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

Alexei Starovoitov
2025-02-11T18:01:10Z

On Tue, Feb 11, 2025 at 2:11 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> 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@infradead.org> 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.

We considered it, but I think it won't tick when irqs are disabled,
since the generic part is jiffies based ?