linux-kernel - Re: [PATCH v2 2/2] mm: initialize deferred pages with interrupts enabled

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Date:   Thu, 2 Apr 2020 09:47:32 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Pavel Tatashin <pasha.tatashin@...een.com>,
        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, jmorris@...ei.org, sashal@...nel.org,
        vbabka@...e.cz
Subject: Re: [PATCH v2 2/2] mm: initialize deferred pages with interrupts
 enabled

On Thu 02-04-20 09:38:54, David Hildenbrand wrote:
> > +	/*
> > +	 * 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;
> > @@ -1809,11 +1816,9 @@ static int __init deferred_init_memmap(void *data)
> >  	 */
> >  	while (spfn < epfn) {
> >  		nr_pages += deferred_init_maxorder(&i, zone, &spfn, &epfn);
> > -		touch_nmi_watchdog();
> > +		cond_resched();
> 
> I do wonder if this change is strictly required in this patch (IOW, if
> we could keep calling touch_nmi_watchdog() also without holding a spinlock)

Exactly. I would go with your patch on top.
 
> Anyhow, it's the right thing to do.
> 
> >  	}
> >  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));
> >  
> > @@ -1855,17 +1860,6 @@ deferred_grow_zone(struct zone *zone, unsigned int order)
> >  
> >  	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.
> > 
> 
> 
> I think we should also look into cleaning up deferred_grow_zone( next),
> we still have that touch_nmi_watchdog() in there. We should rework
> locking. (I think Michal requested that as well)
> 
> For now, this seems to survive my basic testing (RCU stalls gone)
> 
> -- 
> Thanks,
> 
> David / dhildenb
> 

-- 
Michal Hocko
SUSE Labs