Re: [RFC] time: xtime_lock is held too long

Eric Dumazet
2011-05-06T22:30:14Z

Le vendredi 06 mai 2011 à 13:24 -0700, john stultz a écrit :

> So would the easier solution be to just break out timekeeper locking
> from the xtime_lock?
>
> So basically we would just add a timekeeper.lock seqlock and use it to
> protect only the timekeeping code? We can still keep xtime_lock around
> for the tick/jiffies protection (well, until tglx kills jiffies :), but
> gettimeofday and friends wouldn't be blocked for so long.
>
> That should be pretty straight forward now that the timekeeper data is
> completely static to timkeeeping.c.
>

Yes :)

I can see many cpus entering tick_do_update_jiffies64() and all are
calling write_seqlock(&xtime_lock);

Only first one can perform the work, but all others are waiting on the
spinlock, get it, change seqcount, and realize they have nothing to
do...

Meanwhile, a reader must wait that all writers are finished, because of
all seqcount changes storm.

Following patch helps. Of course we might find out why so many cpus (on
my 8 cpus machine !) are calling tick_do_update_jiffies64() at the same
time...

This is basically what I said in my first mail :

Separate logical sections to reduce windows where readers are blocked/spinning.

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index d5097c4..251b2fe 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -56,7 +56,7 @@ static void tick_do_update_jiffies64(ktime_t now)
return;

/* Reevalute with xtime_lock held */
- write_seqlock(&xtime_lock);
+ spin_lock(&xtime_lock.lock);

delta = ktime_sub(now, last_jiffies_update);
if (delta.tv64 >= tick_period.tv64) {
@@ -74,12 +74,15 @@ static void tick_do_update_jiffies64(ktime_t now)
last_jiffies_update = ktime_add_ns(last_jiffies_update,
incr * ticks);
}
+ xtime_lock.sequence++;
+ smp_wmb();
do_timer(++ticks);
-
+ smp_wmb();
+ xtime_lock.sequence++;
/* Keep the tick_next_period variable up to date */
tick_next_period = ktime_add(last_jiffies_update, tick_period);
}
- write_sequnlock(&xtime_lock);
+ spin_unlock(&xtime_lock.lock);
}

/*