Messages in this thread | | | Subject | Re: [RFC] time: xtime_lock is held too long | From | john stultz <> | Date | Fri, 06 May 2011 15:46:40 -0700 |
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On Sat, 2011-05-07 at 00:30 +0200, Eric Dumazet wrote: > 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...
Huh. So who is calling tick_do_update_jiffies64 in your case? I know the sched_tick_timer and tick_nohz_handler checks to make sure tick_do_timer_cpu == cpu to avoid exactly the thundering heard problem on the jiffies update.
There's other spots that call tick_do_update_jiffies64, but I thought those were more rare. So there may be something else wrong going on here.
> 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);
Oof.. No, this is too ugly and really just abuses the seqlock structure.
If you really want to untangle what xtime_lock protects, you need to introduce a new lock (I suggest in the timekeeper structure) to protect the timekeeping data.
Then we can refine xtime_lock to also just protect the jiffies/tick management bits as well if needed.
thanks -john
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