On Thu, Jun 27, 2019 at 11:10:08PM +0200, Thomas Gleixner wrote: > On Thu, 27 Jun 2019, Peter Xu wrote: > > + * @TIMER_PINNED: A pinned timer will not be affected by any timer > > + * placement heuristics (like, NOHZ) and will always be run on the CPU > > + * when the timer was enqueued. > > s/when/on which/
Fixed. > > > + * > > + * Note: Because enqueuing of timers can actually migrate the timer > > + * from one CPU to another, pinned timers are not guaranteed to stay > > + * on the initialy selected CPU. They move to the CPU on which the > > + * enqueue function is invoked via mod_timer() or add_timer(). If the > > + * timer should be placed on a particular CPU, then add_timer_on() has > > + * to be used. It is also suggested that the user should always use > > + * add_timer_on() explicitly for pinned timers. > > That last sentence is not correct. add_timer_on() has limitations over > mod_timer(). As pinned prevents the timer from being queued on a remote CPU > mod timer is perfectly fine for many cases. > > add_timer_on() is really about queueing a timer on a dedicated CPU, which > is often enough a remote CPU. Frankly speaking I still think add_timer_on() is preferred here because mod_timer() users will really need to be careful to make sure they'll pin the timers correctly all the time, and I assume that's why we've tried to find all the TIMER_PINNED users and tried to make sure there's nothing wrong on using them during previous discussion (and more than half of them do use add_timer_on() which seems to be good). In all cases, I'll take your suggestion to drop the last sentence. Thanks for reviewing this document patch. I'll repost. -- Peter Xu
