linux-kernel - [RFC PATCH v2 0/6] SCHED_DEADLINE server infrastructure

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Message-Id: <20200807095051.385985-1-juri.lelli@redhat.com>
Date:   Fri,  7 Aug 2020 11:50:45 +0200
From:   Juri Lelli <juri.lelli@...hat.com>
To:     peterz@...radead.org, mingo@...hat.com
Cc:     rostedt@...dmis.org, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, luca.abeni@...tannapisa.it,
        tommaso.cucinotta@...tannapisa.it, alessio.balsini@...il.com,
        bristot@...hat.com, dietmar.eggemann@....com,
        linux-rt-users@...r.kernel.org, mtosatti@...hat.com,
        williams@...hat.com, valentin.schneider@....com,
        Juri Lelli <juri.lelli@...hat.com>
Subject: [RFC PATCH v2 0/6] SCHED_DEADLINE server infrastructure

Hi,

This is RFC v2 of Peter's SCHED_DEADLINE server infrastructure
implementation [1].

SCHED_DEADLINE servers can help fixing starvation issues of low priority
tasks (e.g., SCHED_OTHER) when higher priority tasks monopolize CPU
cycles. Today we have RT Throttling; DEADLINE servers should be able to
replace and improve that.

I rebased Peter's patches (adding changelogs where needed) on
tip/sched/core as of today and incorporated fixes to issues discussed
during RFC v1. Current set seems to even boot on real HW! :-)

While playing with RFC v1 set (and discussing it further offline with
Daniel) it has emerged the need to slightly change the behavior. Patch
6/6 is a (cumbersome?) attempt to show what's probably needed.
The problem with "original" implementation is that FIFO tasks might
suffer preemption from NORMAL even when spare CPU cycles are available.
In fact, fair deadline server is enqueued right away when NORMAL tasks
wake up and they are first scheduled by the server, thus potentially
preempting a well behaving FIFO task. This is of course not ideal.
So, in patch 6/6 I propose to use some kind of starvation monitor/
watchdog that delays enqueuing of deadline servers to the point when
fair tasks might start to actually suffer from starvation (just randomly
picked HZ/2 for now). One problem I already see with the current
implementation is that it adds overhead to fair paths, so I'm pretty
sure there are better ways to implement the idea (e.g., Daniel already
suggested using a starvation monitor kthread sort of thing).

Receiving comments and suggestions is the sole purpose of this posting
at this stage. Hopefully we can further discuss the idea at Plumbers in
a few weeks. So, please don't focus too much into actual implementation
(which I plan to revise anyway after I'm back from pto :), but try to
see if this might actually fly. The feature seems to be very much needed.

Thanks!

Juri

1 - https://lore.kernel.org/lkml/20190726145409.947503076@infradead.org/

Juri Lelli (1):
  sched/fair: Implement starvation monitor

Peter Zijlstra (5):
  sched: Unify runtime accounting across classes
  sched/deadline: Collect sched_dl_entity initialization
  sched/deadline: Move bandwidth accounting into {en,de}queue_dl_entity
  sched/deadline: Introduce deadline servers
  sched/fair: Add trivial fair server

 include/linux/sched.h    |  28 ++-
 kernel/sched/core.c      |  23 +-
 kernel/sched/deadline.c  | 483 ++++++++++++++++++++++++---------------
 kernel/sched/fair.c      | 136 ++++++++++-
 kernel/sched/rt.c        |  17 +-
 kernel/sched/sched.h     |  50 +++-
 kernel/sched/stop_task.c |  16 +-
 7 files changed, 522 insertions(+), 231 deletions(-)

-- 
2.26.2