[PATCH] sched/fair: fix calc_cfs_shares fixed point arithmetics

[PATCH] sched/fair: fix calc_cfs_shares fixed point arithmetics

[Samuel Thibault]

2159197d6677 ("sched/core: Enable increased load resolution on 64-bit kernels")

exposed yet another miscalculation in calc_cfs_shares: MIN_SHARES is unscaled,
and must thus be scaled before being manipulated against "shares" amounts.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: stable@vger.kernel.org
Fixes: 2159197d6677 ("sched/core: Enable increased load resolution on 64-bit kernels")

---
This should be backported to 4.7 and 4.8 to fix scheduling priorities
miscalculations

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6559d19..be84f72 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2657,8 +2657,8 @@ static long calc_cfs_shares(struct cfs_rq *cfs_rq, struct task_group *tg)
 	if (tg_weight)
 		shares /= tg_weight;
 
-	if (shares < MIN_SHARES)
-		shares = MIN_SHARES;
+	if (shares < scale_load(MIN_SHARES))
+		shares = scale_load(MIN_SHARES);
 	if (shares > tg->shares)
 		shares = tg->shares;
 

Re: [PATCH] sched/fair: fix calc_cfs_shares fixed point arithmetics

[Paul Turner]

On Mon, Dec 19, 2016 at 2:40 PM, Samuel Thibault
<samuel.thibault@ens-lyon.org> wrote:
> 2159197d6677 ("sched/core: Enable increased load resolution on 64-bit kernels")
>
> exposed yet another miscalculation in calc_cfs_shares: MIN_SHARES is unscaled,
> and must thus be scaled before being manipulated against "shares" amounts.

It's actually intentional that MIN_SHARES is un-scaled here, this is
necessary to support the goal of sub-partitioning groups with small
shares.

E.g.  A group with shares=2 and 5 threads will internally provide 2048
units of weight for the load-balancer to account for their
distribution.

>
> Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: stable@vger.kernel.org
> Fixes: 2159197d6677 ("sched/core: Enable increased load resolution on 64-bit kernels")
>
> ---
> This should be backported to 4.7 and 4.8 to fix scheduling priorities
> miscalculations
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 6559d19..be84f72 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2657,8 +2657,8 @@ static long calc_cfs_shares(struct cfs_rq *cfs_rq, struct task_group *tg)
>         if (tg_weight)
>                 shares /= tg_weight;
>
> -       if (shares < MIN_SHARES)
> -               shares = MIN_SHARES;
> +       if (shares < scale_load(MIN_SHARES))
> +               shares = scale_load(MIN_SHARES);
>         if (shares > tg->shares)
>                 shares = tg->shares;
>

Re: [PATCH] sched/fair: fix calc_cfs_shares fixed point arithmetics

[Samuel Thibault]

Paul Turner, on Mon 19 Dec 2016 14:44:38 -0800, wrote:
> On Mon, Dec 19, 2016 at 2:40 PM, Samuel Thibault
> <samuel.thibault@ens-lyon.org> wrote:
> > 2159197d6677 ("sched/core: Enable increased load resolution on 64-bit kernels")
> >
> > exposed yet another miscalculation in calc_cfs_shares: MIN_SHARES is unscaled,
> > and must thus be scaled before being manipulated against "shares" amounts.
> 
> It's actually intentional that MIN_SHARES is un-scaled here, this is
> necessary to support the goal of sub-partitioning groups with small
> shares.

Uh? you mean it's normal that MIN_SHARES is here compared as such
against "shares" while e.g. in sched_group_set_shares or effective_load
it is scaled before comparing with "shares"?

> E.g.  A group with shares=2 and 5 threads will internally provide 2048
> units of weight for the load-balancer to account for their
> distribution.

But here "shares" is already scaled, so

> > -       if (shares < MIN_SHARES)
> > -               shares = MIN_SHARES;
...
> >         return shares;

This will only make sure that the returned shares is 2, not 2048.

Samuel

Re: [PATCH] sched/fair: fix calc_cfs_shares fixed point arithmetics

[Paul Turner]

On Mon, Dec 19, 2016 at 3:07 PM, Samuel Thibault
<samuel.thibault@ens-lyon.org> wrote:
> Paul Turner, on Mon 19 Dec 2016 14:44:38 -0800, wrote:
>> On Mon, Dec 19, 2016 at 2:40 PM, Samuel Thibault
>> <samuel.thibault@ens-lyon.org> wrote:
>> > 2159197d6677 ("sched/core: Enable increased load resolution on 64-bit kernels")
>> >
>> > exposed yet another miscalculation in calc_cfs_shares: MIN_SHARES is unscaled,
>> > and must thus be scaled before being manipulated against "shares" amounts.
>>
>> It's actually intentional that MIN_SHARES is un-scaled here, this is
>> necessary to support the goal of sub-partitioning groups with small
>> shares.
>
> Uh? you mean it's normal that MIN_SHARES is here compared as such
> against "shares" while e.g. in sched_group_set_shares or effective_load
> it is scaled before comparing with "shares"?

Yes.

sched_group_set_shares() controls the amount allocated to the group.

Both calc_cfs_shares() and effective_load() are subdividing this
total.  Which is why it is scaled up from the external value of 2.

>
>> E.g.  A group with shares=2 and 5 threads will internally provide 2048
>> units of weight for the load-balancer to account for their
>> distribution.
>
> But here "shares" is already scaled, so
>
>> > -       if (shares < MIN_SHARES)
>> > -               shares = MIN_SHARES;
> ...
>> >         return shares;
>
> This will only make sure that the returned shares is 2, not 2048.

This is intentional.  The MIN_SHARES you are seeing here is overloaded.
Every "1" unit of share is "SCHED_LOAD_RESOLUTION" bits internally.
We express a minimum of "2" in terms of the unit weight due to larger
numerical errors in the "1" case.
In the unscaled case this needs to be MIN_SHARES, and in the scaled
case, the subdivision of the scaled values must still be >=2.

To make this concrete:
In this case we can then internally say that there are (internally)
~410 units of weight for each of these 5 threads.
Thus, if one cpu has 4 threads and another 1, we see that as a
1640/410 imbalance, not a 2048/2048 balance.

>
> Samuel

Re: [PATCH] sched/fair: fix calc_cfs_shares fixed point arithmetics

[Samuel Thibault]

Paul Turner, on Mon 19 Dec 2016 15:26:19 -0800, wrote:
> >> > -       if (shares < MIN_SHARES)
> >> > -               shares = MIN_SHARES;
> > ...
> >> >         return shares;
> >
> > This will only make sure that the returned shares is 2, not 2048.
> 
> This is intentional.  The MIN_SHARES you are seeing here is overloaded.
> Every "1" unit of share is "SCHED_LOAD_RESOLUTION" bits internally.

I'm not talking about the SCHED_LOAD_RESOLUTION scaling, but about the
SCHED_FIXEDPOINT_SHIFT scaling, which is what
2159197d6677 ("sched/core: Enable increased load resolution on 64-bit kernels")
modified on 64bit platforms.

Samuel

Re: [PATCH] sched/fair: fix calc_cfs_shares fixed point arithmetics

[Paul Turner]

On Mon, Dec 19, 2016 at 3:29 PM, Samuel Thibault
<samuel.thibault@ens-lyon.org> wrote:
> Paul Turner, on Mon 19 Dec 2016 15:26:19 -0800, wrote:
>> >> > -       if (shares < MIN_SHARES)
>> >> > -               shares = MIN_SHARES;
>> > ...
>> >> >         return shares;
>> >
>> > This will only make sure that the returned shares is 2, not 2048.
>>
>> This is intentional.  The MIN_SHARES you are seeing here is overloaded.
>> Every "1" unit of share is "SCHED_LOAD_RESOLUTION" bits internally.
>
> I'm not talking about the SCHED_LOAD_RESOLUTION scaling, but about the
> SCHED_FIXEDPOINT_SHIFT scaling, which is what
> 2159197d6677 ("sched/core: Enable increased load resolution on 64-bit kernels")
> modified on 64bit platforms.

.... From that commit:

"""
-#if 0 /* BITS_PER_LONG > 32 -- currently broken: it increases power
usage under light load  */
+#ifdef CONFIG_64BIT
 # define SCHED_LOAD_RESOLUTION 10
 # define scale_load(w)         ((w) << SCHED_LOAD_RESOLUTION)
 # define scale_load_down(w)    ((w) >> SCHED_LOAD_RESOLUTION)
"""


Please take a deeper look at the scale_load() interactions.

Re: [PATCH] sched/fair: fix calc_cfs_shares fixed point arithmetics

[Samuel Thibault]

Paul Turner, on Mon 19 Dec 2016 15:32:15 -0800, wrote:
> On Mon, Dec 19, 2016 at 3:29 PM, Samuel Thibault
> <samuel.thibault@ens-lyon.org> wrote:
> > Paul Turner, on Mon 19 Dec 2016 15:26:19 -0800, wrote:
> >> >> > -       if (shares < MIN_SHARES)
> >> >> > -               shares = MIN_SHARES;
> >> > ...
> >> >> >         return shares;
> >> >
> >> > This will only make sure that the returned shares is 2, not 2048.
> >>
> >> This is intentional.  The MIN_SHARES you are seeing here is overloaded.
> >> Every "1" unit of share is "SCHED_LOAD_RESOLUTION" bits internally.
> >
> > I'm not talking about the SCHED_LOAD_RESOLUTION scaling, but about the
> > SCHED_FIXEDPOINT_SHIFT scaling, which is what
> > 2159197d6677 ("sched/core: Enable increased load resolution on 64-bit kernels")
> > modified on 64bit platforms.
> 
> .... From that commit:
> 
> """
> -#if 0 /* BITS_PER_LONG > 32 -- currently broken: it increases power
> usage under light load  */
> +#ifdef CONFIG_64BIT
>  # define SCHED_LOAD_RESOLUTION 10
>  # define scale_load(w)         ((w) << SCHED_LOAD_RESOLUTION)
>  # define scale_load_down(w)    ((w) >> SCHED_LOAD_RESOLUTION)

Errgl, sorry, I was referring to the old naming.  This stuff has seen
so much patching over and over in the past revisions...  It though you
were referring to SCHED_CAPACITY_SCALE.  The code I was reading now uses
SCHED_LOAD_RESOLUTION, so that's why I read your "SCHED_LOAD_RESOLUTION"
as "the other scaling".

> The MIN_SHARES you are seeing here is overloaded.
> In the unscaled case this needs to be MIN_SHARES, and in the scaled
> case, the subdivision of the scaled values must still be >=2.

Ok, now I understand.  I have to say this overloading is confusing.

Samuel

Re: [PATCH] sched/fair: fix calc_cfs_shares fixed point arithmetics

[Dietmar Eggemann]

Hi Samuel,

On 12/20/2016 12:45 AM, Samuel Thibault wrote:
> Paul Turner, on Mon 19 Dec 2016 15:32:15 -0800, wrote:
>> On Mon, Dec 19, 2016 at 3:29 PM, Samuel Thibault
>> <samuel.thibault@ens-lyon.org> wrote:
>>> Paul Turner, on Mon 19 Dec 2016 15:26:19 -0800, wrote:

[...]

>> The MIN_SHARES you are seeing here is overloaded.
>> In the unscaled case this needs to be MIN_SHARES, and in the scaled
>> case, the subdivision of the scaled values must still be >=2.
>
> Ok, now I understand.  I have to say this overloading is confusing.
>
> Samuel

this had been already discussed back in August when I posted the 
original patch.


https://lkml.org/lkml/2016/8/22/351
https://lkml.org/lkml/2016/8/23/641

Re: [PATCH] sched/fair: fix calc_cfs_shares fixed point arithmetics

[Peter Zijlstra]

On Tue, Dec 20, 2016 at 02:04:34PM +0100, Dietmar Eggemann wrote:
> Hi Samuel,
> 
> On 12/20/2016 12:45 AM, Samuel Thibault wrote:
> >Paul Turner, on Mon 19 Dec 2016 15:32:15 -0800, wrote:
> >>On Mon, Dec 19, 2016 at 3:29 PM, Samuel Thibault
> >><samuel.thibault@ens-lyon.org> wrote:
> >>>Paul Turner, on Mon 19 Dec 2016 15:26:19 -0800, wrote:
> 
> [...]
> 
> >>The MIN_SHARES you are seeing here is overloaded.
> >>In the unscaled case this needs to be MIN_SHARES, and in the scaled
> >>case, the subdivision of the scaled values must still be >=2.
> >
> >Ok, now I understand.  I have to say this overloading is confusing.
> >
> >Samuel
> 
> this had been already discussed back in August when I posted the original
> patch.

Maybe we should put a comment in to avoid getting more of these ;-)

Re: [PATCH] sched/fair: fix calc_cfs_shares fixed point arithmetics

[Samuel Thibault]

Dietmar Eggemann, on Tue 20 Dec 2016 14:04:34 +0100, wrote:
> On 12/20/2016 12:45 AM, Samuel Thibault wrote:
> >Paul Turner, on Mon 19 Dec 2016 15:32:15 -0800, wrote:
> >>On Mon, Dec 19, 2016 at 3:29 PM, Samuel Thibault
> >><samuel.thibault@ens-lyon.org> wrote:
> >>>Paul Turner, on Mon 19 Dec 2016 15:26:19 -0800, wrote:
> 
> [...]
> 
> >>The MIN_SHARES you are seeing here is overloaded.
> >>In the unscaled case this needs to be MIN_SHARES, and in the scaled
> >>case, the subdivision of the scaled values must still be >=2.
> >
> >Ok, now I understand.  I have to say this overloading is confusing.
> >
> >Samuel
> 
> this had been already discussed back in August when I posted the original
> patch.

But that doesn't show up in the source code or git history.  One
shouldn't have to dig mailing lists to get code comments :)

Samuel

Re: [PATCH] sched/fair: fix calc_cfs_shares fixed point arithmetics

[Dietmar Eggemann]

On 20/12/16 13:15, Peter Zijlstra wrote:
> On Tue, Dec 20, 2016 at 02:04:34PM +0100, Dietmar Eggemann wrote:
>> Hi Samuel,
>>
>> On 12/20/2016 12:45 AM, Samuel Thibault wrote:
>>> Paul Turner, on Mon 19 Dec 2016 15:32:15 -0800, wrote:
>>>> On Mon, Dec 19, 2016 at 3:29 PM, Samuel Thibault
>>>> <samuel.thibault@ens-lyon.org> wrote:
>>>>> Paul Turner, on Mon 19 Dec 2016 15:26:19 -0800, wrote:
>>
>> [...]
>>
>>>> The MIN_SHARES you are seeing here is overloaded.
>>>> In the unscaled case this needs to be MIN_SHARES, and in the scaled
>>>> case, the subdivision of the scaled values must still be >=2.
>>>
>>> Ok, now I understand.  I have to say this overloading is confusing.
>>>
>>> Samuel
>>
>> this had been already discussed back in August when I posted the original
>> patch.
> 
> Maybe we should put a comment in to avoid getting more of these ;-)
> 

Maybe something like this? Mainly what Paul taught us plus an example from
a discussion I had with Vincent.

-- >8 --

Subject: [PATCH] sched/fair: Explain why MIN_SHARES isn't scaled in
 calc_cfs_shares()

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
---
 kernel/sched/fair.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6559d197e08a..a0ca9b11b1b3 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2657,6 +2657,18 @@ static long calc_cfs_shares(struct cfs_rq *cfs_rq, struct task_group *tg)
        if (tg_weight)
                shares /= tg_weight;
 
+       /*
+        * MIN_SHARES has to be unscaled here to support per-cpu partitioning
+        * of a group with small tg->shares value. It is a floor value which is
+        * assigned as a minimum load.weight to the sched_entity representing
+        * the group on a cpu.
+        *
+        * E.g. on 64-bit for a group with tg->shares of scale_load(15)=15*1024
+        * on an 8 core system with 8 tasks each runnable on one cpu shares has
+        * to be 15*1024*1/8=1920 instead of scale_load(MIN_SHARES)=2*1024. In
+        * case no task is runnable on a cpu MIN_SHARES=2 should be returned
+        * instead of 0.
+        */
        if (shares < MIN_SHARES)
                shares = MIN_SHARES;
        if (shares > tg->shares)
-- 
2.11.0

[tip:sched/core] sched/fair: Explain why MIN_SHARES isn't scaled in calc_cfs_shares()

[tip-bot for Dietmar Eggemann]

Commit-ID:  b8fd8423697b9ec729c5bb91737faad84ae19985
Gitweb:     http://git.kernel.org/tip/b8fd8423697b9ec729c5bb91737faad84ae19985
Author:     Dietmar Eggemann <dietmar.eggemann@arm.com>
AuthorDate: Wed, 11 Jan 2017 11:29:47 +0000
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Sat, 14 Jan 2017 11:30:02 +0100

sched/fair: Explain why MIN_SHARES isn't scaled in calc_cfs_shares()

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Turner <pjt@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Samuel Thibault <samuel.thibault@ens-lyon.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/e9a4d858-bcf3-36b9-e3a9-449953e34569@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/sched/fair.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 2b866a2..274c747 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2657,6 +2657,18 @@ static long calc_cfs_shares(struct cfs_rq *cfs_rq, struct task_group *tg)
 	if (tg_weight)
 		shares /= tg_weight;
 
+	/*
+	 * MIN_SHARES has to be unscaled here to support per-CPU partitioning
+	 * of a group with small tg->shares value. It is a floor value which is
+	 * assigned as a minimum load.weight to the sched_entity representing
+	 * the group on a CPU.
+	 *
+	 * E.g. on 64-bit for a group with tg->shares of scale_load(15)=15*1024
+	 * on an 8-core system with 8 tasks each runnable on one CPU shares has
+	 * to be 15*1024*1/8=1920 instead of scale_load(MIN_SHARES)=2*1024. In
+	 * case no task is runnable on a CPU MIN_SHARES=2 should be returned
+	 * instead of 0.
+	 */
 	if (shares < MIN_SHARES)
 		shares = MIN_SHARES;
 	if (shares > tg->shares)