On Mon, Apr 15, 2019 at 05:12:13PM -0300, Marcelo Tosatti wrote: > For isolated CPUs, we'd like to skip awakening ktimersoftd > (the switch to and then back from ktimersoftd takes 10us in > virtualized environments, in addition to other OS overhead, > which exceeds telco requirements for packet forwarding for > 5G) from the sched tick. > > The patch "timers: do not raise softirq unconditionally" from Thomas > attempts to address that by checking, in the sched tick, whether its > necessary to raise the timer softirq. Unfortunately, it attempts to grab > the tvec base spinlock which generates the issue described in the patch > "Revert "timers: do not raise softirq unconditionally"". > > tvec_base->lock protects addition of timers to the wheel versus > timer interrupt execution. > > This patch does not grab the tvec base spinlock from irq context, > but rather performs a lockless access to base->pending_map. > > It handles the the race between timer addition and timer interrupt > execution by unconditionally (in case of isolated CPUs) raising the > timer softirq after making sure the updated bitmap is visible > on remote CPUs. > > This patchset reduces cyclictest latency from 25us to 14us > on my testbox. Patch against v5.0.5-rt3 branch of linux-rt-devel.
