Re: [PATCH V2] clocksource: register cpu notifier to remove timer from dying CPU

[Viresh Kumar]

On 7 May 2014 15:38, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> On Wed, 9 Apr 2014, Viresh Kumar wrote:
>> This patch tries to fix this by registering cpu notifiers from clocksource core,
>> only when we start clocksource-watchdog. And if on the CPU_DEAD notification it
>> is found that dying CPU was the CPU on which this timer is queued on, then it is
>> removed from that CPU and queued to next CPU.
>
> Gah, no. We realy don't want more notifier crap.

Agreed, and could have used the generic ones, probably.

> It's perfectly fine for the watchdog timer to be moved around on cpu
> down.

Functionally? Yes. Then handler doesn't have any CPU specific stuff to
do here and so queuing it on any cpu is fine.

> And the timer itself is not pinned at all. add_timer_on() does
> not set the pinned bit.

The perception I had is this:
- mod_timer() is a more complicated form of add_timer() as it has to
tackle with migration and removal of timers as well. Otherwise they
should work in similar way.
- There is no PINNED bit which can be set, its just a parameter to
__mod_timer() to decide which CPU the timer should fire on.
- And by the 'name add_timer_on()', we must guarantee that timer
fires on the CPU its being added to, otherwise it may break things
for many. There might be users which want to run the handler
on a particular CPU due to some CPU-specific stuff they want to do.
And have used add_timer_on()...

But looking at your reply, it looks that add_timer_on() doesn't
guarantee that timer would fire on the CPU mentioned? Is that the
case for mod_timer_pinned() as well ?

And if that's the case what do we want should we do with these
timers (i.e. ones added with add_timer_on or mod_timer_pinned)
when we try to quiesce a cpu using cpuset.quiesce [1]?

--
viresh

[1] https://lkml.org/lkml/2014/4/4/99
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