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    SubjectRe: [PATCH 12/12] mm, page_alloc: Only enforce watermarks for order-0 allocations
    On Tue, Sep 08, 2015 at 05:26:13PM +0900, Joonsoo Kim wrote:
    > 2015-08-24 21:30 GMT+09:00 Mel Gorman <mgorman@techsingularity.net>:
    > > The primary purpose of watermarks is to ensure that reclaim can always
    > > make forward progress in PF_MEMALLOC context (kswapd and direct reclaim).
    > > These assume that order-0 allocations are all that is necessary for
    > > forward progress.
    > >
    > > High-order watermarks serve a different purpose. Kswapd had no high-order
    > > awareness before they were introduced (https://lkml.org/lkml/2004/9/5/9).
    > > This was particularly important when there were high-order atomic requests.
    > > The watermarks both gave kswapd awareness and made a reserve for those
    > > atomic requests.
    > >
    > > There are two important side-effects of this. The most important is that
    > > a non-atomic high-order request can fail even though free pages are available
    > > and the order-0 watermarks are ok. The second is that high-order watermark
    > > checks are expensive as the free list counts up to the requested order must
    > > be examined.
    > >
    > > With the introduction of MIGRATE_HIGHATOMIC it is no longer necessary to
    > > have high-order watermarks. Kswapd and compaction still need high-order
    > > awareness which is handled by checking that at least one suitable high-order
    > > page is free.
    >
    > I still don't think that this one suitable high-order page is enough.
    > If fragmentation happens, there would be no order-2 freepage. If kswapd
    > prepares only 1 order-2 freepage, one of two successive process forks
    > (AFAIK, fork in x86 and ARM require order 2 page) must go to direct reclaim
    > to make order-2 freepage. Kswapd cannot make order-2 freepage in that
    > short time. It causes latency to many high-order freepage requestor
    > in fragmented situation.
    >

    So what do you suggest instead? A fixed number, some other heuristic?
    You have pushed several times now for the series to focus on the latency
    of standard high-order allocations but again I will say that it is outside
    the scope of this series. If you want to take steps to reduce the latency
    of ordinary high-order allocation requests that can sleep then it should
    be a separate series.

    > > With the patch applied, there was little difference in the allocation
    > > failure rates as the atomic reserves are small relative to the number of
    > > allocation attempts. The expected impact is that there will never be an
    > > allocation failure report that shows suitable pages on the free lists.
    >
    > Due to highatomic pageblock and freepage count mismatch per allocation
    > flag, allocation failure with suitable pages can still be possible.
    >

    An allocation failure of this type would be a !atomic allocation that
    cannot access the reserve. If such allocations requests can access the
    reserve then it defeats the whole point of the pageblock type.

    > > + * Return true if free base pages are above 'mark'. For high-order checks it
    > > + * will return true of the order-0 watermark is reached and there is at least
    > > + * one free page of a suitable size. Checking now avoids taking the zone lock
    > > + * to check in the allocation paths if no pages are free.
    > > */
    > > static bool __zone_watermark_ok(struct zone *z, unsigned int order,
    > > unsigned long mark, int classzone_idx, int alloc_flags,
    > > @@ -2289,7 +2291,7 @@ static bool __zone_watermark_ok(struct zone *z, unsigned int order,
    > > {
    > > long min = mark;
    > > int o;
    > > - long free_cma = 0;
    > > + const bool atomic = (alloc_flags & ALLOC_HARDER);
    > >
    > > /* free_pages may go negative - that's OK */
    > > free_pages -= (1 << order) - 1;
    > > @@ -2301,7 +2303,7 @@ static bool __zone_watermark_ok(struct zone *z, unsigned int order,
    > > * 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)))
    > > + if (likely(!atomic))
    > > free_pages -= z->nr_reserved_highatomic;
    > > else
    > > min -= min / 4;
    > > @@ -2309,22 +2311,30 @@ static bool __zone_watermark_ok(struct zone *z, unsigned int order,
    > > #ifdef CONFIG_CMA
    > > /* If allocation can't use CMA areas don't use free CMA pages */
    > > if (!(alloc_flags & ALLOC_CMA))
    > > - free_cma = zone_page_state(z, NR_FREE_CMA_PAGES);
    > > + free_pages -= zone_page_state(z, NR_FREE_CMA_PAGES);
    > > #endif
    > >
    > > - if (free_pages - free_cma <= min + z->lowmem_reserve[classzone_idx])
    > > + if (free_pages <= min + z->lowmem_reserve[classzone_idx])
    > > return false;
    > > - for (o = 0; o < order; o++) {
    > > - /* At the next order, this order's pages become unavailable */
    > > - free_pages -= z->free_area[o].nr_free << o;
    > >
    > > - /* Require fewer higher order pages to be free */
    > > - min >>= 1;
    > > + /* order-0 watermarks are ok */
    > > + if (!order)
    > > + return true;
    > > +
    > > + /* Check at least one high-order page is free */
    > > + for (o = order; o < MAX_ORDER; o++) {
    > > + struct free_area *area = &z->free_area[o];
    > > + int mt;
    > > +
    > > + if (atomic && area->nr_free)
    > > + return true;
    >
    > How about checking area->nr_free first?
    > In both atomic and !atomic case, nr_free == 0 means
    > there is no appropriate pages.
    >
    > So,
    > if (!area->nr_free)
    > continue;
    > if (atomic)
    > return true;
    > ...
    >
    >
    > > - if (free_pages <= min)
    > > - return false;
    > > + for (mt = 0; mt < MIGRATE_PCPTYPES; mt++) {
    > > + if (!list_empty(&area->free_list[mt]))
    > > + return true;
    > > + }
    >
    > I'm not sure this is really faster than previous.
    > We need to check three lists on each order.
    >
    > Think about order-2 case. I guess order-2 is usually on movable
    > pageblock rather than unmovable pageblock. In this case,
    > we need to check three lists so cost is more.
    >

    Ok, the extra check makes sense. Thanks.

    --
    Mel Gorman
    SUSE Labs


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