[PATCH v3 0/5] support "cpu_isolated" mode for nohz_full

Chris Metcalf
2015-06-03T15:29:20Z

The existing nohz_full mode does a nice job of suppressing extraneous
kernel interrupts for cores that desire it. However, there is a need
for a more deterministic mode that rigorously disallows kernel
interrupts, even at a higher cost in user/kernel transition time:
for example, high-speed networking applications running userspace
drivers that will drop packets if they are ever interrupted.

These changes attempt to provide an initial draft of such a framework;
the changes do not add any overhead to the usual non-nohz_full mode,
and only very small overhead to the typical nohz_full mode. A prctl()
option (PR_SET_CPU_ISOLATED) is added to control whether processes have
requested this stricter semantics, and within that prctl() option we
provide a number of different bits for more precise control.
Additionally, we add a new command-line boot argument to facilitate
debugging where unexpected interrupts are being delivered from.

Code that is conceptually similar has been in use in Tilera's
Multicore Development Environment since 2008, known as Zero-Overhead
Linux, and has seen wide adoption by a range of customers. This patch
series represents the first serious attempt to upstream that
functionality. Although the current state of the kernel isn't quite
ready to run with absolutely no kernel interrupts (for example,
workqueues on cpu_isolated cores still remain to be dealt with), this
patch series provides a way to make dynamic tradeoffs between avoiding
kernel interrupts on the one hand, and making voluntary calls in and
out of the kernel more expensive, for tasks that want it.

The series (based currently on my arch/tile master tree for 4.2,
in turn based on 4.1-rc1) is available at:

git://git.kernel.org/pub/scm/linux/kernel/git/cmetcalf/linux-tile.git dataplane

v3:
remove dependency on cpu_idle subsystem (Thomas Gleixner)
use READ_ONCE instead of ACCESS_ONCE in tick_nohz_cpu_isolated_enter
use seconds for console messages instead of jiffies (Thomas Gleixner)
updated commit description for patch 5/5

v2:
rename "dataplane" to "cpu_isolated"
drop ksoftirqd suppression changes (believed no longer needed)
merge previous "QUIESCE" functionality into baseline functionality
explicitly track syscalls and exceptions for "STRICT" functionality
allow configuring a signal to be delivered for STRICT mode failures
move debug tracking to irq_enter(), not irq_exit()

Note: I have not removed the commit to disable the 1Hz timer tick
fallback that was nack'ed by PeterZ, pending a decision on that thread
as to what to do (https://lkml.org/lkml/2015/5/8/555); also since if
we remove the 1Hz tick, cpu_isolated threads will never re-enter
userspace since a tick will always be pending.

Chris Metcalf (5):
nohz_full: add support for "cpu_isolated" mode
nohz: support PR_CPU_ISOLATED_STRICT mode
nohz: cpu_isolated strict mode configurable signal
nohz: add cpu_isolated_debug boot flag
nohz: cpu_isolated: allow tick to be fully disabled

Documentation/kernel-parameters.txt | 6 +++
arch/tile/kernel/process.c | 9 ++++
arch/tile/kernel/ptrace.c | 6 ++-
arch/tile/mm/homecache.c | 5 +-
arch/x86/kernel/ptrace.c | 2 +
include/linux/context_tracking.h | 11 ++--
include/linux/sched.h | 3 ++
include/linux/tick.h | 28 ++++++++++
include/uapi/linux/prctl.h | 8 +++
kernel/context_tracking.c | 12 +++--
kernel/irq_work.c | 4 +-
kernel/sched/core.c | 18 +++++++
kernel/signal.c | 5 ++
kernel/smp.c | 4 ++
kernel/softirq.c | 6 +++
kernel/sys.c | 8 +++
kernel/time/tick-sched.c | 104 +++++++++++++++++++++++++++++++++++-
17 files changed, 229 insertions(+), 10 deletions(-)

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2.1.2