Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> writes: > Some effective upper bounds for the number of vcpu ids observable in a process: > > - sysconf(3) _SC_NPROCESSORS_CONF, > - the number of threads which exist concurrently in the process, > - the number of cpus in the cpu affinity mask applied by sched_setaffinity, > except in corner-case situations such as cpu hotplug removing all cpus from > the affinity set, > - cgroup cpuset "partition" limits, > > Note that AFAIR non-partition cgroup cpusets allow a cgroup to "borrow" > additional cores from the rest of the system if they are idle, therefore > allowing the number of concurrent threads to go beyond the specified limit. > > AFAIR the sched affinity mask is tweaked independently of the cgroup cpuset. > Those are two mechanisms both affecting the scheduler task placement. > > I would expect the user-space code to use some sensible upper bound as a > hint about how many per-vcpu data structure elements to expect (and how many > to pre-allocate), but have a "lazy initialization" fall-back in case the > vcpu id goes up to the number of configured processors - 1. And I suspect > that even the number of configured processors may change with CRIU. > > If the above explanation makes sense (please let me know if I am wrong > or missed something), I suspect I should add it to the commit message. That helps, thanks. I do think that something like this belongs in the changelog - or, even better, in the upcoming restartable-sequences section in the userspace-api documentation :) Thanks, jon
