The problem: Some places in the kernel implement a parallel programming strategy consisting on local_locks() for most of the work, and some rare remote operations are scheduled on target cpu. This keeps cache bouncing low since cacheline tends to be mostly local, and avoids the cost of locks in non-RT kernels, even though the very few remote operations will be expensive due to scheduling overhead. On the other hand, for RT workloads this can represent a problem: getting an important workload scheduled out to deal with remote requests is sure to introduce unexpected deadline misses. The idea: Currently with PREEMPT_RT=y, local_locks() become per-cpu spinlocks. In this case, instead of scheduling work on a remote cpu, it should be safe to grab that remote cpu's per-cpu spinlock and run the required work locally. Tha major cost, which is un/locking in every local function, already happens in PREEMPT_RT. Also, there is no need to worry about extra cache bouncing: The cacheline invalidation already happens due to schedule_work_on(). This will avoid schedule_work_on(), and thus avoid scheduling-out an RT workload. For patches 2, 3 & 4, I noticed just grabing the lock and executing the function locally is much faster than just scheduling it on a remote cpu. Proposed solution: A new interface called Queue PerCPU Work (QPW), which should replace Work Queue in the above mentioned use case. If PREEMPT_RT=n, this interfaces just wraps the current local_locks + WorkQueue behavior, so no expected change in runtime. If PREEMPT_RT=y, queue_percpu_work_on(cpu,...) will lock that cpu's per-cpu structure and perform work on it locally. This is possible because on functions that can be used for performing remote work on remote per-cpu structures, the local_lock (which is already a this_cpu spinlock()), will be replaced by a qpw_spinlock(), which is able to get the per_cpu spinlock() for the cpu passed as parameter. Patch 1 implements QPW interface, and patches 2, 3 & 4 replaces the current local_lock + WorkQueue interface by the QPW interface in swap, memcontrol & slub interface. Please let me know what you think on that, and please suggest improvements. Thanks a lot! Leo Leonardo Bras (4): Introducing qpw_lock() and per-cpu queue & flush work swap: apply new queue_percpu_work_on() interface memcontrol: apply new queue_percpu_work_on() interface slub: apply new queue_percpu_work_on() interface include/linux/qpw.h | 88 +++++++++++++++++++++++++++++++++++++++++++++ mm/memcontrol.c | 20 ++++++----- mm/slub.c | 26 ++++++++------ mm/swap.c | 26 +++++++------- 4 files changed, 127 insertions(+), 33 deletions(-) create mode 100644 include/linux/qpw.h base-commit: 50736169ecc8387247fe6a00932852ce7b057083 -- 2.45.2
