[PATCH 0/2] timer: Migrate running timers

[Viresh Kumar]

Hi Ingo/Thomas/Peter,

While queuing a timer, we try to migrate it to a non-idle core if the local core
is idle, but we don't try that if the timer is re-armed from its handler.

There were few unsolved problems due to which it was avoided until now. But
there are cases where solving these problems can be useful. When the timer is
always re-armed from its handler, it never migrates to other cores. And many a
times, it ends up waking an idle core to just service the timer, which could
have been handled by a non-idle core.

Peter suggested [1] few changes which can make that work and the first patch
does exactly that. The second one is a minor improvement, that replaces
'running_timer' pointer with 'busy'. That variable was required as part of a
sanity check during CPU hot-unplug operation. I was not sure if we should drop
this extra variable ('running_timer' or 'busy') and the sanity check.

Because we are using another bit from base pointer to keep track of running
status of timer, we get a warning on blackfin, as it doesn't respect
____cacheline_aligned [2].

kernel/time/timer.c: In function 'init_timers':
kernel/time/timer.c:1731:2: error: call to '__compiletime_assert_1731' declared
	with attribute error: BUILD_BUG_ON failed: __alignof__(struct tvec_base)
	& TIMER_FLAG_MASK

--
viresh

[1] https://lkml.org/lkml/2015/3/28/32
[2] https://lkml.org/lkml/2015/3/29/178

Cc: Steven Miao <realmz6@xxxxxxxxx>

Viresh Kumar (2):
  timer: Avoid waking up an idle-core by migrate running timer
  timer: Replace base-> 'running_timer' with 'busy'

 include/linux/timer.h |   3 +-
 kernel/time/timer.c   | 102 ++++++++++++++++++++++++++++++++++++++------------
 2 files changed, 81 insertions(+), 24 deletions(-)

-- 
2.3.0.rc0.44.ga94655d

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