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

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Message-ID: <Z5pTZ0XLFzfM5dic@casper.infradead.org>
Date: Wed, 29 Jan 2025 16:12:23 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Rik van Riel <riel@...riel.com>
Cc: 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>,
	"Paul E. McKenney" <paulmck@...nel.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 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