On Mon, 2019-06-10 at 06:32:29 UTC, Anju T Sudhakar wrote: > Nest and core imc(In-memory Collection counters) assigns a particular > cpu as the designated target for counter data collection. > During system boot, the first online cpu in a chip gets assigned as > the designated cpu for that chip(for nest-imc) and the first online cpu > in a core gets assigned as the designated cpu for that core(for core-imc). > > If the designated cpu goes offline, the next online cpu from the same > chip(for nest-imc)/core(for core-imc) is assigned as the next target, > and the event context is migrated to the target cpu. > Currently, cpumask_any_but() function is used to find the target cpu. > Though this function is expected to return a `random` cpu, this always > returns the next online cpu. > > If all cpus in a chip/core is offlined in a sequential manner, starting > from the first cpu, the event migration has to happen for all the cpus > which goes offline. Since the migration process involves a grace period, > the total time taken to offline all the cpus will be significantly high. > > Example: > In a system which has 2 sockets, with > NUMA node0 CPU(s): 0-87 > NUMA node8 CPU(s): 88-175 > > Time taken to offline cpu 88-175: > real 2m56.099s > user 0m0.191s > sys 0m0.000s > > Use cpumask_last() to choose the target cpu, when the designated cpu > goes online, so the migration will happen only when the last_cpu in the > mask goes offline. This way the time taken to offline all cpus in a > chip/core can be reduced. > > With the patch, > > Time taken to offline cpu 88-175: > real 0m12.207s > user 0m0.171s > sys 0m0.000s > > > Offlining all cpus in reverse order is also taken care because, > cpumask_any_but() is used to find the designated cpu if the last cpu in > the mask goes offline. Since cpumask_any_but() always return the first > cpu in the mask, that becomes the designated cpu and migration will happen > only when the first_cpu in the mask goes offline. > > Example: > With the patch, > > Time taken to offline cpu from 175-88: > real 0m9.330s > user 0m0.110s > sys 0m0.000s > > Signed-off-by: Anju T Sudhakar <a...@linux.vnet.ibm.com> > Reviewed-by: Madhavan Srinivasan <ma...@linux.vnet.ibm.com>
Applied to powerpc next, thanks. https://git.kernel.org/powerpc/c/9c9f8fb71feed144973a70455e0a4ee3da57ed2a cheers
