[PATCH v4 1/2] sched: Add per_cpu cluster domain info and cpus_share_resources API

[Gautham R. Shenoy]

On Wed, Jun 15, 2022 at 07:49:22PM +0530, K Prateek Nayak wrote:

[..snip..]

> 
> - Bisecting:
> 
> When we ran the tests with only Patch 1 of the series, the
> regression was visible and the numbers were worse.
> 
> Clients:       tip                     cluster              Patch 1 Only
>     8    3263.81 (0.00 pct)      3086.81 (-5.42 pct)     3018.63 (-7.51 pct)
>    16    6011.19 (0.00 pct)      5360.28 (-10.82 pct)    4869.26 (-18.99 pct)
>    32    12058.31 (0.00 pct)     8769.08 (-27.27 pct)    8159.60 (-32.33 pct)
>    64    21258.21 (0.00 pct)     19021.09 (-10.52 pct)   13161.92 (-38.08 pct)
> 
> We further bisected the hunks to narrow down the cause to the per CPU
> variable declarations. 
> 
> 
> >
> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index 01259611beb9..b9bcfcf8d14d 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -1753,7 +1753,9 @@ static inline struct sched_domain *lowest_flag_domain(int cpu, int flag)
> >  DECLARE_PER_CPU(struct sched_domain __rcu *, sd_llc);
> >  DECLARE_PER_CPU(int, sd_llc_size);
> >  DECLARE_PER_CPU(int, sd_llc_id);
> > +DECLARE_PER_CPU(int, sd_share_id);
> >  DECLARE_PER_CPU(struct sched_domain_shared __rcu *, sd_llc_shared);
> > +DECLARE_PER_CPU(struct sched_domain __rcu *, sd_cluster);
> 
> The main reason for the regression seems to be the above declarations.

I think you meant that the regressions are due to the DEFINE_PER_CPU()
instances from the following hunk:

> > @@ -664,6 +664,8 @@ static void destroy_sched_domains(struct sched_domain *sd)
> >  DEFINE_PER_CPU(struct sched_domain __rcu *, sd_llc);
> >  DEFINE_PER_CPU(int, sd_llc_size);
> >  DEFINE_PER_CPU(int, sd_llc_id);
> > +DEFINE_PER_CPU(int, sd_share_id);
> > +DEFINE_PER_CPU(struct sched_domain __rcu *, sd_cluster);
> >  DEFINE_PER_CPU(struct sched_domain_shared __rcu *, sd_llc_shared);
> >  DEFINE_PER_CPU(struct sched_domain __rcu *, sd_numa);
> >  DEFINE_PER_CPU(struct sched_domain __rcu *, sd_asym_packing);
> >


The System.map diff for these variables between tip vs tip +
cluster-sched-v4 on your test system looks as follows:

 0000000000020520 D sd_asym_packing
 0000000000020528 D sd_numa
-0000000000020530 D sd_llc_shared
-0000000000020538 D sd_llc_id
-000000000002053c D sd_llc_size
-0000000000020540 D sd_llc
+0000000000020530 D sd_cluster
+0000000000020538 D sd_llc_shared
+0000000000020540 D sd_share_id
+0000000000020544 D sd_llc_id
+0000000000020548 D sd_llc_size
+0000000000020550 D sd_llc

The allocations are in the reverse-order of the definitions.

That perhaps explains why you no longer see the regression when you
define the sd_share_id and sd_cluster per-cpu definitions at the
beginning as indicated by the following

> - Move the declarations of sd_share_id and sd_cluster to the top
> 
>   Clients:       tip                    Patch 1            Patch 1 (Declarion on Top)
>     8      3255.69 (0.00 pct)      3018.63 (-7.28 pct)     3072.30 (-5.63 pct)
>    16      6092.67 (0.00 pct)      4869.26 (-20.08 pct)    5586.59 (-8.30 pct)
>    32      11156.56 (0.00 pct)     8159.60 (-26.86 pct)    11184.17 (0.24 pct)
>    64      21019.97 (0.00 pct)     13161.92 (-37.38 pct)   20289.70 (-3.47 pct)


--
Thanks and Regards
gautham.