Messages in this thread Patch in this message | | | From | Jason Low <> | Subject | [PATCH v2 2/5] sched, numa: Document usages of mm->numa_scan_seq | Date | Tue, 28 Apr 2015 13:00:21 -0700 |
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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.
Signed-off-by: Jason Low <jason.low2@hp.com> --- kernel/sched/fair.c | 12 ++++++++++++ 1 files changed, 12 insertions(+), 0 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 5a44371..794f7d7 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 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) return; @@ -2107,6 +2112,13 @@ 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. + * That's not much of an issue though, since this is just used + * for statistical sampling. Use WRITE_ONCE and READ_ONCE, which + * are not expensive, to avoid load/store tearing. + */ WRITE_ONCE(p->mm->numa_scan_seq, READ_ONCE(p->mm->numa_scan_seq) + 1); p->mm->numa_scan_offset = 0; } -- 1.7.2.5
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