linux-kernel - Re: [linus:master] [x86] 4817f70c25: stress-ng.mmapaddr.ops_per

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Message-ID: <05da0ae9-073e-4578-b65c-f837c28eead8@paulmck-laptop>
Date: Wed, 29 Jan 2025 08:36:12 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Rik van Riel <riel@...riel.com>
Cc: Matthew Wilcox <willy@...radead.org>,
	Qi Zheng <zhengqi.arch@...edance.com>,
	Peter Zijlstra <peterz@...radead.org>,
	David Hildenbrand <david@...hat.com>,
	kernel test robot <oliver.sang@...el.com>, oe-lkp@...ts.linux.dev,
	lkp@...el.com, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Andy Lutomirski <luto@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	David Rientjes <rientjes@...gle.com>,
	Hugh Dickins <hughd@...gle.com>, Jann Horn <jannh@...gle.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Mel Gorman <mgorman@...e.de>, Muchun Song <muchun.song@...ux.dev>,
	Peter Xu <peterx@...hat.com>, Will Deacon <will@...nel.org>,
	Zach O'Keefe <zokeefe@...gle.com>,
	Dan Carpenter <dan.carpenter@...aro.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>
Subject: Re: [linus:master] [x86] 4817f70c25: stress-ng.mmapaddr.ops_per_sec
 63.0% regression

On Wed, Jan 29, 2025 at 11:14:29AM -0500, Rik van Riel wrote:
> On Wed, 2025-01-29 at 16:12 +0000, Matthew Wilcox wrote:
> > On Wed, Jan 29, 2025 at 10:59:20AM -0500, Rik van Riel wrote:
> > > Below is a tentative fix for the issue. It is kind of a big hammer,
> > > and maybe the RCU people have a better idea on how to solve this
> > > problem, but it may be worth giving this a try to see if it helps
> > > with the regression you identified.
> > 
> > Perhaps better to do:
> > 
> > +++ b/mm/page_alloc.c
> > @@ -97,8 +97,8 @@ static DEFINE_MUTEX(pcp_batch_high_lock);
> >   * On SMP, spin_trylock is sufficient protection.
> >   * On PREEMPT_RT, spin_trylock is equivalent on both SMP and UP.
> >   */
> > -#define pcp_trylock_prepare(flags)     do { } while (0)
> > -#define pcp_trylock_finish(flag)       do { } while (0)
> > +#define pcp_trylock_prepare(flags)     rcu_read_lock()
> > +#define pcp_trylock_finish(flag)       rcu_reada_unlock()
> >  #else
> > 
> >  /* UP spin_trylock always succeeds so disable IRQs to prevent re-
> > entrancy. */
> > 
> > with appropriate comment changes
> > 
> Agreed. Assuming this change even works :)
> 
> Paul, does this look like it could do the trick,
> or do we need something else to make RCU freeing
> happy again?

I don't claim to fully understand the issue, but this would prevent
any RCU grace periods starting subsequently from completing.  It would
not prevent RCU callbacks from being invoked for RCU grace periods that
started earlier.

So it won't prevent RCU callbacks from being invoked.

It *will* ensure that only a finite number of RCU callbacks get invoked.
For some perhaps rather large value of "finite".

Does that help, or is more required?

							Thanx, Paul