Re: [PATCH v9 19/32] timers: add_timer_on(): Make sure TIMER_PINNED flag is set

[Anna-Maria Behnsen]

Sebastian Siewior <bigeasy@xxxxxxxxxxxxx> writes:

> On 2023-12-06 10:57:57 [+0100], Anna-Maria Behnsen wrote:
>> Sebastian Siewior <bigeasy@xxxxxxxxxxxxx> writes:
>> 
>> > On 2023-12-01 10:26:41 [+0100], Anna-Maria Behnsen wrote:
>> >> When adding a timer to the timer wheel using add_timer_on(), it is an
>> >> implicitly pinned timer. With the timer pull at expiry time model in place,
>> >> TIMER_PINNED flag is required to make sure timers end up in proper base.
>> >> 
>> >> Add TIMER_PINNED flag unconditionally when add_timer_on() is executed.
>> >
>> > This is odd. I have some vague memory that this was already the case.
>> > Otherwise all add_timer_on() users without TIMER_PINNED may get it wrong
>> > due to optimisation.
>> 
>> Which optimisation are you talking about? Are you talking about the
>> heuristic for finding the best CPU in get_target_base()? This heuristic
>> is not used for add_timer_on().
>
> Yes, true. And nobody probably mixes add_timer_on() and mod_timer().

Workqueue mixes add_timer_on() and add_timer(). But therefore the
add_timer_global() function is introduced in patch 17 which drops the
TIMER_PINNED flag. In patch 18 the add_timer() in workqueue code is
replaced by the new function.

Thanks,

        Anna-Maria