Messages in this thread Patch in this message | | | Subject | Re: [RFC] time: xtime_lock is held too long | From | Eric Dumazet <> | Date | Sat, 07 May 2011 00:30:04 +0200 |
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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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