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Date: Tue, 11 Feb 2025 11:11:46 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Kumar Kartikeya Dwivedi <memxor@gmail.com>, bpf <bpf@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Barret Rhoden <brho@google.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Will Deacon <will@kernel.org>, Waiman Long <llong@redhat.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Andrii Nakryiko <andrii@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Martin KaFai Lau <martin.lau@kernel.org>,
	Eduard Zingerman <eddyz87@gmail.com>,
	"Paul E. McKenney" <paulmck@kernel.org>, Tejun Heo <tj@kernel.org>,
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	linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
	Kernel Team <kernel-team@meta.com>
Subject: Re: [PATCH bpf-next v2 07/26] rqspinlock: Add support for timeouts
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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.