Messages in this thread | | | Subject | Re: [PATCH v2 2/5] sched, numa: Document usages of mm->numa_scan_seq | From | Davidlohr Bueso <> | Date | Thu, 30 Apr 2015 11:54:07 -0700 |
| |
On Thu, 2015-04-30 at 14:42 -0400, Waiman Long wrote: > On 04/29/2015 02:45 PM, Jason Low wrote: > > On Wed, 2015-04-29 at 14:14 -0400, Waiman Long wrote: > >> On 04/28/2015 04:00 PM, Jason Low wrote: > >>> 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; > >>> } > >> READ_ONCE followed by a WRITE_ONCE won't stop load/store tearing from > >> happening unless you use an atomic instruction to do the increment. So I > >> think your comment may be a bit misleading. > > Right, the READ and WRITE operations will still be done separately and > > won't be atomic. Here, we're saying that this prevents load/store > > tearing on each of those individual write/read operations. Please let me > > know if you prefer this to be worded differently. > > I do have a question of what kind of tearing you are talking about. Do > you mean the tearing due to mm being changed in the middle of the > access? The reason why I don't like this kind of construct is that I am > not sure if > the address translation p->mm->numa_scan_seq is being done once or > twice. I looked at the compiled code and the translation is done only once. > > Anyway, the purpose of READ_ONCE and WRITE_ONCE is not for eliminating > data tearing. They are to make sure that the compiler won't compile away > data access and they are done in the order they appear in the program. I > don't think it is a good idea to associate tearing elimination with > those macros. So I would suggest removing the last sentence in your comment.
I agree. Related, Linus also had some thoughts about the _very specific_ purposes of these macros: http://www.spinics.net/lists/linux-next/msg32494.html
I also wonder why this patch is included in a set called "sched, timer: Improve scalability of itimers" ;)
Thanks, Davidlohr
| |