mm/page_alloc: batch page freeing in free_frozen_page_commit

Before returning, free_frozen_page_commit calls free_pcppages_bulk using
nr_pcp_free to determine how many pages can appropritately be freed, based
on the tunable parameters stored in pcp.  While this number is an accurate
representation of how many pages should be freed in total, it is not an
appropriate number of pages to free at once using free_pcppages_bulk,
since we have seen the value consistently go above 2000 in the Meta fleet
on larger machines.

As such, perform batched page freeing in free_pcppages_bulk by using
pcp->batch.  In order to ensure that other processes are not starved of
the zone lock, free both the zone lock and pcp lock to yield to other
threads.

Note that because free_frozen_page_commit now performs a spinlock inside
the function (and can fail), the function may now return with a freed pcp.
To handle this, return true if the pcp is locked on exit and false
otherwise.

In addition, since free_frozen_page_commit must now be aware of what UP
flags were stored at the time of the spin lock, and because we must be
able to report new UP flags to the callers, add a new unsigned long*
parameter UP_flags to keep track of this.

The following are a few synthetic benchmarks, made on three machines.  The
first is a large machine with 754GiB memory and 316 processors.  The
second is a relatively smaller machine with 251GiB memory and 176
processors.  The third and final is the smallest of the three, which has
62GiB memory and 36 processors.

On all machines, I kick off a kernel build with -j$(nproc).  Negative
delta is better (faster compilation)

Large machine (754GiB memory, 316 processors)
make -j$(nproc)
+------------+---------------+-----------+
| Metric (s) | Variation (%) | Delta(%)  |
+------------+---------------+-----------+
| real       |        0.8070 |  - 1.4865 |
| user       |        0.2823 |  + 0.4081 |
| sys        |        5.0267 |  -11.8737 |
+------------+---------------+-----------+

Medium machine (251GiB memory, 176 processors)
make -j$(nproc)
+------------+---------------+----------+
| Metric (s) | Variation (%) | Delta(%) |
+------------+---------------+----------+
| real       |        0.2806 |  +0.0351 |
| user       |        0.0994 |  +0.3170 |
| sys        |        0.6229 |  -0.6277 |
+------------+---------------+----------+

Small machine (62GiB memory, 36 processors)
make -j$(nproc)
+------------+---------------+----------+
| Metric (s) | Variation (%) | Delta(%) |
+------------+---------------+----------+
| real       |        0.1503 |  -2.6585 |
| user       |        0.0431 |  -2.2984 |
| sys        |        0.1870 |  -3.2013 |
+------------+---------------+----------+

Here, variation is the coefficient of variation, i.e.  standard deviation
/ mean.

Link: https://lkml.kernel.org/r/20251014145011.3427205-4-joshua.hahnjy@gmail.com
Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com>
Suggested-by: Chris Mason <clm@fb.com>
Co-developed-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Michal Hocko <mhocko@suse.com>
Cc: SeongJae Park <sj@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 52480f513ba210d238e57e2b99f17ce916253d4d..58cb279928a59ca7c655bcdc70c192368e0db442 100644 (file)
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2818,12 +2818,22 @@ static int nr_pcp_high(struct per_cpu_pages *pcp, struct zone *zone,
        return high;
 }
 
-static void free_frozen_page_commit(struct zone *zone,
+/*
+ * Tune pcp alloc factor and adjust count & free_count. Free pages to bring the
+ * pcp's watermarks below high.
+ *
+ * May return a freed pcp, if during page freeing the pcp spinlock cannot be
+ * reacquired. Return true if pcp is locked, false otherwise.
+ */
+static bool free_frozen_page_commit(struct zone *zone,
                struct per_cpu_pages *pcp, struct page *page, int migratetype,
-               unsigned int order, fpi_t fpi_flags)
+               unsigned int order, fpi_t fpi_flags, unsigned long *UP_flags)
 {
        int high, batch;
+       int to_free, to_free_batched;
        int pindex;
+       int cpu = smp_processor_id();
+       int ret = true;
        bool free_high = false;
 
        /*
@@ -2861,15 +2871,46 @@ static void free_frozen_page_commit(struct zone *zone,
                 * Do not attempt to take a zone lock. Let pcp->count get
                 * over high mark temporarily.
                 */
-               return;
+               return true;
        }
 
        high = nr_pcp_high(pcp, zone, batch, free_high);
        if (pcp->count < high)
-               return;
+               return true;
+
+       to_free = nr_pcp_free(pcp, batch, high, free_high);
+       while (to_free > 0 && pcp->count > 0) {
+               to_free_batched = min(to_free, batch);
+               free_pcppages_bulk(zone, to_free_batched, pcp, pindex);
+               to_free -= to_free_batched;
+
+               if (to_free <= 0 || pcp->count <= 0)
+                       break;
+
+               pcp_spin_unlock(pcp);
+               pcp_trylock_finish(*UP_flags);
+
+               pcp_trylock_prepare(*UP_flags);
+               pcp = pcp_spin_trylock(zone->per_cpu_pageset);
+               if (!pcp) {
+                       pcp_trylock_finish(*UP_flags);
+                       ret = false;
+                       break;
+               }
+
+               /*
+                * Check if this thread has been migrated to a different CPU.
+                * If that is the case, give up and indicate that the pcp is
+                * returned in an unlocked state.
+                */
+               if (smp_processor_id() != cpu) {
+                       pcp_spin_unlock(pcp);
+                       pcp_trylock_finish(*UP_flags);
+                       ret = false;
+                       break;
+               }
+       }
 
-       free_pcppages_bulk(zone, nr_pcp_free(pcp, batch, high, free_high),
-                          pcp, pindex);
        if (test_bit(ZONE_BELOW_HIGH, &zone->flags) &&
            zone_watermark_ok(zone, 0, high_wmark_pages(zone),
                              ZONE_MOVABLE, 0)) {
@@ -2887,6 +2928,7 @@ static void free_frozen_page_commit(struct zone *zone,
                    next_memory_node(pgdat->node_id) < MAX_NUMNODES)
                        atomic_set(&pgdat->kswapd_failures, 0);
        }
+       return ret;
 }
 
 /*
@@ -2934,7 +2976,9 @@ static void __free_frozen_pages(struct page *page, unsigned int order,
        pcp_trylock_prepare(UP_flags);
        pcp = pcp_spin_trylock(zone->per_cpu_pageset);
        if (pcp) {
-               free_frozen_page_commit(zone, pcp, page, migratetype, order, fpi_flags);
+               if (!free_frozen_page_commit(zone, pcp, page, migratetype,
+                                               order, fpi_flags, &UP_flags))
+                       return;
                pcp_spin_unlock(pcp);
        } else {
                free_one_page(zone, page, pfn, order, fpi_flags);
@@ -3034,8 +3078,11 @@ void free_unref_folios(struct folio_batch *folios)
                        migratetype = MIGRATE_MOVABLE;
 
                trace_mm_page_free_batched(&folio->page);
-               free_frozen_page_commit(zone, pcp, &folio->page, migratetype,
-                                       order, FPI_NONE);
+               if (!free_frozen_page_commit(zone, pcp, &folio->page,
+                               migratetype, order, FPI_NONE, &UP_flags)) {
+                       pcp = NULL;
+                       locked_zone = NULL;
+               }
        }
 
        if (pcp) {