When a deferrable work (INIT_DEFERRABLE_WORK, etc.) is queued via
queue_delayed_work() it's probably intended to run the work item on any
CPU that isn't idle. However, we queue the work to run at a later time
by starting a deferrable timer that binds to whatever CPU the work is
queued on which is same with queue_delayed_work_on(smp_processor_id())
effectively.
As a result WORK_CPU_UNBOUND work items aren't really cpu unbound now.
In fact this is perfectly fine with UP kernel and also won't affect much a
system without dyntick with SMP kernel too as every cpus run timers
periodically. But on SMP systems with dyntick current implementation leads
deferrable timers not very scalable because the timer's base which has
queued the deferrable timer won't wake up till next non-deferrable timer
expires even though there are possible other non idle cpus are running
which are able to run expired deferrable timers.
The deferrable work is a good example of the current implementation's
victim like below.
INIT_DEFERRABLE_WORK(&dwork, fn);
CPU 0 CPU 1
queue_delayed_work(wq, &dwork, HZ);
queue_delayed_work_on(WORK_CPU_UNBOUND);
...
__mod_timer() -> queues timer to the
current cpu's timer
base.
...
tick_nohz_idle_enter() -> cpu enters idle.
A second later
cpu 0 is now in idle. cpu 1 exits idle or wasn't in idle so
now it's in active but won't
cpu 0 won't wake up till next handle cpu unbound deferrable timer
non-deferrable timer expires. as it's in cpu 0's timer base.
To make all cpu unbound deferrable timers are scalable, introduce a common
timer base which is only for cpu unbound deferrable timers to make those
are indeed cpu unbound so that can be scheduled by tick_do_timer_cpu.
This common timer fixes scalability issue of delayed work and all other cpu
unbound deferrable timer using implementations.
Change-Id: I8b6c57d8b6445a76fa02a8cb598a8ef22aef7200
CC: Thomas Gleixner <tglx@linutronix.de>
CC: John Stultz <john.stultz@linaro.org>
CC: Tejun Heo <tj@kernel.org>
[joonwoop@codeaurora.org: timer->base replaced with CPU index so get
the deferrable timer wheel from lock_timer_base() instead of
do_init_timer().]
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>
646bf5125d