linux-kernel - Re: [PATCH 2/6] mm/page_alloc: Disassociate the pcp->high from pcp->batch

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Date:   Fri, 28 May 2021 12:27:58 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Hillf Danton <hdanton@...a.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Michal Hocko <mhocko@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH 2/6] mm/page_alloc: Disassociate the pcp->high from
 pcp->batch

On 5/27/21 12:52 PM, Mel Gorman wrote:
> On Wed, May 26, 2021 at 08:14:13PM +0200, Vlastimil Babka wrote:
>> > @@ -6698,11 +6717,10 @@ static void __zone_set_pageset_high_and_batch(struct zone *zone, unsigned long h
>> >   */
>> >  static void zone_set_pageset_high_and_batch(struct zone *zone)
>> >  {
>> > -	unsigned long new_high, new_batch;
>> > +	int new_high, new_batch;
>> >  
>> > -	new_batch = zone_batchsize(zone);
>> > -	new_high = 6 * new_batch;
>> > -	new_batch = max(1UL, 1 * new_batch);
>> > +	new_batch = max(1, zone_batchsize(zone));
>> > +	new_high = zone_highsize(zone, new_batch);
>> >  
>> >  	if (zone->pageset_high == new_high &&
>> >  	    zone->pageset_batch == new_batch)
>> > @@ -8170,6 +8188,12 @@ static void __setup_per_zone_wmarks(void)
>> >  		zone->_watermark[WMARK_LOW]  = min_wmark_pages(zone) + tmp;
>> >  		zone->_watermark[WMARK_HIGH] = min_wmark_pages(zone) + tmp * 2;
>> >  
>> > +		/*
>> > +		 * The watermark size have changed so update the pcpu batch
>> > +		 * and high limits or the limits may be inappropriate.
>> > +		 */
>> > +		zone_set_pageset_high_and_batch(zone);
>> 
>> Hm so this puts the call in the path of various watermark related sysctl
>> handlers, but it's not protected by pcp_batch_high_lock. The zone lock won't
>> help against zone_pcp_update() from a hotplug handler. On the other hand, since
>> hotplug handlers also call __setup_per_zone_wmarks(), the zone_pcp_update()
>> calls there are now redundant and could be removed, no?
>> But later there will be a new sysctl in patch 6/6 using pcp_batch_high_lock,
>> thus that one will not be protected against the watermark related sysctl
>> handlers that reach here.
>> 
>> To solve all this, seems like the static lock in setup_per_zone_wmarks() could
>> become a top-level visible lock and pcp high/batch updates could switch to that
>> one instead of own pcp_batch_high_lock. And zone_pcp_update() calls from hotplug
>> handlers could be removed.
>> 
> 
> Hmm, the locking has very different hold times. The static lock in
> setup_per_zone_wmarks is a spinlock that protects against parallel updates
> of watermarks and is held for a short duration. The pcp_batch_high_lock
> is a mutex that is held for a relatively long time while memory is being
> offlined and can sleep. Memory hotplug updates the watermarks without
> pcp_batch_high_lock held so overall, unifying the locking there should
> be a separate series.
> 
> How about this as a fix for this patch?
> 
> ---8<---
> mm/page_alloc: Disassociate the pcp->high from pcp->batch -fix
> 
> Vlastimil Babka noted that __setup_per_zone_wmarks updating pcp->high
> did not protect watermark-related sysctl handlers from a parallel
> memory hotplug operations. This patch moves the PCP update to
> setup_per_zone_wmarks and updates the PCP high value while protected
> by the pcp_batch_high_lock mutex.
> 
> This is a fix to the mmotm patch mm-page_alloc-disassociate-the-pcp-high-from-pcp-batch.patch.
> It'll cause a conflict with mm-page_alloc-adjust-pcp-high-after-cpu-hotplug-events.patch
> but the resolution is simply to change the caller in setup_per_zone_wmarks
> to zone_pcp_update(zone, 0)
> 
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>

Looks fine. But I would also remove the redudancy introduced by this patch+fix,
as part of the patch:

online_pages()
  zone_pcp_update(zone); <- this predates the patch
  init_per_zone_wmark_min()
    setup_per_zone_wmarks()
      for_each_zone(zone)
         zone_pcp_update(zone); <- new in this patch

offline_pages() similarly

In any case, for the fixed version:
Acked-by: Vlastimil Babka <vbabka@...e.cz>

> ---
>  mm/page_alloc.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 329b71e41db4..b1b3c66e9d88 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -8199,12 +8199,6 @@ static void __setup_per_zone_wmarks(void)
>  		zone->_watermark[WMARK_LOW]  = min_wmark_pages(zone) + tmp;
>  		zone->_watermark[WMARK_HIGH] = min_wmark_pages(zone) + tmp * 2;
>  
> -		/*
> -		 * The watermark size have changed so update the pcpu batch
> -		 * and high limits or the limits may be inappropriate.
> -		 */
> -		zone_set_pageset_high_and_batch(zone);
> -
>  		spin_unlock_irqrestore(&zone->lock, flags);
>  	}
>  
> @@ -8221,11 +8215,19 @@ static void __setup_per_zone_wmarks(void)
>   */
>  void setup_per_zone_wmarks(void)
>  {
> +	struct zone *zone;
>  	static DEFINE_SPINLOCK(lock);
>  
>  	spin_lock(&lock);
>  	__setup_per_zone_wmarks();
>  	spin_unlock(&lock);
> +
> +	/*
> +	 * The watermark size have changed so update the pcpu batch
> +	 * and high limits or the limits may be inappropriate.
> +	 */
> +	for_each_zone(zone)
> +		zone_pcp_update(zone);
>  }
>  
>  /*
>