Messages in this thread | | | From | "Rafael J. Wysocki" <> | Subject | Re: [patch 00/10] timer: Move from a push remote at enqueue to a pull at expiry model | Date | Tue, 18 Apr 2017 15:57:20 +0200 |
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On Monday, April 17, 2017 08:32:41 PM Thomas Gleixner wrote: > Placing timers at enqueue time on a target CPU based on dubious heuristics > does not make any sense: > > 1) Most timer wheel timers are canceled or rearmed before they expire. > > 2) The heuristics to predict which CPU will be busy when the timer expires > are wrong by definition. > > So we waste precious cycles to place timers at enqueue time. > > The proper solution to this problem is to always queue the timers on the > local CPU and allow the non pinned timers to be pulled onto a busy CPU at > expiry time. > > To achieve this the timer storage has been split into local pinned and > global timers. Local pinned timers are always expired on the CPU on which > they have been queued. Global timers can be expired on any CPU. > > As long as a CPU is busy it expires both local and global timers. When a > CPU goes idle it arms for the first expiring local timer. If the first > expiring pinned (local) timer is before the first expiring movable timer, > then no action is required because the CPU will wake up before the first > movable timer expires. If the first expiring movable timer is before the > first expiring pinned (local) timer, then this timer is queued into a idle > timerqueue and eventually expired by some other active CPU. > > To avoid global locking the timerqueues are implemented as a hierarchy. The > lowest level of the hierarchy holds the CPUs. The CPUs are associated to > groups of 8, which are seperated per node. If more than one CPU group > exist, then a second level in the hierarchy collects the groups. Depending > on the size of the system more than 2 levels are required. Each group has a > "migrator" which checks the timerqueue during the tick for remote expirable > timers. > > If the last CPU in a group goes idle it reports the first expiring event in > the group up to the next group(s) in the hierarchy. If the last CPU goes > idle it arms its timer for the first system wide expiring timer to ensure > that no timer event is missed. > > The series is also available from git: > > git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git WIP.timers
No concerns from me FWIW.
Thanks, Rafael
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