Re: [PATCH 1/2] timekeeping: Avoid taking lock in NMI path with CONFIG_DEBUG_TIMEKEEPING

[Alexei Starovoitov]

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 <rostedt@xxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: stable <stable@xxxxxxxxxxxxxxx> # 4.1+
> Reported-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Reported-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
> ---
>  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 <bgregg@xxxxxxxxxxx>

# 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)