Commit 7e5a2c17 authored Apr 30, 2015 by Jason Low Committed by Ingo Molnar May 08, 2015

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@kernel.org>
Signed-off-by: Jason Low <jason.low2@hp.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Aswin Chandramouleeswaran <aswin@hp.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Preeti U Murthy <preeti@linux.vnet.ibm.com>
Cc: Scott J Norton <scott.norton@hp.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Waiman Long <waiman.long@hp.com>
Link: http://lkml.kernel.org/r/1430440094.2475.61.camel@j-VirtualBox


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 316c1608

kernel/sched/fair.c

+13 −0

Original line number	Diff line number	Diff line
		@@ -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)
		return;
		@@ -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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