sched/core: Add debugging code to catch missing update_rq_clock() calls

There's no diagnostic checks for figuring out when we've accidentally
missed update_rq_clock() calls. Let's add some by piggybacking on the
rq_*pin_lock() wrappers.

The idea behind the diagnostic checks is that upon pining rq lock the
rq clock should be updated, via update_rq_clock(), before anybody
reads the clock with rq_clock() or rq_clock_task().

The exception to this rule is when updates have explicitly been
disabled with the rq_clock_skip_update() optimisation.

There are some functions that only unpin the rq lock in order to grab
some other lock and avoid deadlock. In that case we don't need to
update the clock again and the previous diagnostic state can be
carried over in rq_repin_lock() by saving the state in the rq_flags
context.

Since this patch adds a new clock update flag and some already exist
in rq::clock_skip_update, that field has now been renamed. An attempt
has been made to keep the flag manipulation code small and fast since
it's used in the heart of the __schedule() fast path.

For the !CONFIG_SCHED_DEBUG case the only object code change (other
than addresses) is the following change to reset RQCF_ACT_SKIP inside
of __schedule(),

  -       c7 83 38 09 00 00 00    movl   $0x0,0x938(%rbx)
  -       00 00 00
  +       83 a3 38 09 00 00 fc    andl   $0xfffffffc,0x938(%rbx)

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Byungchul Park <byungchul.park@lge.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Luca Abeni <luca.abeni@unitn.it>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Wanpeng Li <wanpeng.li@hotmail.com>
Cc: Yuyang Du <yuyang.du@intel.com>
Link: http://lkml.kernel.org/r/20160921133813.31976-8-matt@codeblueprint.co.uk
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 7 insertions, 4 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d2338927773a..a129b34b8206 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -102,9 +102,12 @@ void update_rq_clock(struct rq *rq)
 
 	lockdep_assert_held(&rq->lock);
 
-	if (rq->clock_skip_update & RQCF_ACT_SKIP)
+	if (rq->clock_update_flags & RQCF_ACT_SKIP)
 		return;
 
+#ifdef CONFIG_SCHED_DEBUG
+	rq->clock_update_flags |= RQCF_UPDATED;
+#endif
 	delta = sched_clock_cpu(cpu_of(rq)) - rq->clock;
 	if (delta < 0)
 		return;
@@ -2889,7 +2892,7 @@ context_switch(struct rq *rq, struct task_struct *prev,
 		rq->prev_mm = oldmm;
 	}
 
-	rq->clock_skip_update = 0;
+	rq->clock_update_flags &= ~(RQCF_ACT_SKIP|RQCF_REQ_SKIP);
 
 	/*
 	 * Since the runqueue lock will be released by the next
@@ -3364,7 +3367,7 @@ static void __sched notrace __schedule(bool preempt)
 	raw_spin_lock(&rq->lock);
 	rq_pin_lock(rq, &rf);
 
-	rq->clock_skip_update <<= 1; /* promote REQ to ACT */
+	rq->clock_update_flags <<= 1; /* promote REQ to ACT */
 
 	switch_count = &prev->nivcsw;
 	if (!preempt && prev->state) {
@@ -3405,7 +3408,7 @@ static void __sched notrace __schedule(bool preempt)
 		trace_sched_switch(preempt, prev, next);
 		rq = context_switch(rq, prev, next, &rf); /* unlocks the rq */
 	} else {
-		rq->clock_skip_update = 0;
+		rq->clock_update_flags &= ~(RQCF_ACT_SKIP|RQCF_REQ_SKIP);
 		rq_unpin_lock(rq, &rf);
 		raw_spin_unlock_irq(&rq->lock);
 	}