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

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

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