linux-kernel - Re: [PATCH 15/18] sched: Set preferred NUMA node based on number of private faults

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Date:	Wed, 31 Jul 2013 10:29:38 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 15/18] sched: Set preferred NUMA node based on number of
 private faults

On Fri, Jul 26, 2013 at 01:20:50PM +0200, Peter Zijlstra wrote:
> On Mon, Jul 15, 2013 at 04:20:17PM +0100, Mel Gorman wrote:
> > diff --git a/mm/mprotect.c b/mm/mprotect.c
> > index cacc64a..04c9469 100644
> > --- a/mm/mprotect.c
> > +++ b/mm/mprotect.c
> > @@ -37,14 +37,15 @@ static inline pgprot_t pgprot_modify(pgprot_t oldprot, pgprot_t newprot)
> >  
> >  static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
> >  		unsigned long addr, unsigned long end, pgprot_t newprot,
> > -		int dirty_accountable, int prot_numa, bool *ret_all_same_node)
> > +		int dirty_accountable, int prot_numa, bool *ret_all_same_nidpid)
> >  {
> >  	struct mm_struct *mm = vma->vm_mm;
> >  	pte_t *pte, oldpte;
> >  	spinlock_t *ptl;
> >  	unsigned long pages = 0;
> > -	bool all_same_node = true;
> > +	bool all_same_nidpid = true;
> >  	int last_nid = -1;
> > +	int last_pid = -1;
> >  
> >  	pte = pte_offset_map_lock(mm, pmd, addr, &ptl);
> >  	arch_enter_lazy_mmu_mode();
> > @@ -64,10 +65,17 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
> >  				page = vm_normal_page(vma, addr, oldpte);
> >  				if (page) {
> >  					int this_nid = page_to_nid(page);
> > +					int nidpid = page_nidpid_last(page);
> > +					int this_pid = nidpid_to_pid(nidpid);
> > +
> >  					if (last_nid == -1)
> >  						last_nid = this_nid;
> > -					if (last_nid != this_nid)
> > -						all_same_node = false;
> > +					if (last_pid == -1)
> > +						last_pid = this_pid;
> > +					if (last_nid != this_nid ||
> > +					    last_pid != this_pid) {
> > +						all_same_nidpid = false;
> > +					}
> 
> At this point I would've expected something like:
> 
> 		int nidpid = page_nidpid_last(page);
> 		int thisnid = nidpid_to_nid(nidpid);
> 		int thispid = nidpit_to_pid(nidpit);
> 
> It seems 'weird' to mix the state like you did; is there a reason the
> above is incorrect?
> 

No there isn't and it looks like a brain fart. I've changed it to what
you suggested.

> >  
> >  					if (!pte_numa(oldpte)) {
> >  						ptent = pte_mknuma(ptent);
> > @@ -106,7 +114,7 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
> >  	arch_leave_lazy_mmu_mode();
> >  	pte_unmap_unlock(pte - 1, ptl);
> >  
> > -	*ret_all_same_node = all_same_node;
> > +	*ret_all_same_nidpid = all_same_nidpid;
> >  	return pages;
> >  }
> >  
> > @@ -133,7 +141,7 @@ static inline unsigned long change_pmd_range(struct vm_area_struct *vma,
> >  	pmd_t *pmd;
> >  	unsigned long next;
> >  	unsigned long pages = 0;
> > -	bool all_same_node;
> > +	bool all_same_nidpid;
> >  
> >  	pmd = pmd_offset(pud, addr);
> >  	do {
> > @@ -151,7 +159,7 @@ static inline unsigned long change_pmd_range(struct vm_area_struct *vma,
> >  		if (pmd_none_or_clear_bad(pmd))
> >  			continue;
> >  		pages += change_pte_range(vma, pmd, addr, next, newprot,
> > -				 dirty_accountable, prot_numa, &all_same_node);
> > +				 dirty_accountable, prot_numa, &all_same_nidpid);
> >  
> >  		/*
> >  		 * If we are changing protections for NUMA hinting faults then
> > @@ -159,7 +167,7 @@ static inline unsigned long change_pmd_range(struct vm_area_struct *vma,
> >  		 * node. This allows a regular PMD to be handled as one fault
> >  		 * and effectively batches the taking of the PTL
> >  		 */
> > -		if (prot_numa && all_same_node)
> > +		if (prot_numa && all_same_nidpid)
> >  			change_pmd_protnuma(vma->vm_mm, addr, pmd);
> >  	} while (pmd++, addr = next, addr != end);
> >  
> 
> Hurmph I just stumbled upon this PMD 'trick' and I'm not at all sure I
> like it. If an application would pre-fault/initialize its memory with
> the main thread we'll collapse it into a PMDs and forever thereafter (by
> virtue of do_pmd_numa_page()) they'll all stay the same. Resulting in
> PMD granularity.
> 

Potentially yes. When that PMD trick was introduced it was because the cost
of faults was very high due to a high scanning rate. The trick mitigated
worse-case scenarios until faults were properly accounted for and the scan
rates were better controlled. As these *should* be addressed by the series
I think I will be adding a patch to kick away this PMD crutch and see how
it looks in profiles.

-- 
Mel Gorman
SUSE Labs
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