[tip:sched/core] sched/numa: Do not trap hinting faults for shared libraries

[tip-bot for Mel Gorman]

Commit-ID:  4591ce4f2d22dc9de7a6719161ce409b5fd1caac
Gitweb:     http://git.kernel.org/tip/4591ce4f2d22dc9de7a6719161ce409b5fd1caac
Author:     Mel Gorman <mgorman@xxxxxxx>
AuthorDate: Mon, 7 Oct 2013 11:29:13 +0100
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Wed, 9 Oct 2013 12:40:42 +0200

sched/numa: Do not trap hinting faults for shared libraries

NUMA hinting faults will not migrate a shared executable page mapped by
multiple processes on the grounds that the data is probably in the CPU
cache already and the page may just bounce between tasks running on multipl
nodes. Even if the migration is avoided, there is still the overhead of
trapping the fault, updating the statistics, making scheduler placement
decisions based on the information etc. If we are never going to migrate
the page, it is overhead for no gain and worse a process may be placed on
a sub-optimal node for shared executable pages. This patch avoids trapping
faults for shared libraries entirely.

Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
Reviewed-by: Rik van Riel <riel@xxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/1381141781-10992-36-git-send-email-mgorman@xxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 kernel/sched/fair.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index de9b4d8..fbc0c84 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1231,6 +1231,16 @@ void task_numa_work(struct callback_head *work)
 		if (!vma_migratable(vma) || !vma_policy_mof(p, vma))
 			continue;
 
+		/*
+		 * Shared library pages mapped by multiple processes are not
+		 * migrated as it is expected they are cache replicated. Avoid
+		 * hinting faults in read-only file-backed mappings or the vdso
+		 * as migrating the pages will be of marginal benefit.
+		 */
+		if (!vma->vm_mm ||
+		    (vma->vm_file && (vma->vm_flags & (VM_READ|VM_WRITE)) == (VM_READ)))
+			continue;
+
 		do {
 			start = max(start, vma->vm_start);
 			end = ALIGN(start + (pages << PAGE_SHIFT), HPAGE_SIZE);
--
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