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Subject[RFD PATCH 0/4] cpu: Bulk CPU Hotplug support.
From
Date

Hi,

(NOTE: This is an RFD. Patches are not for inclusion)

The current CPU-Hotplug infrastructure enables us to hotplug one CPU at any
given time. However, with newer machines which have multiple-cores and
multi-threads, it makes much sense to change the unit of hotplug to a core or
a package. We might want to evacuate a core or a package to reduce the avg
power/to manage the temperature of the system/to dynamically provision
cores/packages to a running system. But performing a series of CPU-Hotplug
is relatively slower.

Currently on a ppc64 box with 16 CPUs, the time taken for
a individual cpu-hotplug operation is as follows.

# time echo 0 > /sys/devices/system/cpu/cpu2/online
real 0m0.025s
user 0m0.000s
sys 0m0.002s
# time echo 1 > /sys/devices/system/cpu/cpu2/online
real 0m0.021s
user 0m0.000s
sys 0m0.000s
(The online time used to be ~200ms. It has been reduced after applying patch 1
of the series which reduces the polling interval from 200ms to 1ms.)

Of the this, the time taken for sending the notifications and performing
the actual cpu-hotplug operation (detailed profile is appended at the end of
the text) is:

12.645925 ms on the offline path.
21.019581 ms on the online path.

(The 10ms discrepancy that we observe in the total time taken for cpu-offline
Vs the time accounted for notifiers and cpu-hotplug operation is because of a
synchronize_sched() performed after clearing the active_cpu_mask.)

So, of the accounted time, a major chunk of time is consumed by
cpuset_track_online_cpus() while handling CPU_DEAD and CPU_ONLINE
notifications.

11.320205 ms: cpuset_track_online_cpus : CPU_DEAD
12.767882 ms: cpuset_track_online_cpus : CPU_ONLINE

cpuset_trace_online_cpus() among other things performs the task of rebuilding
the sched_domains for every online CPU in the system.

The operations performed within the cpuset_track_online_cpus()
depends only on the cpu_online_map and not on the CPU which has been
hotplugged. The other notifiers which behave similarly are
- ratelimit_handler(),
- vmstat_cpuup_callback()
- vmscan: cpu_callback()

Thus if we bunch up multiple cpu-offlines/onlines, we can reduce the overall
time taken by optimizing notifiers such as these, so that they can
perform the necessary functions only once, after the completion of the
CPU-Hotplug operation. This would cut down the CPU hotplug time substantially.

The whole approach would require the Cpu-Hotplug notifiers to work on
cpumask_t instead of cpu. A similar proposal has been once proposed before by
Shaohua Li (http://lkml.org/lkml/2006/5/8/18)

In this patch series, we extend the existing cpu online/offline
interface to enable the user to offline/online a bunch of CPUs
at the same time.

The proposed interface to do so are the sysfs file:
/sys/devices/system/cpu/online
/sys/devices/system/cpu/online

The usage is:
echo 4,6,7 > /sys/devices/system/cpu/offline
echo 5 > /sys/devices/system/cpu/offline
echo 4-7 > /sys/devices/system/cpu/online
As of now, this patch series does no optimizations to the CPU-Hotplug core
but serially hotplugs the CPUs in the list provided by the user.

The interface provided in this patch series has been tested on a
16-way ppc64 box.


Still TODO:
- Enhance the subsystem notifiers to work on a cpumask_var_t instead of a cpu
id.

- Optimize the subsystem notifiers to reduce the time consumed while
handling CPU_[DOWN_PREPARE/DEAD/UP_PREPARE/ONLINE] events for the
cpumask_var_t.

- Define the Rollback Semantics for the notifiers which fail to handle
a CPU_* event correctly.

- Send the kobject-events for the corresponding device entries of each of the
CPUs present in the list to maintain ABI compatibility.

Any feedback is much appreciated

---
Gautham R Shenoy (4):
cpu: measure time taken by subsystem notifiers during cpu-hotplug
cpu: Define new functions cpu_down_mask and cpu_up_mask
cpu: sysfs interface for hotplugging bunch of CPUs.
powerpc: cpu: Reduce the polling interval in __cpu_up()


arch/powerpc/kernel/smp.c | 5 +-
drivers/base/cpu.c | 76 ++++++++++++++++++++++++++++--
include/linux/cpu.h | 2 +
include/trace/notifier_trace.h | 32 ++++++++++++
kernel/cpu.c | 103 ++++++++++++++++++++++++++++++----------
kernel/notifier.c | 23 +++++++--
6 files changed, 203 insertions(+), 38 deletions(-)
create mode 100644 include/trace/notifier_trace.h
--
Thanks and Regards
gautham


****************** Cpu-Hotplug profile ********************************
=============================================================================
statistics for CPU_DOWN_PREPARE
=============================================================================
379 ns: buffer_cpu_notify : CPU_DOWN_PREPARE
457 ns: topology_cpu_callback : CPU_DOWN_PREPARE
504 ns: flow_cache_cpu : CPU_DOWN_PREPARE
517 ns: cpu_callback : CPU_DOWN_PREPARE
533 ns: hotplug_cfd : CPU_DOWN_PREPARE
546 ns: dev_cpu_callback : CPU_DOWN_PREPARE
547 ns: timer_cpu_notify : CPU_DOWN_PREPARE
562 ns: page_alloc_cpu_notify : CPU_DOWN_PREPARE
564 ns: cpuset_track_online_cpus : CPU_DOWN_PREPARE
594 ns: blk_cpu_notify : CPU_DOWN_PREPARE
623 ns: hotplug_hrtick : CPU_DOWN_PREPARE
623 ns: radix_tree_callback : CPU_DOWN_PREPARE
715 ns: remote_softirq_cpu_notify : CPU_DOWN_PREPARE
777 ns: rb_cpu_notify : CPU_DOWN_PREPARE
777 ns: sysfs_cpu_notify : CPU_DOWN_PREPARE
807 ns: rcu_cpu_notify : CPU_DOWN_PREPARE
820 ns: ratelimit_handler : CPU_DOWN_PREPARE
822 ns: pageset_cpuup_callback : CPU_DOWN_PREPARE
898 ns: cpu_callback : CPU_DOWN_PREPARE
898 ns: relay_hotcpu_callback : CPU_DOWN_PREPARE
929 ns: hrtimer_cpu_notify : CPU_DOWN_PREPARE
930 ns: cpu_callback : CPU_DOWN_PREPARE
1096 ns: cpu_numa_callback : CPU_DOWN_PREPARE
1096 ns: percpu_counter_hotcpu_callback: CPU_DOWN_PREPARE
1111 ns: slab_cpuup_callback : CPU_DOWN_PREPARE
1139 ns: update_runtime : CPU_DOWN_PREPARE
1143 ns: rcu_barrier_cpu_hotplug : CPU_DOWN_PREPARE
2725 ns: workqueue_cpu_callback : CPU_DOWN_PREPARE
2852 ns: migration_call : CPU_DOWN_PREPARE
4497 ns: vmstat_cpuup_callback : CPU_DOWN_PREPARE
=========================================================================
Total time for CPU_DOWN_PREPARE = .030481000 ms
=========================================================================
=============================================================================
statistics for CPU_DYING
=============================================================================
349 ns: cpu_callback : CPU_DYING
349 ns: hotplug_hrtick : CPU_DYING
349 ns: remote_softirq_cpu_notify : CPU_DYING
351 ns: timer_cpu_notify : CPU_DYING
363 ns: vmstat_cpuup_callback : CPU_DYING
364 ns: rb_cpu_notify : CPU_DYING
365 ns: blk_cpu_notify : CPU_DYING
365 ns: cpu_callback : CPU_DYING
365 ns: cpu_numa_callback : CPU_DYING
365 ns: cpuset_track_online_cpus : CPU_DYING
365 ns: dev_cpu_callback : CPU_DYING
365 ns: hotplug_cfd : CPU_DYING
365 ns: page_alloc_cpu_notify : CPU_DYING
365 ns: radix_tree_callback : CPU_DYING
365 ns: relay_hotcpu_callback : CPU_DYING
365 ns: topology_cpu_callback : CPU_DYING
365 ns: update_runtime : CPU_DYING
366 ns: pageset_cpuup_callback : CPU_DYING
367 ns: sysfs_cpu_notify : CPU_DYING
378 ns: flow_cache_cpu : CPU_DYING
380 ns: rcu_cpu_notify : CPU_DYING
381 ns: buffer_cpu_notify : CPU_DYING
381 ns: cpu_callback : CPU_DYING
383 ns: slab_cpuup_callback : CPU_DYING
455 ns: ratelimit_handler : CPU_DYING
502 ns: workqueue_cpu_callback : CPU_DYING
699 ns: percpu_counter_hotcpu_callback: CPU_DYING
1370 ns: rcu_barrier_cpu_hotplug : CPU_DYING
1583 ns: migration_call : CPU_DYING
2971 ns: hrtimer_cpu_notify : CPU_DYING
=========================================================================
Total time for CPU_DYING = .016356000 ms
=========================================================================
=============================================================================
statistics for CPU_DOWN_CANCELED
=============================================================================
=========================================================================
Total time for CPU_DOWN_CANCELED = 0 ms
=========================================================================
=============================================================================
statistics for __stop_machine
=============================================================================
556214 ns: __stop_machine :
=========================================================================
Total time for __stop_machine = .556214000 ms
=========================================================================
=============================================================================
statistics for CPU_DEAD
=============================================================================
352 ns: update_runtime : CPU_DEAD
363 ns: rb_cpu_notify : CPU_DEAD
364 ns: relay_hotcpu_callback : CPU_DEAD
367 ns: hotplug_cfd : CPU_DEAD
396 ns: cpu_callback : CPU_DEAD
411 ns: hotplug_hrtick : CPU_DEAD
426 ns: rcu_barrier_cpu_hotplug : CPU_DEAD
489 ns: remote_softirq_cpu_notify : CPU_DEAD
517 ns: ratelimit_handler : CPU_DEAD
533 ns: workqueue_cpu_callback : CPU_DEAD
626 ns: dev_cpu_callback : CPU_DEAD
867 ns: cpu_numa_callback : CPU_DEAD
1430 ns: rcu_cpu_notify : CPU_DEAD
1827 ns: blk_cpu_notify : CPU_DEAD
1933 ns: buffer_cpu_notify : CPU_DEAD
2194 ns: pageset_cpuup_callback : CPU_DEAD
2613 ns: vmstat_cpuup_callback : CPU_DEAD
2902 ns: radix_tree_callback : CPU_DEAD
4373 ns: hrtimer_cpu_notify : CPU_DEAD
5799 ns: timer_cpu_notify : CPU_DEAD
9468 ns: flow_cache_cpu : CPU_DEAD
12579 ns: cpu_callback : CPU_DEAD
13855 ns: cpu_callback : CPU_DEAD
25095 ns: topology_cpu_callback : CPU_DEAD
29020 ns: page_alloc_cpu_notify : CPU_DEAD
66894 ns: percpu_counter_hotcpu_callback: CPU_DEAD
118473 ns: slab_cpuup_callback : CPU_DEAD
153415 ns: sysfs_cpu_notify : CPU_DEAD
159933 ns: migration_call : CPU_DEAD
11320205 ns: cpuset_track_online_cpus : CPU_DEAD
=========================================================================
Total time for CPU_DEAD = 11.937719000 ms
=========================================================================
=============================================================================
statistics for CPU_POST_DEAD
=============================================================================
332 ns: remote_softirq_cpu_notify : CPU_POST_DEAD
334 ns: hotplug_hrtick : CPU_POST_DEAD
334 ns: hrtimer_cpu_notify : CPU_POST_DEAD
334 ns: radix_tree_callback : CPU_POST_DEAD
334 ns: relay_hotcpu_callback : CPU_POST_DEAD
334 ns: topology_cpu_callback : CPU_POST_DEAD
334 ns: update_runtime : CPU_POST_DEAD
335 ns: buffer_cpu_notify : CPU_POST_DEAD
348 ns: pageset_cpuup_callback : CPU_POST_DEAD
348 ns: slab_cpuup_callback : CPU_POST_DEAD
349 ns: rcu_barrier_cpu_hotplug : CPU_POST_DEAD
350 ns: cpu_callback : CPU_POST_DEAD
350 ns: flow_cache_cpu : CPU_POST_DEAD
350 ns: rb_cpu_notify : CPU_POST_DEAD
350 ns: sysfs_cpu_notify : CPU_POST_DEAD
350 ns: timer_cpu_notify : CPU_POST_DEAD
351 ns: page_alloc_cpu_notify : CPU_POST_DEAD
352 ns: cpuset_track_online_cpus : CPU_POST_DEAD
365 ns: hotplug_cfd : CPU_POST_DEAD
365 ns: vmstat_cpuup_callback : CPU_POST_DEAD
366 ns: cpu_callback : CPU_POST_DEAD
367 ns: cpu_numa_callback : CPU_POST_DEAD
368 ns: cpu_callback : CPU_POST_DEAD
395 ns: blk_cpu_notify : CPU_POST_DEAD
396 ns: rcu_cpu_notify : CPU_POST_DEAD
397 ns: dev_cpu_callback : CPU_POST_DEAD
442 ns: migration_call : CPU_POST_DEAD
563 ns: percpu_counter_hotcpu_callback: CPU_POST_DEAD
778 ns: ratelimit_handler : CPU_POST_DEAD
94184 ns: workqueue_cpu_callback : CPU_POST_DEAD
=========================================================================
Total time for CPU_POST_DEAD = .105155000 ms
=========================================================================
=============================================================================
statistics for CPU_UP_PREPARE
=============================================================================
334 ns: hotplug_hrtick : CPU_UP_PREPARE
336 ns: update_runtime : CPU_UP_PREPARE
350 ns: flow_cache_cpu : CPU_UP_PREPARE
350 ns: radix_tree_callback : CPU_UP_PREPARE
365 ns: cpuset_track_online_cpus : CPU_UP_PREPARE
365 ns: page_alloc_cpu_notify : CPU_UP_PREPARE
365 ns: sysfs_cpu_notify : CPU_UP_PREPARE
367 ns: hrtimer_cpu_notify : CPU_UP_PREPARE
381 ns: buffer_cpu_notify : CPU_UP_PREPARE
381 ns: rb_cpu_notify : CPU_UP_PREPARE
383 ns: cpu_callback : CPU_UP_PREPARE
410 ns: rcu_barrier_cpu_hotplug : CPU_UP_PREPARE
413 ns: remote_softirq_cpu_notify : CPU_UP_PREPARE
426 ns: blk_cpu_notify : CPU_UP_PREPARE
475 ns: vmstat_cpuup_callback : CPU_UP_PREPARE
518 ns: hotplug_cfd : CPU_UP_PREPARE
594 ns: percpu_counter_hotcpu_callback: CPU_UP_PREPARE
731 ns: ratelimit_handler : CPU_UP_PREPARE
805 ns: relay_hotcpu_callback : CPU_UP_PREPARE
1007 ns: dev_cpu_callback : CPU_UP_PREPARE
1690 ns: rcu_cpu_notify : CPU_UP_PREPARE
1875 ns: timer_cpu_notify : CPU_UP_PREPARE
2083 ns: pageset_cpuup_callback : CPU_UP_PREPARE
5016 ns: cpu_numa_callback : CPU_UP_PREPARE
6944 ns: topology_cpu_callback : CPU_UP_PREPARE
7064 ns: slab_cpuup_callback : CPU_UP_PREPARE
20964 ns: cpu_callback : CPU_UP_PREPARE
36301 ns: cpu_callback : CPU_UP_PREPARE
38337 ns: migration_call : CPU_UP_PREPARE
139963 ns: workqueue_cpu_callback : CPU_UP_PREPARE
=========================================================================
Total time for CPU_UP_PREPARE = .269593000 ms
=========================================================================
=============================================================================
statistics for CPU_UP_CANCELED
=============================================================================
=========================================================================
Total time for CPU_UP_CANCELED = 0 ms
=========================================================================
=============================================================================
statistics for __cpu_up
=============================================================================
7881152 ns: __cpu_up :
=========================================================================
Total time for __cpu_up = 7.881152000 ms
=========================================================================
=============================================================================
statistics for CPU_STARTING
=============================================================================
318 ns: cpu_callback : CPU_STARTING
334 ns: hotplug_cfd : CPU_STARTING
334 ns: hotplug_hrtick : CPU_STARTING
334 ns: hrtimer_cpu_notify : CPU_STARTING
336 ns: remote_softirq_cpu_notify : CPU_STARTING
336 ns: topology_cpu_callback : CPU_STARTING
348 ns: cpu_callback : CPU_STARTING
348 ns: flow_cache_cpu : CPU_STARTING
349 ns: cpu_callback : CPU_STARTING
349 ns: update_runtime : CPU_STARTING
350 ns: dev_cpu_callback : CPU_STARTING
350 ns: rb_cpu_notify : CPU_STARTING
351 ns: sysfs_cpu_notify : CPU_STARTING
352 ns: cpuset_track_online_cpus : CPU_STARTING
365 ns: vmstat_cpuup_callback : CPU_STARTING
381 ns: blk_cpu_notify : CPU_STARTING
393 ns: page_alloc_cpu_notify : CPU_STARTING
395 ns: timer_cpu_notify : CPU_STARTING
396 ns: relay_hotcpu_callback : CPU_STARTING
396 ns: slab_cpuup_callback : CPU_STARTING
397 ns: cpu_numa_callback : CPU_STARTING
397 ns: pageset_cpuup_callback : CPU_STARTING
397 ns: radix_tree_callback : CPU_STARTING
410 ns: buffer_cpu_notify : CPU_STARTING
410 ns: rcu_cpu_notify : CPU_STARTING
412 ns: rcu_barrier_cpu_hotplug : CPU_STARTING
426 ns: percpu_counter_hotcpu_callback: CPU_STARTING
549 ns: ratelimit_handler : CPU_STARTING
549 ns: workqueue_cpu_callback : CPU_STARTING
592 ns: migration_call : CPU_STARTING
=========================================================================
Total time for CPU_STARTING = .011654000 ms
=========================================================================
=============================================================================
statistics for CPU_ONLINE
=============================================================================
334 ns: hotplug_cfd : CPU_ONLINE
334 ns: relay_hotcpu_callback : CPU_ONLINE
334 ns: remote_softirq_cpu_notify : CPU_ONLINE
335 ns: hrtimer_cpu_notify : CPU_ONLINE
349 ns: topology_cpu_callback : CPU_ONLINE
352 ns: flow_cache_cpu : CPU_ONLINE
352 ns: slab_cpuup_callback : CPU_ONLINE
365 ns: dev_cpu_callback : CPU_ONLINE
365 ns: rb_cpu_notify : CPU_ONLINE
379 ns: pageset_cpuup_callback : CPU_ONLINE
381 ns: page_alloc_cpu_notify : CPU_ONLINE
381 ns: rcu_cpu_notify : CPU_ONLINE
381 ns: timer_cpu_notify : CPU_ONLINE
395 ns: hotplug_hrtick : CPU_ONLINE
410 ns: blk_cpu_notify : CPU_ONLINE
426 ns: rcu_barrier_cpu_hotplug : CPU_ONLINE
455 ns: cpu_numa_callback : CPU_ONLINE
459 ns: radix_tree_callback : CPU_ONLINE
473 ns: buffer_cpu_notify : CPU_ONLINE
504 ns: ratelimit_handler : CPU_ONLINE
639 ns: percpu_counter_hotcpu_callback: CPU_ONLINE
791 ns: update_runtime : CPU_ONLINE
1052 ns: cpu_callback : CPU_ONLINE
1282 ns: cpu_callback : CPU_ONLINE
1845 ns: cpu_callback : CPU_ONLINE
2502 ns: vmstat_cpuup_callback : CPU_ONLINE
4332 ns: migration_call : CPU_ONLINE
14505 ns: workqueue_cpu_callback : CPU_ONLINE
54588 ns: sysfs_cpu_notify : CPU_ONLINE
12767882 ns: cpuset_track_online_cpus : CPU_ONLINE
=========================================================================
Total time for CPU_ONLINE = 12.857182000 ms
=========================================================================


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