On Thu, Apr 06, 2017 at 11:46:33AM +0200, Sebastian Andrzej Siewior wrote: > On 2017-04-06 11:32:24 [+0200], Peter Zijlstra wrote: > > On Tue, Apr 04, 2017 at 08:42:02PM +0200, Sebastian Andrzej Siewior wrote: > > > While converting the existing users I tried to stick with the rules > > > above however… well mostly CPUFREQ tries to temporary switch the CPU > > > mask to do something on a certain CPU and then switches the mask back it > > > its original value. > > > > > > There's a bunch of that through ancient and rotten parts of the kernel. > > All those sites are broken. > > > > Nothing stops userspace from setting a different affinity right after > > the kernel does for those threads. > > Good. So you are saying I should convert them to something like > queue_work_on()?
Not sure; iirc there were a few variants. Some can indeed simply do queue_work_on() and possibly wait for completion. some should maybe be a per-cpu kthread, others will be more 'interesting'. IIRC MIPS has a case where only 1 in N cores has an FPU. And once a task uses FPU, it gets affined to the core that has one or something like that. Of course, nothing then stops someone else breaking that affinity. But I suspect it will simply fault on the next FPU instruction and 'reset' the mask or something. I've no clue and no real desire to know.