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a=lCpzRmAYbLLaTzLvsPZ7Mbvzbb8=:19 a=xqWC_Br6kY4A:10 a=IkcTkHD0fZMA:10 a=FelO9ux0wxsA:10 a=GoEa3M9JfhUA:10 a=VkNPw1HP01LnGYTKEx00:22 a=jiCTI4zE5U7BLdzWsZGv:22 a=BqU2WV_vvsyTyxaotp0D:22 a=3ws6EuzwyXv_bdi9qswA:9 a=QEXdDO2ut3YA:10 a=WmVTiCyuxqgg3mnwYu6p:22 X-Proofpoint-ORIG-GUID: _Dayw2J-G755dFEotP6NX-zTUGoMlkhe X-Proofpoint-GUID: _Dayw2J-G755dFEotP6NX-zTUGoMlkhe X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwNjIzMDA2NiBTYWx0ZWRfX/kwBLbAQaoEe V5XWvjsMC6dhdWuSQrQoUwVujbEaQwF5tpmkYSVSYUX7tk0EInSVLwJDY0Ft3D9FeUkLks/pn/N 6PrtU0aue0inS2hMma9hCINXbih68mhgQWL9qyWAwvxAU1blEGRqtjEpuzLbBQtUeVSd1o0bZhH zRjJccy5UAjpGQfiOEMe/qqnffYnuhtcidEE27w1zhpvgxQFqiRBoMvB8dvDaQdDLX1t6RavCjO hwFAlKPg9Ethc3wWI4AnyPm16KD6tviP4eLUjlIEzr3kIyTs5pUAaTGGsu5FgByUFGCUqtnPqqE xCgcMZVPYUhP4h1iGkSrMfincsI347xkW8gzmaHR2WEl4ClQYSeFfxSmIkzJXn6yYBJ0DKPCexh SFlQDQrJU5HEkcQrbEphljnsPgpryNV/MN8khjll8zsWrnUgrzSA4ebFKaDS8wqKxSlBITsr7OW RDl/8HkFsdV13lvMwBQ== X-Proofpoint-Spam-Info: AW1haW4tMjYwNjIzMDA2NiBTYWx0ZWRfX0f0EQA6+RyHb mduweRO5TY89y0v6aa0w7vATFK8TeKI1SPw+8UzmKLNNekdu0kDMC4dy6AVUwMT3cTuW+Nt89aZ 3yiKyUKZokYXs86SF5HIUYfcdiHDakGgk5c8/1SRXpCDwbxWvW4T On 19-06-2026 21:50, Thomas Gleixner wrote: > On Mon, Jun 08 2026 at 19:22, Partha Satapathy wrote: >> RDS already checks cpu_online() before calling add_timer_on(). >> However, that still races with CPU hotplug and can leave a timer >> queued on a CPU which goes offline before the enqueue completes. > I can't find any invocation of add_timer_on() in the RDS code and the > commit you mentioned does not exist and there is no trace of it on > lore. But Gemini told me it's Oracle private hackery. So what the heck > are you telling me about RDS checking something and having a race > condition? > >> The current workaround is to serialize the operation with >> cpus_read_lock(), but doing so in the networking path adds overhead >> to every timer enqueue and increases CPU hotplug latency. > I buy the cpus_read_lock() overhead, but who cares about hotplug latency? > > Aside of that if you just want to temporary prevent a CPU from vanishing > while queueing the timer, then preempt_disable() does the trick too and > that's definitely more lightweight than cpus_read_lock(). > >>> On Thu, Apr 23 2026 at 09:19, Partha Satapathy wrote: >>>> /** >>>> * __timer_delete - Internal function: Deactivate a timer >>>> * @timer: The timer to be deactivated >>>> @@ -2507,6 +2543,7 @@ int timers_prepare_cpu(unsigned int cpu) >>>> base->next_expiry_recalc = false; >>>> base->timers_pending = false; >>>> base->is_idle = false; >>>> + base->is_active = true; >>> That's just wrong. The base is active only when the CPU reaches online state. >> Setting base->is_active = true in timers_prepare_cpu() was >> intentional as a small optimization to avoid an additional hotplug >> callback. Timers queued in this window will be serviced as the CPU >> completes bring-up. > It's still intentionally wrong. > >> However, to keep the state strictly aligned with CPU online state, I >> can move this activation logic to a dedicated callback under >> CPUHP_AP_ONLINE_DYN instead if you prefer. > What's wrong with checking cpu_online() under the CPU's base lock? When > the lock is held the CPU can't go away. > >> The RDS use case treats the target CPU primarily as a locality hint >> rather than a strict execution requirement. >> >> Given your outline above, this may be better addressed by a generic >> placement-hint interface than by strict CPU placement semantics. > Well, a placement hint interface still needs to ensure that the target > is online and does not vanish before the timer or work has been queued, > which means the whole operation needs to be protected against hotplug no > matter what. So you end up with something like this: > > scoped_guard(preempt) { > if (!cpu_online(cpu)) > cpu = pick_online_cpu(); > add_timer_on(.....); > } > > The simplest variant for pick_online_cpu() is obviously > cpumask_first(onlinemask), but you can go wild there and find the > nearest one. > > But making this a generic thing is not necessarily a good idea because > pick_online_cpu() could turn out to be expensive and you might want to > cache the result once your preferred CPU vanished. > > Alternatively you implement a hotplug callback for RDS which changes the > preferred CPU before the CPU goes offline. That's cheap and a one time > effort and you don't have to do anything on the timer/workqueue > side. There you can even justify to search for the nearest neighbor or > whatever preference you have. > > Thanks, > > tglx > > Thanks for the detailed explanation. I'll drop this timer core change and work on the RDS-side fix instead, most likely by updating the preferred CPU from a CPU hotplug callback. Sorry for the noise. Thanks, Partha