@@ -676,12 +676,15 @@ enum zone_watermarks {
#define high_wmark_pages(z) (z->_watermark[WMARK_HIGH] + z->watermark_boost)
#define wmark_pages(z, i) (z->_watermark[i] + z->watermark_boost)
+#define PCPF_PREV_FREE_HIGH_ORDER 0x01
+
struct per_cpu_pages {
spinlock_t lock; /* Protects lists field */
int count; /* number of pages in the list */
int high; /* high watermark, emptying needed */
int batch; /* chunk size for buddy add/remove */
- short free_factor; /* batch scaling factor during free */
+ u8 flags; /* protected by pcp->lock */
+ u8 free_factor; /* batch scaling factor during free */
#ifdef CONFIG_NUMA
short expire; /* When 0, remote pagesets are drained */
#endif
@@ -2370,7 +2370,7 @@ static void free_unref_page_commit(struct zone *zone, struct per_cpu_pages *pcp,
{
int high;
int pindex;
- bool free_high;
+ bool free_high = false;
__count_vm_events(PGFREE, 1 << order);
pindex = order_to_pindex(migratetype, order);
@@ -2383,8 +2383,13 @@ static void free_unref_page_commit(struct zone *zone, struct per_cpu_pages *pcp,
* freeing without allocation. The remainder after bulk freeing
* stops will be drained from vmstat refresh context.
*/
- free_high = (pcp->free_factor && order && order <= PAGE_ALLOC_COSTLY_ORDER);
-
+ if (order && order <= PAGE_ALLOC_COSTLY_ORDER) {
+ free_high = (pcp->free_factor &&
+ (pcp->flags & PCPF_PREV_FREE_HIGH_ORDER));
+ pcp->flags |= PCPF_PREV_FREE_HIGH_ORDER;
+ } else if (pcp->flags & PCPF_PREV_FREE_HIGH_ORDER) {
+ pcp->flags &= ~PCPF_PREV_FREE_HIGH_ORDER;
+ }
high = nr_pcp_high(pcp, zone, free_high);
if (pcp->count >= high) {
free_pcppages_bulk(zone, nr_pcp_free(pcp, high, free_high), pcp, pindex);
In commit f26b3fa04611 ("mm/page_alloc: limit number of high-order pages on PCP during bulk free"), the PCP (Per-CPU Pageset) will be drained when PCP is mostly used for high-order pages freeing to improve the cache-hot pages reusing between page allocation and freeing CPUs. But, the PCP draining mechanism may be triggered unexpectedly when process exits. With some customized trace point, it was found that PCP draining (free_high == true) was triggered with the order-1 page freeing with the following call stack, => free_unref_page_commit => free_unref_page => __mmdrop => exit_mm => do_exit => do_group_exit => __x64_sys_exit_group => do_syscall_64 Checking the source code, this is the page table PGD freeing (mm_free_pgd()). It's a order-1 page freeing if CONFIG_PAGE_TABLE_ISOLATION=y. Which is a common configuration for security. Just before that, page freeing with the following call stack was found, => free_unref_page_commit => free_unref_page_list => release_pages => tlb_batch_pages_flush => tlb_finish_mmu => exit_mmap => __mmput => exit_mm => do_exit => do_group_exit => __x64_sys_exit_group => do_syscall_64 So, when a process exits, - a large number of user pages of the process will be freed without page allocation, it's highly possible that pcp->free_factor becomes > 0. - after freeing all user pages, the PGD will be freed, which is a order-1 page freeing, PCP will be drained. All in all, when a process exits, it's high possible that the PCP will be drained. This is an unexpected behavior. To avoid this, in the patch, the PCP draining will only be triggered for 2 consecutive high-order page freeing. On a 2-socket Intel server with 224 logical CPU, we run 8 kbuild instances in parallel (each with `make -j 28`) in 8 cgroup. This simulates the kbuild server that is used by 0-Day kbuild service. With the patch, the cycles% of the spinlock contention (mostly for zone lock) decreases from 13.5% to 10.6% (with PCP size == 361). The number of PCP draining for high order pages freeing (free_high) decreases 80.8%. This helps network workload too for reduced zone lock contention. On a 2-socket Intel server with 128 logical CPU, with the patch, the network bandwidth of the UNIX (AF_UNIX) test case of lmbench test suite with 16-pair processes increase 17.1%. The cycles% of the spinlock contention (mostly for zone lock) decreases from 50.0% to 45.8%. The number of PCP draining for high order pages freeing (free_high) decreases 27.4%. The cache miss rate keeps 0.3%. Signed-off-by: "Huang, Ying" <ying.huang@intel.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Mel Gorman <mgorman@techsingularity.net> Cc: Vlastimil Babka <vbabka@suse.cz> Cc: David Hildenbrand <david@redhat.com> Cc: Johannes Weiner <jweiner@redhat.com> Cc: Dave Hansen <dave.hansen@linux.intel.com> Cc: Michal Hocko <mhocko@suse.com> Cc: Pavel Tatashin <pasha.tatashin@soleen.com> Cc: Matthew Wilcox <willy@infradead.org> Cc: Christoph Lameter <cl@linux.com> --- include/linux/mmzone.h | 5 ++++- mm/page_alloc.c | 11 ++++++++--- 2 files changed, 12 insertions(+), 4 deletions(-)