On Mon, Jun 24, 2024 at 11:57:57PM -0300, Leonardo Bras wrote: > On Mon, Jun 24, 2024 at 03:54:14PM -0700, Boqun Feng wrote: > > 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. > > > > Thanks! > > > > > Link to full series: > > > https://lore.kernel.org/all/20240622035815.569665-1-leobras@xxxxxxxxxx/ > > > > > > > And apologies to Leonardo... I think this is a follow-up of: > > > > https://lpc.events/event/17/contributions/1484/ > > > > and I did remember we had a quick chat after that which I suggested it's > > better to change to a different name, sorry that I never found time to > > write a proper rely to your previous seriese [1] as promised. > > > > [1]: https://lore.kernel.org/lkml/20230729083737.38699-2-leobras@xxxxxxxxxx/ > > That's correct, I commented about this in the end of above presentation. > Don't worry, and thanks for suggesting the per-cpu naming, it was very > helpful on designing this solution. > > > > > > 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... > > > > > > > I think it's a good idea, especially the new name is less confusing ;-) > > So I wonder Thomas' thoughts as well. > > Thanks! > > > > > And I think a few (micro-)benchmark numbers will help. > > Last year I got some numbers on how replacing local_locks with > spinlocks would impact memcontrol.c cache operations: > > https://lore.kernel.org/all/20230125073502.743446-1-leobras@xxxxxxxxxx/ > > tl;dr: It increased clocks spent in the most common this_cpu operations, > while reducing clocks spent in remote operations (drain_all_stock). > > In RT case, since local locks are already spinlocks, this cost is > already paid, so we can get results like these: > > drain_all_stock > cpus Upstream Patched Diff (cycles) Diff(%) > 1 44331.10831 38978.03581 -5353.072507 -12.07520567 > 8 43992.96512 39026.76654 -4966.198572 -11.2886198 > 128 156274.6634 58053.87421 -98220.78915 -62.85138425 > > Upstream: Clocks to schedule work on remote CPU (performing not accounted) > Patched: Clocks to grab remote cpu's spinlock and perform the needed work > locally. This looks good as a micro-benchmark. And it answers why we need patch #3 in this series. It'll be better if we have something similar for patch #2 and #4. Besides, micro-benchmarks are usually a bit artifical IMO, it's better if we have the data to prove that your changes improve the performance from a more global view. For example, could you find or create a use case where flush_slab() becomes somewhat a hot path? And we can then know the performance gain from your changes in that use case. Maybe Vlastimil has something in his mind already? ;-) Also keep in mind that your changes apply to RT, so a natural follow-up question would be: will it hurt the system latency? I know litte about this area, so I must defer this to experts. The above concern brings another opportunity: would it make sense to use real locks instead of queuing work on a remote CPU in the case when RT is not needed, but CPU isolation is important? I.e. nohz_full situations? > > Do you have other suggestions to use as (micro-) benchmarking? > My overall suggestion is that you do find a valuable pattern where queuing remote work may not be the best option, but usually a real world usage would make more sense for the extra complexity that we will pay. Does this make sense? Regards, Boqun > Thanks! > Leo > > > > > > Regards, > > Boqun > > > > > > 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 > > > > > >
