Re: [External] Re: [PATCH 0/2] Fix nohz_full vs rt bandwidth

[Hao Jia]

On 2023/9/7 Phil Auld wrote:
> Hi Hao,
>
> On Wed, Sep 06, 2023 at 02:45:39PM +0800 Hao Jia wrote:
> > Friendly ping...
> >
> > On 2023/8/21 Hao Jia wrote:
> > > Since the commit 88c56cfeaec4 ("sched/fair: Block nohz tick_stop
> > > when cfs bandwidth in use") was merged, it handles conflicts between
> > > NOHZ full and cfs_bandwidth well, and the scheduler feature HZ_BW
> > > allows us to choose which one to prefer.
> > >
> > > This conflict also exists between NOHZ full and rt_bandwidth,
> > > these two patches try to handle it in a similar way.
>
> Are you actually hitting this in the real world?
>
> We, for example, no longer enable RT_GROUP_SCHED so this is a non-issue
> for our use cases.  I'd recommend considering that. (Does it even
> work with cgroup2?)

Yes, it has always been there. Regardless of whether RT_GROUP_SCHED is 
enabled or not, rt bandwidth is always enabled. If RT_GROUP_SCHED is not 
enabled, all rt tasks in the system are a group, and rt_runtime is 
950000, and rt_period is 1000000.So rt bandwidth is always enabled by 
default.


Thanks,
Hao

> In some ways what you have is a simplification of code, but it also
> obfuscates the stop_tick conditions by hiding them all in the class
> specific functions.  It was easier to see why the tick didn't stop
> looking at the original code.
>
> It would be better to do this only if it is really needed, in my opinion.
>
>
> Cheers,
> Phil
>
> > > patch1: Extracts a can_stop_tick() callback function for each
> > > sched_class from sched_can_stop_tick(), it will make things clearer
> > > and also convenient to handle the conflict between NOHZ full
> > > and rt_bandwidth.
> > >
> > > patch2: If the HZ_BW scheduler feature is enabled, and the RT task
> > > to be run is constrained by rt_bandwidth runtime. Then it will
> > > prevent NO_HZ full from stopping tick.
> > >
> > > Hao Jia (2):
> > >     sched/core: Introduce sched_class::can_stop_tick()
> > >     sched/rt: Block nohz tick_stop when rt bandwidth in use
> > >
> > >    kernel/sched/core.c     | 67 +++++--------------------------
> > >    kernel/sched/deadline.c | 16 ++++++++
> > >    kernel/sched/fair.c     | 56 +++++++++++++++++++++++---
> > >    kernel/sched/rt.c       | 89 ++++++++++++++++++++++++++++++++++++++++-
> > >    kernel/sched/sched.h    |  5 ++-
> > >    5 files changed, 168 insertions(+), 65 deletions(-)