linux-kernel - Re: [patch v5] mm: lru_cache_disable: replace work queue synchronization with synchronize

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Message-Id: <20220528141809.732788db257253396462c79e@linux-foundation.org>
Date:   Sat, 28 May 2022 14:18:09 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     Borislav Petkov <bp@...en8.de>, michael@...haellarabel.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Minchan Kim <minchan@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Nicolas Saenz Julienne <nsaenzju@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [patch v5] mm: lru_cache_disable: replace work queue
 synchronization with synchronize_rcu

On Thu, 28 Apr 2022 15:00:11 -0300 Marcelo Tosatti <mtosatti@...hat.com> wrote:

> On Thu, Mar 31, 2022 at 03:52:45PM +0200, Borislav Petkov wrote:
> > On Thu, Mar 10, 2022 at 10:22:12AM -0300, Marcelo Tosatti wrote:
> > > 
> ...
>
> > 
> > Someone pointed me at this:
> > 
> > https://www.phoronix.com/scan.php?page=news_item&px=Linux-518-Stress-NUMA-Goes-Boom
> > 
> > which says this one causes a performance regression with stress-ng's
> > NUMA test...
> 
> Michael,
> 
> This is probably do_migrate_pages that is taking too long due to
> synchronize_rcu().
> 
> Switching to synchronize_rcu_expedited() should probably fix it...
> Can you give it a try, please?

I guess not.

Is anyone else able to demonstrate a stress-ng performance regression
due to ff042f4a9b0508?  And if so, are they able to try Marcelo's
one-liner?

> diff --git a/mm/swap.c b/mm/swap.c
> index bceff0cb559c..04a8bbf9817a 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -879,7 +879,7 @@ void lru_cache_disable(void)
>  	 * lru_disable_count = 0 will have exited the critical
>  	 * section when synchronize_rcu() returns.
>  	 */
> -	synchronize_rcu();
> +	synchronize_rcu_expedited();
>  #ifdef CONFIG_SMP
>  	__lru_add_drain_all(true);
>  #else
> 
>