Re: [PATCH 1/9] slub: Make PREEMPT_RT support less convoluted

From: Hyeonggon Yoo <42.hyeyoo@gmail.com>
To: Christoph Lameter <cl@gentwo.de>
Cc: Vlastimil Babka <vbabka@suse.cz>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	linux-kernel@vger.kernel.org,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Matthew Wilcox <willy@infradead.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	David Rientjes <rientjes@google.com>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	Pekka Enberg <penberg@kernel.org>,
	linux-mm@kvack.org
Subject: Re: [PATCH 1/9] slub: Make PREEMPT_RT support less convoluted
Date: Thu, 25 Aug 2022 14:15:06 +0900	[thread overview]
Message-ID: <YwcFWsO/iNE534dp@hyeyoo> (raw)
In-Reply-To: <alpine.DEB.2.22.394.2208191655540.2139415@gentwo.de>

On Fri, Aug 19, 2022 at 05:04:31PM +0200, Christoph Lameter wrote:
> On Thu, 18 Aug 2022, Vlastimil Babka wrote:
> 
> > On 8/18/22 11:42, Christoph Lameter wrote:
> > > On Wed, 17 Aug 2022, Sebastian Andrzej Siewior wrote:
> > >
> > >> + *   On PREEMPT_RT, the local lock neither disables interrupts nor preemption
> > >> + *   which means the lockless fastpath cannot be used as it might interfere with
> > >> + *   an in-progress slow path operations. In this case the local lock is always
> > >> + *   taken but it still utilizes the freelist for the common operations.
> > >
> > > The slub fastpath does not interfere with slow path operations and the
> >
> > That's true on !PREEMPT_RT because a slowpath operation under
> > local_lock_irqsave() will disable interrupts, so there can't be a
> > fastpath operation in an interrupt handler appearing in the middle of a
> > slowpath operation.
> >
> > On PREEMPT_RT local_lock_irqsave() doesn't actually disable interrupts,
> > so that can happen. IIRC we learned that the hard way when Mike
> > Galbraith was testing early versions of my PREEMPT_RT changes for SLUB.
> 
> Well yes if you enable interrupts during the slowpath then interrupts may
> use the fastpath. That is a basic design change to the way concurrency is
> handled in the allocators.
> 
> There needs to be some fix here to restore the exclusion of the fastpath
> during slow path processing. This could be
> 
> A) Exclude the fastpath during slowpath operations
> 
> This can be accomplished by setting things up like in the debug mode
> that also excludes the fastpath.

I think we can do that by disabling preemption (for a short period, I think)
in slowpath on RT (like disabling irq in non-RT)

But I wonder if RT guys will prefer that?

> B) Force interrupt allocations to the slowpath.
> 
> Check some flag that indicates an interrupt allocation is occurring and
> then bypass the fastpath.

There is nothing special about interrupt allocation on RT.
All users of SLUB on RT must not be in hardirq context.

So I don't think it is possible to distingush between a thread being preempted
and another thread that preempts it.

-- 
Thanks,
Hyeonggon