From: Paul Jackson For systems that aren't using cpusets, but have them CONFIG_CPUSET enabled in their kernel (eventually this may be most distribution kernels), this patch removes even the minimal rcu_read_lock() from the memory page allocation path. Actually, it removes that rcu call for any task that is in the root cpuset (top_cpuset), which on systems not actively using cpusets, is all tasks. We don't need the rcu check for tasks in the top_cpuset, because the top_cpuset is statically allocated, so at no risk of being freed out from underneath us. Signed-off-by: Paul Jackson Signed-off-by: Andrew Morton --- kernel/cpuset.c | 13 +++++++++---- 1 files changed, 9 insertions(+), 4 deletions(-) diff -puN kernel/cpuset.c~cpuset-skip-rcu-check-if-task-is-in-root-cpuset kernel/cpuset.c --- devel/kernel/cpuset.c~cpuset-skip-rcu-check-if-task-is-in-root-cpuset 2005-12-14 02:48:44.000000000 -0800 +++ devel-akpm/kernel/cpuset.c 2005-12-14 02:48:44.000000000 -0800 @@ -647,10 +647,15 @@ void cpuset_update_task_memory_state() struct task_struct *tsk = current; struct cpuset *cs; - rcu_read_lock(); - cs = rcu_dereference(tsk->cpuset); - my_cpusets_mem_gen = cs->mems_generation; - rcu_read_unlock(); + if (tsk->cpuset == &top_cpuset) { + /* Don't need rcu for top_cpuset. It's never freed. */ + my_cpusets_mem_gen = top_cpuset.mems_generation; + } else { + rcu_read_lock(); + cs = rcu_dereference(tsk->cpuset); + my_cpusets_mem_gen = cs->mems_generation; + rcu_read_unlock(); + } if (my_cpusets_mem_gen != tsk->cpuset_mems_generation) { down(&callback_sem); _