linux-kernel - Re: [PATCH] mm: initialize deferred pages with interrupts enabled

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Message-ID: <20200401195721.GZ22681@dhcp22.suse.cz>
Date:   Wed, 1 Apr 2020 21:57:21 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Pavel Tatashin <pasha.tatashin@...een.com>
Cc:     linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        linux-mm@...ck.org, dan.j.williams@...el.com,
        shile.zhang@...ux.alibaba.com, daniel.m.jordan@...cle.com,
        ktkhai@...tuozzo.com, david@...hat.com, jmorris@...ei.org,
        sashal@...nel.org
Subject: Re: [PATCH] mm: initialize deferred pages with interrupts enabled

On Wed 01-04-20 15:32:38, Pavel Tatashin wrote:
> Initializing struct pages is a long task and keeping interrupts disabled
> for the duration of this operation introduces a number of problems.
> 
> 1. jiffies are not updated for long period of time, and thus incorrect time
>    is reported. See proposed solution and discussion here:
>    lkml/20200311123848.118638-1-shile.zhang@...ux.alibaba.com

http://lkml.kernel.org/r/20200311123848.118638-1-shile.zhang@linux.alibaba.com

> 2. It prevents farther improving deferred page initialization by allowing
>    inter-node multi-threading.
> 
> We are keeping interrupts disabled to solve a rather theoretical problem
> that was never observed in real world (See 3a2d7fa8a3d5).
> 
> Lets keep interrupts enabled. In case we ever encounter a scenario where
> an interrupt thread wants to allocate large amount of memory this early in
> boot we can deal with that by growing zone (see deferred_grow_zone()) by
> the needed amount before starting deferred_init_memmap() threads.
>
> Before:
> [    1.232459] node 0 initialised, 12058412 pages in 1ms
> 
> After:
> [    1.632580] node 0 initialised, 12051227 pages in 436ms
> 

Fixes: 3a2d7fa8a3d5 ("mm: disable interrupts while initializing deferred pages")
> Signed-off-by: Pavel Tatashin <pasha.tatashin@...een.com>

I would much rather see pgdat_resize_lock completely out of both the
allocator and deferred init path altogether but this can be done in a
separate patch. This one looks slightly safer for stable backports.

To be completely honest I would love to see the resize lock go away
completely. That might need a deeper thought but I believe it is
something that has never been done properly.

Acked-by: Michal Hocko <mhocko@...e.com>

Thanks!

> ---
>  mm/page_alloc.c | 21 +++++++--------------
>  1 file changed, 7 insertions(+), 14 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 3c4eb750a199..4498a13b372d 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1792,6 +1792,13 @@ static int __init deferred_init_memmap(void *data)
>  	BUG_ON(pgdat->first_deferred_pfn > pgdat_end_pfn(pgdat));
>  	pgdat->first_deferred_pfn = ULONG_MAX;
>  
> +	/*
> +	 * Once we unlock here, the zone cannot be grown anymore, thus if an
> +	 * interrupt thread must allocate this early in boot, zone must be
> +	 * pre-grown prior to start of deferred page initialization.
> +	 */
> +	pgdat_resize_unlock(pgdat, &flags);
> +
>  	/* Only the highest zone is deferred so find it */
>  	for (zid = 0; zid < MAX_NR_ZONES; zid++) {
>  		zone = pgdat->node_zones + zid;
> @@ -1812,8 +1819,6 @@ static int __init deferred_init_memmap(void *data)
>  	while (spfn < epfn)
>  		nr_pages += deferred_init_maxorder(&i, zone, &spfn, &epfn);
>  zone_empty:
> -	pgdat_resize_unlock(pgdat, &flags);
> -
>  	/* Sanity check that the next zone really is unpopulated */
>  	WARN_ON(++zid < MAX_NR_ZONES && populated_zone(++zone));
>  
> @@ -1854,18 +1859,6 @@ deferred_grow_zone(struct zone *zone, unsigned int order)
>  		return false;
>  
>  	pgdat_resize_lock(pgdat, &flags);
> -
> -	/*
> -	 * If deferred pages have been initialized while we were waiting for
> -	 * the lock, return true, as the zone was grown.  The caller will retry
> -	 * this zone.  We won't return to this function since the caller also
> -	 * has this static branch.
> -	 */
> -	if (!static_branch_unlikely(&deferred_pages)) {
> -		pgdat_resize_unlock(pgdat, &flags);
> -		return true;
> -	}
> -
>  	/*
>  	 * If someone grew this zone while we were waiting for spinlock, return
>  	 * true, as there might be enough pages already.
> -- 
> 2.17.1
> 

-- 
Michal Hocko
SUSE Labs