On Wed, Dec 14, 2005 at 12:40:37AM -0800, Paul Jackson wrote:
> Optimize the cpuset impact on page allocation, the most
> performance critical cpuset hook in the kernel.
>
> On each page allocation, the cpuset hook needs to check for a
> possible change in the current tasks cpuset. It can now handle
> the common case, of no change, without taking any spinlock or
> semaphore, thanks to RCU.
>
> Convert a spinlock on the current task to an rcu_read_lock(),
> saving approximately a memory barrier and an atomic op, depending
> on architecture.
>
> This is done by adding rcu_assign_pointer() and synchronize_rcu()
> calls to the write side of the task->cpuset pointer, in
> cpuset.c:attach_task(), to delay freeing up a detached cpuset
> until after any critical sections referencing that pointer.
>
> Thanks to Andi Kleen, Nick Piggin and Eric Dumazet for ideas.
Looks good to me from an RCU perspective!
Moving from synchronize_rcu() to call_rcu() would be tricky, since
the check_for_release() function can block in kmalloc(). If updates
become a bottleneck, one approach would be to invoke work queues
from within the RCU callback.
Thanx, Paul
Acked-by: <[email protected]>
> Signed-off-by: Paul Jackson <[email protected]>
>
> ---
>
> kernel/cpuset.c | 40 ++++++++++++++++++++++++++++++----------
> 1 files changed, 30 insertions(+), 10 deletions(-)
>
> --- 2.6.15-rc3-mm1.orig/kernel/cpuset.c 2005-12-13 16:49:01.767509666 -0800
> +++ 2.6.15-rc3-mm1/kernel/cpuset.c 2005-12-13 17:19:37.989982316 -0800
> @@ -39,6 +39,7 @@
> #include <linux/namei.h>
> #include <linux/pagemap.h>
> #include <linux/proc_fs.h>
> +#include <linux/rcupdate.h>
> #include <linux/sched.h>
> #include <linux/seq_file.h>
> #include <linux/slab.h>
> @@ -248,6 +249,11 @@ static struct super_block *cpuset_sb;
> * a tasks cpuset pointer we use task_lock(), which acts on a spinlock
> * (task->alloc_lock) already in the task_struct routinely used for
> * such matters.
> + *
> + * P.S. One more locking exception. RCU is used to guard the
> + * update of a tasks cpuset pointer by attach_task() and the
> + * access of task->cpuset->mems_generation via that pointer in
> + * the routine cpuset_update_task_memory_state().
> */
>
> static DECLARE_MUTEX(manage_sem);
> @@ -610,12 +616,24 @@ static void guarantee_online_mems(const
> * cpuset pointer. This routine also might acquire callback_sem and
> * current->mm->mmap_sem during call.
> *
> - * The task_lock() is required to dereference current->cpuset safely.
> - * Without it, we could pick up the pointer value of current->cpuset
> - * in one instruction, and then attach_task could give us a different
> - * cpuset, and then the cpuset we had could be removed and freed,
> - * and then on our next instruction, we could dereference a no longer
> - * valid cpuset pointer to get its mems_generation field.
> + * Reading current->cpuset->mems_generation doesn't need task_lock
> + * to guard the current->cpuset derefence, because it is guarded
> + * from concurrent freeing of current->cpuset by attach_task(),
> + * using RCU.
> + *
> + * The rcu_dereference() is technically probably not needed,
> + * as I don't actually mind if I see a new cpuset pointer but
> + * an old value of mems_generation. However this really only
> + * matters on alpha systems using cpusets heavily. If I dropped
> + * that rcu_dereference(), it would save them a memory barrier.
> + * For all other arch's, rcu_dereference is a no-op anyway, and for
> + * alpha systems not using cpusets, another planned optimization,
> + * avoiding the rcu critical section for tasks in the root cpuset
> + * which is statically allocated, so can't vanish, will make this
> + * irrelevant. Better to use RCU as intended, than to engage in
> + * some cute trick to save a memory barrier that is impossible to
> + * test, for alpha systems using cpusets heavily, which might not
> + * even exist.
> *
> * This routine is needed to update the per-task mems_allowed data,
> * within the tasks context, when it is trying to allocate memory
> @@ -627,11 +645,12 @@ void cpuset_update_task_memory_state()
> {
> int my_cpusets_mem_gen;
> struct task_struct *tsk = current;
> - struct cpuset *cs = tsk->cpuset;
> + struct cpuset *cs;
>
> - task_lock(tsk);
> + rcu_read_lock();
> + cs = rcu_dereference(tsk->cpuset);
> my_cpusets_mem_gen = cs->mems_generation;
> - task_unlock(tsk);
> + rcu_read_unlock();
>
> if (my_cpusets_mem_gen != tsk->cpuset_mems_generation) {
> down(&callback_sem);
> @@ -1131,7 +1150,7 @@ static int attach_task(struct cpuset *cs
> return -ESRCH;
> }
> atomic_inc(&cs->count);
> - tsk->cpuset = cs;
> + rcu_assign_pointer(tsk->cpuset, cs);
> task_unlock(tsk);
>
> guarantee_online_cpus(cs, &cpus);
> @@ -1151,6 +1170,7 @@ static int attach_task(struct cpuset *cs
> if (is_memory_migrate(cs))
> do_migrate_pages(tsk->mm, &from, &to, MPOL_MF_MOVE_ALL);
> put_task_struct(tsk);
> + synchronize_rcu();
> if (atomic_dec_and_test(&oldcs->count))
> check_for_release(oldcs, ppathbuf);
> return 0;
>
> --
> I won't rest till it's the best ...
> Programmer, Linux Scalability
> Paul Jackson <[email protected]> 1.650.933.1373
> -
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