Re: [PATCH v4 08/13] mm/mempolicy: Create a page allocator for policy

[Feng Tang]

On Wed, Apr 14, 2021 at 03:08:19PM +0200, Michal Hocko wrote:
> On Wed 17-03-21 11:40:05, Feng Tang wrote:
> > From: Ben Widawsky <ben.widawsky@xxxxxxxxx>
> > 
> > Add a helper function which takes care of handling multiple preferred
> > nodes. It will be called by future patches that need to handle this,
> > specifically VMA based page allocation, and task based page allocation.
> > Huge pages don't quite fit the same pattern because they use different
> > underlying page allocation functions. This consumes the previous
> > interleave policy specific allocation function to make a one stop shop
> > for policy based allocation.
> > 
> > With this, MPOL_PREFERRED_MANY's semantic is more like MPOL_PREFERRED
> > that it will first try the preferred node/nodes, and fallback to all
> > other nodes when first try fails. Thanks to Michal Hocko for suggestions
> > on this.
> > 
> > For now, only interleaved policy will be used so there should be no
> > functional change yet. However, if bisection points to issues in the
> > next few commits, it was likely the fault of this patch.
> 
> I am not sure this is helping much. Let's see in later patches but I
> would keep them separate and rather create a dedicated function for the
> new policy allocation mode.
 
Thanks for the suggestion, we will rethink the implementations. 

- Feng

> > Similar functionality is offered via policy_node() and
> > policy_nodemask(). By themselves however, neither can achieve this
> > fallback style of sets of nodes.
> > 
> > [ Feng: for the first try, add NOWARN flag, and skip the direct reclaim
> >   to speedup allocation in some case ]
> > 
> > Link: https://lore.kernel.org/r/20200630212517.308045-9-ben.widawsky@xxxxxxxxx
> > Signed-off-by: Ben Widawsky <ben.widawsky@xxxxxxxxx>
> > Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
> > ---
> >  mm/mempolicy.c | 65 ++++++++++++++++++++++++++++++++++++++++++++++------------
> >  1 file changed, 52 insertions(+), 13 deletions(-)
> > 
> > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> > index d945f29..d21105b 100644
> > --- a/mm/mempolicy.c
> > +++ b/mm/mempolicy.c
> > @@ -2187,22 +2187,60 @@ bool mempolicy_nodemask_intersects(struct task_struct *tsk,
> >  	return ret;
> >  }
> >  
> > -/* Allocate a page in interleaved policy.
> > -   Own path because it needs to do special accounting. */
> > -static struct page *alloc_page_interleave(gfp_t gfp, unsigned order,
> > -					unsigned nid)
> > +/* Handle page allocation for all but interleaved policies */
> > +static struct page *alloc_pages_policy(struct mempolicy *pol, gfp_t gfp,
> > +				       unsigned int order, int preferred_nid)
> >  {
> >  	struct page *page;
> > +	gfp_t gfp_mask = gfp;
> >  
> > -	page = __alloc_pages(gfp, order, nid);
> > -	/* skip NUMA_INTERLEAVE_HIT counter update if numa stats is disabled */
> > -	if (!static_branch_likely(&vm_numa_stat_key))
> > +	if (pol->mode == MPOL_INTERLEAVE) {
> > +		page = __alloc_pages(gfp, order, preferred_nid);
> > +		/* skip NUMA_INTERLEAVE_HIT counter update if numa stats is disabled */
> > +		if (!static_branch_likely(&vm_numa_stat_key))
> > +			return page;
> > +		if (page && page_to_nid(page) == preferred_nid) {
> > +			preempt_disable();
> > +			__inc_numa_state(page_zone(page), NUMA_INTERLEAVE_HIT);
> > +			preempt_enable();
> > +		}
> >  		return page;
> > -	if (page && page_to_nid(page) == nid) {
> > -		preempt_disable();
> > -		__inc_numa_state(page_zone(page), NUMA_INTERLEAVE_HIT);
> > -		preempt_enable();
> >  	}
> > +
> > +	VM_BUG_ON(preferred_nid != NUMA_NO_NODE);
> > +
> > +	preferred_nid = numa_node_id();
> > +
> > +	/*
> > +	 * There is a two pass approach implemented here for
> > +	 * MPOL_PREFERRED_MANY. In the first pass we try the preferred nodes
> > +	 * but allow the allocation to fail. The below table explains how
> > +	 * this is achieved.
> > +	 *
> > +	 * | Policy                        | preferred nid | nodemask   |
> > +	 * |-------------------------------|---------------|------------|
> > +	 * | MPOL_DEFAULT                  | local         | NULL       |
> > +	 * | MPOL_PREFERRED                | best          | NULL       |
> > +	 * | MPOL_INTERLEAVE               | ERR           | ERR        |
> > +	 * | MPOL_BIND                     | local         | pol->nodes |
> > +	 * | MPOL_PREFERRED_MANY           | best          | pol->nodes |
> > +	 * | MPOL_PREFERRED_MANY (round 2) | local         | NULL       |
> > +	 * +-------------------------------+---------------+------------+
> > +	 */
> > +	if (pol->mode == MPOL_PREFERRED_MANY) {
> > +		gfp_mask |=  __GFP_NOWARN;
> > +
> > +		/* Skip direct reclaim, as there will be a second try */
> > +		gfp_mask &= ~__GFP_DIRECT_RECLAIM;
> > +	}
> > +
> > +	page = __alloc_pages_nodemask(gfp_mask, order,
> > +				      policy_node(gfp, pol, preferred_nid),
> > +				      policy_nodemask(gfp, pol));
> > +
> > +	if (unlikely(!page && pol->mode == MPOL_PREFERRED_MANY))
> > +		page = __alloc_pages_nodemask(gfp, order, preferred_nid, NULL);
> > +
> >  	return page;
> >  }
> >  
> > @@ -2244,8 +2282,8 @@ alloc_pages_vma(gfp_t gfp, int order, struct vm_area_struct *vma,
> >  		unsigned nid;
> >  
> >  		nid = interleave_nid(pol, vma, addr, PAGE_SHIFT + order);
> > +		page = alloc_pages_policy(pol, gfp, order, nid);
> >  		mpol_cond_put(pol);
> > -		page = alloc_page_interleave(gfp, order, nid);
> >  		goto out;
> >  	}
> >  
> > @@ -2329,7 +2367,8 @@ struct page *alloc_pages_current(gfp_t gfp, unsigned order)
> >  	 * nor system default_policy
> >  	 */
> >  	if (pol->mode == MPOL_INTERLEAVE)
> > -		page = alloc_page_interleave(gfp, order, interleave_nodes(pol));
> > +		page = alloc_pages_policy(pol, gfp, order,
> > +					  interleave_nodes(pol));
> >  	else
> >  		page = __alloc_pages_nodemask(gfp, order,
> >  				policy_node(gfp, pol, numa_node_id()),
> > -- 
> > 2.7.4
> 
> -- 
> Michal Hocko
> SUSE Labs