Re: [RFC PATCH v1 0/4] Introduce QPW for per-cpu operations

[Leonardo Bras <leobras@redhat.com>]

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@redhat.com/

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
>