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

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Message-ID: <CAADnVQJjNzMapuJvOni2gPHhxRo=C7_FCXBpG2bb9ctN8L7rcA@mail.gmail.com>
Date: Tue, 11 Feb 2025 10:00:57 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Kumar Kartikeya Dwivedi <memxor@...il.com>, bpf <bpf@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, Barret Rhoden <brho@...gle.com>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Will Deacon <will@...nel.org>, 
	Waiman Long <llong@...hat.com>, Alexei Starovoitov <ast@...nel.org>, Andrii Nakryiko <andrii@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Martin KaFai Lau <martin.lau@...nel.org>, 
	Eduard Zingerman <eddyz87@...il.com>, "Paul E. McKenney" <paulmck@...nel.org>, Tejun Heo <tj@...nel.org>, 
	Josh Don <joshdon@...gle.com>, Dohyun Kim <dohyunkim@...gle.com>, 
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>, Kernel Team <kernel-team@...a.com>
Subject: Re: [PATCH bpf-next v2 07/26] rqspinlock: Add support for timeouts

On Tue, Feb 11, 2025 at 2:11 AM Peter Zijlstra <peterz@...radead.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@...radead.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 ?