Re: [PATCH bpf-next v2 07/26] rqspinlock: Add support for timeouts

[Peter Zijlstra]

On Mon, Feb 10, 2025 at 08:55:56PM -0800, Alexei Starovoitov wrote:
> On Mon, Feb 10, 2025 at 1:56 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > On Thu, Feb 06, 2025 at 02:54:15AM -0800, Kumar Kartikeya Dwivedi wrote:
> > > @@ -68,6 +71,44 @@
> > >
> > >  #include "mcs_spinlock.h"
> > >
> > > +struct rqspinlock_timeout {
> > > +     u64 timeout_end;
> > > +     u64 duration;
> > > +     u16 spin;
> > > +};
> > > +
> > > +static noinline int check_timeout(struct rqspinlock_timeout *ts)
> > > +{
> > > +     u64 time = ktime_get_mono_fast_ns();
> >
> > This is only sane if you have a TSC clocksource. If you ever manage to
> > hit the HPET fallback, you're *really* sad.
> 
> ktime_get_mono_fast_ns() is the best NMI safe time source we're aware of.
> perf, rcu, even hardlockup detector are using it.

perf is primarily using local_clock(), as is the scheduler.