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?
