On Tue, Aug 23, 2016 at 04:08:21PM -0700, John Stultz wrote: > When I added some extra sanity checking in timekeeping_get_ns() under > CONFIG_DEBUG_TIMEKEEPING, I missed that the NMI safe __ktime_get_fast_ns() > method was using timekeeping_get_ns(). > > Thus the locking added to the debug checks broke the NMI-safety of > __ktime_get_fast_ns(). > > This patch open-codes the timekeeping_get_ns() logic for > __ktime_get_fast_ns(), so can avoid any deadlocks in NMI. > > Cc: Steven Rostedt <[email protected]> > Cc: Peter Zijlstra <[email protected]> > Cc: Ingo Molnar <[email protected]> > Cc: Thomas Gleixner <[email protected]> > Cc: stable <[email protected]> # 4.1+ > Reported-by: Steven Rostedt <[email protected]> > Reported-by: Peter Zijlstra <[email protected]> > Signed-off-by: John Stultz <[email protected]> > --- > kernel/time/timekeeping.c | 5 ++++- > 1 file changed, 4 insertions(+), 1 deletion(-) > > diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c > index 3b65746..e07fb09 100644 > --- a/kernel/time/timekeeping.c > +++ b/kernel/time/timekeeping.c > @@ -401,7 +401,10 @@ static __always_inline u64 __ktime_get_fast_ns(struct > tk_fast *tkf) > do { > seq = raw_read_seqcount_latch(&tkf->seq); > tkr = tkf->base + (seq & 0x01); > - now = ktime_to_ns(tkr->base) + timekeeping_get_ns(tkr); > + now = ktime_to_ns(tkr->base); > + > + now += clocksource_delta(tkr->read(tkr->clock), > + tkr->cycle_last, tkr->mask);
we're seeing the time jumping backwards between __ktime_get_fast_ns calls. and looks like this patch broke it, since delta is being added to ns. It seems it should be: now += timekeeping_delta_to_ns(clocksource_delta(...)); or better fix possible? Reported-by: Brendan Gregg <[email protected]> # perf record -F 9 --cpu 0 --clockid CLOCK_MONOTONIC -- sleep 1 # perf script swapper 0 [000] 2942480.468981: 111111111 cpu-clock: ffffffff810013aa xen_hypercall_sched_op+0xa (vmlinux) swapper 0 [000] 2942480.471468: 111111111 cpu-clock: ffffffff810013aa xen_hypercall_sched_op+0xa (vmlinux) swapper 0 [000] 2942480.473959: 111111111 cpu-clock: ffffffff810013aa xen_hypercall_sched_op+0xa (vmlinux) swapper 0 [000] 2942480.475202: 111111111 cpu-clock: ffffffff810013aa xen_hypercall_sched_op+0xa (vmlinux) swapper 0 [000] 2942480.477691: 111111111 cpu-clock: ffffffff810013aa xen_hypercall_sched_op+0xa (vmlinux) swapper 0 [000] 2942481.470226: 111111111 cpu-clock: ffffffff810013aa xen_hypercall_sched_op+0xa (vmlinux) swapper 0 [000] 2942481.472569: 111111111 cpu-clock: ffffffff810013aa xen_hypercall_sched_op+0xa (vmlinux) swapper 0 [000] 2942481.476460: 111111111 cpu-clock: ffffffff810013aa xen_hypercall_sched_op+0xa (vmlinux) swapper 0 [000] 2942481.478943: 111111111 cpu-clock: ffffffff810013aa xen_hypercall_sched_op+0xa (vmlinux) Note the timestamp jump. fixed (rolled back patch): # perf record -F 9 --cpu 0 --clockid CLOCK_MONOTONIC -- sleep 1 # perf script swapper 0 [000] 589.729663: 111111111 cpu-clock: ffffffff810013aa xen_hypercall_sched_op+0xa (vmlinux) swapper 0 [000] 589.840772: 111111111 cpu-clock: ffffffff810013aa xen_hypercall_sched_op+0xa (vmlinux) swapper 0 [000] 589.951883: 111111111 cpu-clock: ffffffff810013aa xen_hypercall_sched_op+0xa (vmlinux) swapper 0 [000] 590.062994: 111111111 cpu-clock: ffffffff810013aa xen_hypercall_sched_op+0xa (vmlinux) swapper 0 [000] 590.174075: 111111111 cpu-clock: ffffffff810013aa xen_hypercall_sched_op+0xa (vmlinux) swapper 0 [000] 590.285217: 111111111 cpu-clock: ffffffff810013aa xen_hypercall_sched_op+0xa (vmlinux) swapper 0 [000] 590.396328: 111111111 cpu-clock: ffffffff810013aa xen_hypercall_sched_op+0xa (vmlinux) swapper 0 [000] 590.507439: 111111111 cpu-clock: ffffffff810013aa xen_hypercall_sched_op+0xa (vmlinux) swapper 0 [000] 590.618550: 111111111 cpu-clock: ffffffff810013aa xen_hypercall_sched_op+0xa (vmlinux)
