Messages in this thread | | | Date | Mon, 17 Oct 2011 23:00:35 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: Linux 3.1-rc9 |
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On Mon, 17 Oct 2011, Peter Zijlstra wrote:
> On Mon, 2011-10-17 at 11:31 -0700, Linus Torvalds wrote: > > On Mon, Oct 17, 2011 at 10:54 AM, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote: > > > > > > I could of course propose this... but I really won't since I'm half > > > retching by now.. ;-) > > > > Wow. Is this "ugly and fragile code week" and I just didn't get the memo? > > Do I get a price? > > > I do wonder if we might not fix the problem by just taking the > > *existing* lock in the right order? > > > > IOW, how nasty would be it be to make "scheduler_tick()" just get the > > cputimer->lock outside or rq->lock? > > > > Sure, we'd hold that lock *much* longer than we need, but how much do > > we care? Is that a lock that gets contention? It migth be the simple > > solution for now - I *would* like to get 3.1 out.. > > Ah, sadly the tick isn't the only one with the inverted callchain, > pretty much every callchain in the scheduler ends up in update_curr() > one way or another. > > The easier way around might be something like this... even when two > threads in a process race to enable this clock the the wasted time is > pretty much of the same order as we would otherwise have wasted spinning > on the lock and the update_gt_cputime() think would end up moving the > clock fwd to the latest outcome any which way. > > Humm,. Thomas anything?
No, that should work. It does not make that call path more racy against exit, which is another trainwreck at least on 32bit machines which I discovered while looking for the problems with your patch.
thread_group_cputime() reads task->signal->utime/stime/sum_sched_runtime
These fields are updated in __exit_signal() w/o holding task->signal->cputimer.lock. So nothing prevents that these values change while we read them.
All callers of thread_group_cputime() except the scheduler callpath hold sighand lock, which is also taken in __exit_signal().
So your patch does not make that particular case worse.
That said, I really need some sleep before I can make a final judgement on that horror. The call paths are such an intermingled mess that it's not funny anymore. I do that tomorrow morning first thing.
Thanks,
tglx
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