Re: [PATCH 09/10] mm, page_alloc: Reserve pageblocks for high-order atomic allocations on demand

[Mel Gorman <mgorman@techsingularity.net>]

On Fri, Jul 31, 2015 at 10:28:52AM +0200, Vlastimil Babka wrote:
> On 07/29/2015 02:53 PM, Mel Gorman wrote:
> >On Wed, Jul 29, 2015 at 01:35:17PM +0200, Vlastimil Babka wrote:
> >>On 07/20/2015 10:00 AM, Mel Gorman wrote:
> >>>+/*
> >>>+ * Used when an allocation is about to fail under memory pressure. This
> >>>+ * potentially hurts the reliability of high-order allocations when under
> >>>+ * intense memory pressure but failed atomic allocations should be easier
> >>>+ * to recover from than an OOM.
> >>>+ */
> >>>+static void unreserve_highatomic_pageblock(const struct alloc_context *ac)
> >>>+{
> >>>+	struct zonelist *zonelist = ac->zonelist;
> >>>+	unsigned long flags;
> >>>+	struct zoneref *z;
> >>>+	struct zone *zone;
> >>>+	struct page *page;
> >>>+	int order;
> >>>+
> >>>+	for_each_zone_zonelist_nodemask(zone, z, zonelist, ac->high_zoneidx,
> >>>+								ac->nodemask) {
> >>
> >>This fixed order might bias some zones over others wrt unreserving. Is it OK?
> >
> >I could not think of a situation where it mattered. It'll always be
> >preferring highest zone over lower zones. Allocation requests that can
> >use any zone that do not care. Allocation requests that are limited to
> >lower zones are protected as long as possible.
> 
> Hmm... allocation requests will follow fair zone policy and thus the
> highatomic reservations will be spread fairly among all zones? Unless the
> allocations require lower zones of course.
> 

Does that matter?

> But for unreservations, normal/high allocations failing under memory
> pressure will lead to unreserving highatomic pageblocks first in the higher
> zones and only then the lower zones, and that was my concern. But it's true
> that failing allocations that require lower zones will lead to unreserving
> the lower zones, so it might be ok in the end.
> 

Again, I don't think it matters.

> >
> >>
> >>>+		/* Preserve at least one pageblock */
> >>>+		if (zone->nr_reserved_highatomic <= pageblock_nr_pages)
> >>>+			continue;
> >>>+
> >>>+		spin_lock_irqsave(&zone->lock, flags);
> >>>+		for (order = 0; order < MAX_ORDER; order++) {
> >>
> >>Would it make more sense to look in descending order for a higher chance of
> >>unreserving a pageblock that's mostly free? Like the traditional page stealing does?
> >>
> >
> >I don't think it's worth the search cost. Traditional page stealing is
> >searching because it's trying to minimise events that cause external
> >fragmentation. Here we'd gain very little. We are under some memory
> >pressure here, if enough pages are not free then another one will get
> >freed shortly. Either way, I doubt the difference is measurable.
> 
> Hmm, I guess...
> 
> >
> >>>+			struct free_area *area = &(zone->free_area[order]);
> >>>+
> >>>+			if (list_empty(&area->free_list[MIGRATE_HIGHATOMIC]))
> >>>+				continue;
> >>>+
> >>>+			page = list_entry(area->free_list[MIGRATE_HIGHATOMIC].next,
> >>>+						struct page, lru);
> >>>+
> >>>+			zone->nr_reserved_highatomic -= pageblock_nr_pages;
> >>>+			set_pageblock_migratetype(page, ac->migratetype);
> >>
> >>Would it make more sense to assume MIGRATE_UNMOVABLE, as high-order allocations
> >>present in the pageblock typically would be, and apply the traditional page
> >>stealing heuristics to decide if it should be changed to ac->migratetype (if
> >>that differs)?
> >>
> >
> >Superb spot, I had to think about this for a while and initially I was
> >thinking your suggestion was a no-brainer and obviously the right thing
> >to do.
> >
> >On the pro side, it preserves the fragmentation logic because it'll force
> >the normal page stealing logic to be applied.
> >
> >On the con side, we may reassign the pageblock twice -- once to
> >MIGRATE_UNMOVABLE and once to ac->migratetype. That one does not matter
> >but the second con is that we inadvertly increase the number of unmovable
> >blocks in some cases.
> >
> >Lets say we default to MIGRATE_UNMOVABLE, ac->migratetype is MIGRATE_MOVABLE
> >and there are enough free pages to satisfy the allocation but not steal
> >the whole pageblock. The end result is that we have a new unmovable
> >pageblock that may not be necessary. The next unmovable allocation
> >potentially is forever. They key observation is that previously the
> >pageblock could have been short-lived high-order allocations that could
> >be completely free soon if it was assigned MIGRATE_MOVABLE. This may not
> >apply when SLUB is using high-order allocations but the point still
> >holds.
> 
> Yeah, I see the point. The obvious counterexample is a pageblock that we
> design as MOVABLE and yet it contains some long-lived unmovable allocation.
> More unmovable allocations could lead to choosing another movable block as a
> fallback, while if we marked this pageblock as unmovable, they could go here
> and not increase fragmentation.
> 
> The problem is, we can't known which one is the case. I've toyed with an
> idea of MIGRATE_MIXED blocks that would be for cases where the heuristics
> decide that e.g. an UNMOVABLE block is empty enough to change it to MOVABLE,
> but still it may contain some unmovable allocations. Such pageblocks should
> be preferred fallbacks for future unmovable allocations before truly
> pristine movable pageblocks.
> 

I tried that once upon a time. The number of blocks simply increased
over time and there was no sensible way to recover from it. I never
found a solution that worked for very long.

> >>>+
> >>>+	/*
> >>>+	 * If the caller is not atomic then discount the reserves. This will
> >>>+	 * over-estimate how the atomic reserve but it avoids a search
> >>>+	 */
> >>>+	if (likely(!(alloc_flags & ALLOC_HARDER)))
> >>>+		free_pages -= z->nr_reserved_highatomic;
> >>
> >>Hm, so in the case the maximum of 10% reserved blocks is already full, we deny
> >>the allocation access to another 10% of the memory and push it to reclaim. This
> >>seems rather excessive.
> >
> >It's necessary. If normal callers can use it then the reserve fills with
> >normal pages, the memory gets fragmented and high-order atomic allocations
> >fail due to fragmentation. Similarly, the number of MIGRATE_HIGHORDER
> >pageblocks cannot be unbound or everything else will be continually pushed
> >into reclaim even if there is plenty of memory free.
> 
> I understand denying normal allocations access to highatomic reserves via
> watermarks is necessary. But my concern is that for each reserved pageblock
> we effectively deny up to two pageblocks-worth-of-pages to normal
> allocations. One pageblock that is marked as MIGRATE_HIGHATOMIC, and once it
> becomes full, free_pages above are decreased twice - once by the pageblock
> becoming full, and then again by subtracting z->nr_reserved_highatomic. This
> extra gap is still usable by highatomic allocations, but they will also
> potentially mark more pageblocks highatomic and further increase the gap. In
> the worst case we have 10% of pageblocks marked highatomic and full, and
> another 10% that is only usable by highatomic allocations (but won't be
> marked as such), and if no more highatomic allocations come then the 10% is
> wasted.

Similar to what I said to Joosoo, I'll drop the size of the reserves in
the next revision. It brings things back in line with MIGRATE_RESERVE in
terms of the amount of memory we reserve while still removing the need
for high-order watermark checks. It means that the rate of atomic
high-order allocation failure will remain the same after the series as
before but that should not matter.

-- 
Mel Gorman
SUSE Labs

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