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Message-ID: <20250211101146.GB29593@noisy.programming.kicks-ass.net>
Date: Tue, 11 Feb 2025 11:11:46 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
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 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.