On Thu, 28 Apr 2022 11:15:27 +0200, Vincent Ray said: > Then I guess it could be preempted at any time, especially with aggressive versions of preemptions ? > And if so, are we not at risk that our thread is migrated to an other CPU just after smp_processor_id returned ? Often, we don't actually *care* if it gets migrated. A *lot* of uses of smp_processor_id() are just to make statistic gathering more efficient. Rather than all the CPUs do all sorts of locking to avoid race conditions between different threads updating the same variable, and avoid massive cache line ping-ponging, the code just gets a pointer to a per_cpu area where we update some statistic or counter without worrying about that stuff because we know no other processor should be updating *this* processor's per_cpu area. Then anything that cares about the *total* across all CPUs can iterate across all the per_cpu numbers - and that (a) usually happens much less frequently than updates and (b) only needs to read all the per_cpu area without updating them. Yes, there's a very small chance that the "wrong" CPU's stats will be updated. But let's be realistic - for a lot of statistics, you'll never notice. _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
