > would work as it is logically defined. But I don't really understand
> > > > > > how much of a burden adding an extra branch would impose, so I don't know if
> > > > > > this tradeoff is worth it.
> > > > >
> > > > > It ultimately depends on the governor, which is why I think that the
> > > > > negative value check should be done by the governor, if needed, and
> > > > > not by the function called by it, because in the latter case the check
> > > > > may be redundant and we end up with an extra branch (or two branches
> > > > > in this particular case) for no good reason whatsoever.
> > > > >
> > > > > Yes, there are governors which simply can do the negative value check
> > > > > upfront right after calling that function and ensure that they will
> > > > > not deal with negative values going forward.  This is probably what
> > > > > I'll do in the menu case.
> > > > >
> > > > > However, if the governor is simple enough and it can avoid doing the
> > > > > explicit negative value check, I don't see a reason to do that check
> > > > > elsewhere "just in case".
> > > >
> > > > Makes sense. I will submit my patch to fix this issue in menu and TEO.
> > >
> > > Well, I have patches for that already and they are not
> > > super-straightforward.  Though If you want to try to fix this
> > > yourself, I'll wait for your submission.
> >
> > Thanks! I really like this subsystem, so I hope to contribute a little.
> > I still have some questions.
> >
> > For TEO governor:
> >     Even if we change some datatypes as your patch did before, some explicit
> >     type conversions still need to be added to prevent wrong results.
> 
> That's true.  That patch was way incomplete and I would start with
> changing the data type of exit_latency_ns and target_residency_ns in
> struct cpuidle_state to s64 for this reason.
> 
> >     For example:
> >         line 276 (because 1u > -1 will be false)
> >         line 329
> >         line 422
> >
> >     Since some of them are in a loop, does the overhead caused by the
> >     type conversions worth it? or do I need to do some pre-processing to avoid
> >     duplicate conversions (which may cause additional space overhead) ?
> 
> Well, it looks like it may be better if I send my patches after all,
> because it will take less time overall than me explaining here what I
> would do.  Let me do that.
> 
> > For menu governor:
> >     If we simply change the datatypes, the conversions required are even more.
> 
> I would add a negative return value check to menu as I said before.
> 
> Not really AFAICS, but again let me send a patch to illustrate my point.

Sure! I will study your patches carefully.