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

[Marcelo Tosatti]

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/
> 
> 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...

Thomas?