Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752195AbbD2SpK (ORCPT ); Wed, 29 Apr 2015 14:45:10 -0400 Received: from g2t2353.austin.hp.com ([15.217.128.52]:49311 "EHLO g2t2353.austin.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751324AbbD2SpH (ORCPT ); Wed, 29 Apr 2015 14:45:07 -0400 Message-ID: <1430333101.8722.32.camel@j-VirtualBox> Subject: Re: [PATCH v2 2/5] sched, numa: Document usages of mm->numa_scan_seq From: Jason Low To: Waiman Long Cc: Peter Zijlstra , Ingo Molnar , Thomas Gleixner , linux-kernel@vger.kernel.org, "Paul E. McKenney" , Andrew Morton , Oleg Nesterov , Frederic Weisbecker , Mel Gorman , Rik van Riel , Steven Rostedt , Preeti U Murthy , Mike Galbraith , Davidlohr Bueso , Aswin Chandramouleeswaran , Scott J Norton , jason.low2@hp.com Date: Wed, 29 Apr 2015 11:45:01 -0700 In-Reply-To: <55411F91.6050101@hp.com> References: <1430251224-5764-1-git-send-email-jason.low2@hp.com> <1430251224-5764-3-git-send-email-jason.low2@hp.com> <55411F91.6050101@hp.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.2.3-0ubuntu6 Content-Transfer-Encoding: 7bit Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2404 Lines: 56 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 > > --- > > 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/