[tip:sched/core] sched/numa: Document usages of mm->numa_scan_seq

From: tip-bot for Jason Low
Date: Fri May 08 2015 - 09:23:24 EST

Commit-ID: 7e5a2c1729f1612618ed236249a15bf15f309325
Gitweb: http://git.kernel.org/tip/7e5a2c1729f1612618ed236249a15bf15f309325
Author: Jason Low <jason.low2@xxxxxx>
AuthorDate: Thu, 30 Apr 2015 17:28:14 -0700
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Fri, 8 May 2015 12:13:13 +0200

sched/numa: Document usages of mm->numa_scan_seq

The p->mm->numa_scan_seq is accessed using READ_ONCE/WRITE_ONCE
and modified without exclusive access. It is not clear why it is
accessed this way. This patch provides some documentation on that.

Suggested-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Jason Low <jason.low2@xxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Acked-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Acked-by: Rik van Riel <riel@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Aswin Chandramouleeswaran <aswin@xxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Mike Galbraith <umgwanakikbuti@xxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Preeti U Murthy <preeti@xxxxxxxxxxxxxxxxxx>
Cc: Scott J Norton <scott.norton@xxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Waiman Long <waiman.long@xxxxxx>
Link: http://lkml.kernel.org/r/1430440094.2475.61.camel@j-VirtualBox
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
kernel/sched/fair.c | 13 +++++++++++++
1 file changed, 13 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d6915a0..f18ddb7 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1794,6 +1794,11 @@ static void task_numa_placement(struct task_struct *p)
u64 runtime, period;
spinlock_t *group_lock = NULL;

+ /*
+ * The p->mm->numa_scan_seq field gets updated without
+ * exclusive access. Use READ_ONCE() here to ensure
+ * that the field is read in a single access:
+ */
seq = READ_ONCE(p->mm->numa_scan_seq);
if (p->numa_scan_seq == seq)
@@ -2107,6 +2112,14 @@ void task_numa_fault(int last_cpupid, int mem_node, int pages, int flags)

static void reset_ptenuma_scan(struct task_struct *p)
+ /*
+ * We only did a read acquisition of the mmap sem, so
+ * p->mm->numa_scan_seq is written to without exclusive access
+ * and the update is not guaranteed to be atomic. That's not
+ * much of an issue though, since this is just used for
+ * statistical sampling. Use READ_ONCE/WRITE_ONCE, which are not
+ * expensive, to avoid any form of compiler optimizations:
+ */
WRITE_ONCE(p->mm->numa_scan_seq, READ_ONCE(p->mm->numa_scan_seq) + 1);
p->mm->numa_scan_offset = 0;
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