Re: [PATCH 1/1] sched/fair: allow disabling newidle_balance with sched_relax_domain_level

[Vincent Guittot]

On Thu, 28 Mar 2024 at 17:27, Vitalii Bursov <vitaly@xxxxxxxxxx> wrote:
>
>
> On 28.03.24 16:43, Vincent Guittot wrote:
> > On Thu, 28 Mar 2024 at 01:31, Vitalii Bursov <vitaly@xxxxxxxxxx> wrote:
> >>
> >> Change relax_domain_level checks so that it would be possible
> >> to exclude all domains from newidle balancing.
> >>
> >> This matches the behavior described in the documentation:
> >>   -1   no request. use system default or follow request of others.
> >>    0   no search.
> >>    1   search siblings (hyperthreads in a core).
> >>
> >> "2" enables levels 0 and 1, level_max excludes the last (level_max)
> >> level, and level_max+1 includes all levels.
> >
> > I was about to say that max+1 is useless because it's the same as -1
> > but it's not exactly the same because it can supersede the system wide
> > default_relax_domain_level. I wonder if one should be able to enable
> > more levels than what the system has set by default.
>
> I don't know is such systems exist, but cpusets.rst suggests that
> increasing it beyoud the default value is possible:
> > If your situation is:
> >
> >  - The migration costs between each cpu can be assumed considerably
> >    small(for you) due to your special application's behavior or
> >    special hardware support for CPU cache etc.
> >  - The searching cost doesn't have impact(for you) or you can make
> >    the searching cost enough small by managing cpuset to compact etc.
> >  - The latency is required even it sacrifices cache hit rate etc.
> >    then increasing 'sched_relax_domain_level' would benefit you.

Fair enough. The doc should be updated as we can now clear the flags
but not set them

>
>
> >>
> >> Signed-off-by: Vitalii Bursov <vitaly@xxxxxxxxxx>
> >> ---
> >>  kernel/cgroup/cpuset.c  | 2 +-
> >>  kernel/sched/debug.c    | 1 +
> >>  kernel/sched/topology.c | 2 +-
> >>  3 files changed, 3 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> >> index 4237c874871..da24187c4e0 100644
> >> --- a/kernel/cgroup/cpuset.c
> >> +++ b/kernel/cgroup/cpuset.c
> >> @@ -2948,7 +2948,7 @@ bool current_cpuset_is_being_rebound(void)
> >>  static int update_relax_domain_level(struct cpuset *cs, s64 val)
> >>  {
> >>  #ifdef CONFIG_SMP
> >> -       if (val < -1 || val >= sched_domain_level_max)
> >> +       if (val < -1 || val > sched_domain_level_max + 1)
> >>                 return -EINVAL;
> >>  #endif
> >>
> >> diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> >> index 8d5d98a5834..8454cd4e5e1 100644
> >> --- a/kernel/sched/debug.c
> >> +++ b/kernel/sched/debug.c
> >> @@ -423,6 +423,7 @@ static void register_sd(struct sched_domain *sd, struct dentry *parent)
> >>
> >>  #undef SDM
> >>
> >> +       debugfs_create_u32("level", 0444, parent, (u32 *)&sd->level);
> >
> > IMO, this should be a separate patch as it's not part of the fix
>
> Thanks, I'll split it.
>
> >>         debugfs_create_file("flags", 0444, parent, &sd->flags, &sd_flags_fops);
> >>         debugfs_create_file("groups_flags", 0444, parent, &sd->groups->flags, &sd_flags_fops);
> >>  }
> >> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> >> index 99ea5986038..3127c9b30af 100644
> >> --- a/kernel/sched/topology.c
> >> +++ b/kernel/sched/topology.c
> >> @@ -1468,7 +1468,7 @@ static void set_domain_attribute(struct sched_domain *sd,
> >>         } else
> >>                 request = attr->relax_domain_level;
> >>
> >> -       if (sd->level > request) {
> >> +       if (sd->level >= request) {
> >
> > good catch and worth :
> > Fixes: 9ae7ab20b483 ("sched/topology: Don't set SD_BALANCE_WAKE on
> > cpuset domain relax")
> >
> Will add this.
> Thanks.
>
> >
> >>                 /* Turn off idle balance on this domain: */
> >>                 sd->flags &= ~(SD_BALANCE_WAKE|SD_BALANCE_NEWIDLE);
> >>         }
> >> --
> >> 2.20.1
> >>