On Wed, Mar 30, 2016 at 07:55:47AM -0700, Paul E. McKenney wrote:
> On Tue, Mar 29, 2016 at 06:49:08AM -0700, Paul E. McKenney wrote:
> > On Mon, Mar 28, 2016 at 05:28:14PM -0700, Paul E. McKenney wrote:
> > > On Mon, Mar 28, 2016 at 05:25:18PM -0700, Paul E. McKenney wrote:
> > > > On Mon, Mar 28, 2016 at 06:08:41AM -0700, Paul E. McKenney wrote:
> > > > > On Mon, Mar 28, 2016 at 08:25:47AM +0200, Peter Zijlstra wrote:
> > > > > > On Sun, Mar 27, 2016 at 02:06:41PM -0700, Paul E. McKenney wrote:
> > > >
> > > > [ . . . ]
> > > >
> > > > > > > OK, so I should instrument migration_call() if I get the repro
> > > > > > > rate up?
> > > > > >
> > > > > > Can do, maybe try the below first. (yes I know how long it all
> > > > > > takes :/)
> > > > >
> > > > > OK, will run this today, then run calibration for last night's run
> > > > > this
> > > > > evening.
> >
> > And of 18 two-hour runs, there were five failures, or about 28%.
> > That said, I don't have even one significant digit on the failure rate,
> > as 5 of 18 is within the 95% confidence limits for a failure probability
> > as low as 12.5% and as high as 47%.
>
> And after last night's run, this is narrowed down to between 23% and 38%,
> which is close enough. Average is 30%, 18 failures in 60 runs.
>
> Next step is to test Peter's patch some more. Might take a couple of
> night's worth of runs to get statistical significance. After which
> it will be time to rebase to 4.6-rc1.
And the first night was not so good: 6 failures out of 24 runs. Adding
this to the 1-of-10 earlier gets 7 failures of 34. Here are how things
stack up given the range of base failure estimates:
Low 95% bound of 23%: 84% confidence.
Actual measurement of 30%: 92% confidence.
High 95% bound of 38%: 98% confidence.
So there is still some chance that Peter's patch is helping. I will
run for one more evening, after which it will be time to move forward
to 4.6-rc1.
Thanx, Paul
> > However, the previous night's runs gave 7 failures in 24 two-hour runs,
> > for about a 29% failure rate. There is thus a good probability that my
> > disabling of TIF_POLLING_NRFLAG had no effect whatsoever, tantalizing
> > though that possibility might have been.
> >
> > (FWIW, I use the pdf_binomial() and quantile_binomial() functions in
> > maxima for computing this stuff. Similar stuff is no doubt available
> > in other math/stat packages as well.)
> >
> > So we have bugs, but not much idea where they are. Situation normal.
> >
> > Other thoughts?
> >
> > Thanx, Paul
> >
> > > > And there was one failure out of ten runs. If last night's failure rate
> > > > was typical (7 of 24), then I believe we can be about 87% confident that
> > > > this change helped. That isn't all that confident, but...
> > >
> > > And, as Murphy would have it, the instrumentation didn't trigger. I just
> > > got the usual stall-warning messages with a starving RCU grace-period
> > > kthread.
> > >
> > > Thanx, Paul
> > >
> > > > Tested-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
> > > >
> > > > So what to run tonight?
> > > >
> > > > The most sane approach would be to run stock in order to get a baseline
> > > > failure rate. It is tempting to run more of Peter's patch, but part of
> > > > the problem is that we don't know the current baseline.
> > > >
> > > > So baseline it is...
> > > >
> > > > Thanx, Paul
> > > >
> > > > > Speaking of which, last night's run (disabling TIF_POLLING_NRFLAG)
> > > > > consisted of 24 two-hour runs. Six of them had hard hangs, and
> > > > > another
> > > > > had a hang that eventually unhung of its own accord. I believe that
> > > > > this
> > > > > is significantly fewer failures than from a stock kernel, but I could
> > > > > be wrong, and it will take some serious testing to give statistical
> > > > > confidence for whatever conclusion is correct.
> > > > >
> > > > > > > > The other interesting case would be resched_cpu(), which uses
> > > > > > > > set_nr_and_not_polling() to kick a remote cpu to call
> > > > > > > > schedule(). It
> > > > > > > > atomically sets TIF_NEED_RESCHED and returns if
> > > > > > > > TIF_POLLING_NRFLAG was
> > > > > > > > not set. If indeed not, it will send an IPI.
> > > > > > > >
> > > > > > > > This assumes the idle 'exit' path will do the same as the IPI
> > > > > > > > does; and
> > > > > > > > if you look at cpu_idle_loop() it does indeed do both
> > > > > > > > preempt_fold_need_resched() and sched_ttwu_pending().
> > > > > > > >
> > > > > > > > Note that one cannot rely on irq_enter()/irq_exit() being
> > > > > > > > called for the
> > > > > > > > scheduler IPI.
> > > > > > >
> > > > > > > OK, thank you for the info! Any specific debug actions?
> > > > > >
> > > > > > Dunno, something like the below should bring visibility into the
> > > > > > (lockless) wake_list thingy.
> > > > > >
> > > > > > So these trace_printk()s should happen between trace_sched_waking()
> > > > > > and
> > > > > > trace_sched_wakeup() (I've not fully read the thread, but ISTR you
> > > > > > had
> > > > > > some traces with these here thingies on).
> > > > > >
> > > > > > ---
> > > > > > arch/x86/include/asm/bitops.h | 6 ++++--
> > > > > > kernel/sched/core.c | 9 +++++++++
> > > > > > 2 files changed, 13 insertions(+), 2 deletions(-)
> > > > > >
> > > > > > diff --git a/arch/x86/include/asm/bitops.h
> > > > > > b/arch/x86/include/asm/bitops.h
> > > > > > index 7766d1cf096e..5345784d5e41 100644
> > > > > > --- a/arch/x86/include/asm/bitops.h
> > > > > > +++ b/arch/x86/include/asm/bitops.h
> > > > > > @@ -112,11 +112,13 @@ clear_bit(long nr, volatile unsigned long
> > > > > > *addr)
> > > > > > if (IS_IMMEDIATE(nr)) {
> > > > > > asm volatile(LOCK_PREFIX "andb %1,%0"
> > > > > > : CONST_MASK_ADDR(nr, addr)
> > > > > > - : "iq" ((u8)~CONST_MASK(nr)));
> > > > > > + : "iq" ((u8)~CONST_MASK(nr))
> > > > > > + : "memory");
> > > > > > } else {
> > > > > > asm volatile(LOCK_PREFIX "btr %1,%0"
> > > > > > : BITOP_ADDR(addr)
> > > > > > - : "Ir" (nr));
> > > > > > + : "Ir" (nr)
> > > > > > + : "memory");
> > > > > > }
> > > > > > }
> > > > >
> > > > > Is the above addition of "memory" strictly for the debug below, or is
> > > > > it also a potential fix?
> > > > >
> > > > > Starting it up regardless, but figured I should ask!
> > > > >
> > > > > Thanx, Paul
> > > > >
> > > > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > > > > index 0b21e7a724e1..b446f73c530d 100644
> > > > > > --- a/kernel/sched/core.c
> > > > > > +++ b/kernel/sched/core.c
> > > > > > @@ -1669,6 +1669,7 @@ void sched_ttwu_pending(void)
> > > > > > while (llist) {
> > > > > > p = llist_entry(llist, struct task_struct, wake_entry);
> > > > > > llist = llist_next(llist);
> > > > > > + trace_printk("waking %d\n", p->pid);
> > > > > > ttwu_do_activate(rq, p, 0);
> > > > > > }
> > > > > >
> > > > > > @@ -1719,6 +1720,7 @@ static void ttwu_queue_remote(struct
> > > > > > task_struct *p, int cpu)
> > > > > > struct rq *rq = cpu_rq(cpu);
> > > > > >
> > > > > > if (llist_add(&p->wake_entry, &cpu_rq(cpu)->wake_list)) {
> > > > > > + trace_printk("queued %d for waking on %d\n", p->pid,
> > > > > > cpu);
> > > > > > if (!set_nr_if_polling(rq->idle))
> > > > > > smp_send_reschedule(cpu);
> > > > > > else
> > > > > > @@ -5397,10 +5399,17 @@ migration_call(struct notifier_block *nfb,
> > > > > > unsigned long action, void *hcpu)
> > > > > > migrate_tasks(rq);
> > > > > > BUG_ON(rq->nr_running != 1); /* the migration thread */
> > > > > > raw_spin_unlock_irqrestore(&rq->lock, flags);
> > > > > > +
> > > > > > + /* really bad m'kay */
> > > > > > + WARN_ON(!llist_empty(&rq->wake_list));
> > > > > > +
> > > > > > break;
> > > > > >
> > > > > > case CPU_DEAD:
> > > > > > calc_load_migrate(rq);
> > > > > > +
> > > > > > + /* more bad */
> > > > > > + WARN_ON(!llist_empty(&rq->wake_list));
> > > > > > break;
> > > > > > #endif
> > > > > > }
> > > > > >
