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

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Message-ID: <ZpVsow629seGUXRz@tpad>
Date: Mon, 15 Jul 2024 15:38:27 -0300
From: Marcelo Tosatti <mtosatti@...hat.com>
To: Vlastimil Babka <vbabka@...e.cz>, Thomas Gleixner <tglx@...utronix.de>
Cc: Leonardo Bras <leobras@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...nel.org>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Shakeel Butt <shakeel.butt@...ux.dev>,
	Muchun Song <muchun.song@...ux.dev>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux.com>, Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Hyeonggon Yoo <42.hyeyoo@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
	Waiman Long <longman@...hat.com>, Boqun Feng <boqun.feng@...il.com>,
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [RFC PATCH v1 0/4] Introduce QPW for per-cpu operations

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/
> 
> 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?