linux-kernel - Re: [RFC] time: xtime

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Date:	Sat, 07 May 2011 00:30:04 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	john stultz <johnstul@...ibm.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	lkml <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Anton Blanchard <anton@...ba.org>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [RFC] time: xtime_lock is held too long

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);
 }
 
 /*





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