On Mon, Jun 24, 2024 at 09:31:51AM +0200, Vlastimil Babka wrote: > Hi, > > you've included tglx, which is great, but there's also LOCKING PRIMITIVES > section in MAINTAINERS so I've added folks from there in my reply. > Link to full series: > https://lore.kernel.org/all/20240622035815.569665-1-leobras@xxxxxxxxxx/ Thanks Vlastimil! > > On 6/22/24 5:58 AM, Leonardo Bras wrote: > > 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. > > I've also noticed this a while ago (likely in the context of rewriting SLUB > to use local_lock) and asked about it on IRC, and IIRC tglx wasn't fond of > the idea. But I forgot the details about why, so I'll let the the locking > experts reply... > > > 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 >
