linux-kernel - Re: [PATCH v8 04/12] arm64: support WFET in smp_cond_relaxed

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Message-ID: <aW-KIVQ_H1mVpGHx@willie-the-truck>
Date: Tue, 20 Jan 2026 13:58:57 +0000
From: Will Deacon <will@...nel.org>
To: Ankur Arora <ankur.a.arora@...cle.com>
Cc: linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-pm@...r.kernel.org,
	bpf@...r.kernel.org, arnd@...db.de, catalin.marinas@....com,
	peterz@...radead.org, akpm@...ux-foundation.org,
	mark.rutland@....com, harisokn@...zon.com, cl@...two.org,
	ast@...nel.org, rafael@...nel.org, daniel.lezcano@...aro.org,
	memxor@...il.com, zhenglifeng1@...wei.com,
	xueshuai@...ux.alibaba.com, joao.m.martins@...cle.com,
	boris.ostrovsky@...cle.com, konrad.wilk@...cle.com
Subject: Re: [PATCH v8 04/12] arm64: support WFET in
 smp_cond_relaxed_timeout()

On Fri, Jan 09, 2026 at 11:05:06AM -0800, Ankur Arora wrote:
> 
> Will Deacon <will@...nel.org> writes:
> 
> > On Sun, Dec 14, 2025 at 08:49:11PM -0800, Ankur Arora wrote:
> >> Extend __cmpwait_relaxed() to __cmpwait_relaxed_timeout() which takes
> >> an additional timeout value in ns.
> >>
> >> Lacking WFET, or with zero or negative value of timeout we fallback
> >> to WFE.
> >>
> >> Cc: Arnd Bergmann <arnd@...db.de>
> >> Cc: Catalin Marinas <catalin.marinas@....com>
> >> Cc: Will Deacon <will@...nel.org>
> >> Cc: linux-arm-kernel@...ts.infradead.org
> >> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
> >> ---
> >>  arch/arm64/include/asm/barrier.h |  8 ++--
> >>  arch/arm64/include/asm/cmpxchg.h | 72 ++++++++++++++++++++++----------
> >>  2 files changed, 55 insertions(+), 25 deletions(-)
> >
> > Sorry, just spotted something else on this...
> >
> >> diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
> >> index 6190e178db51..fbd71cd4ef4e 100644
> >> --- a/arch/arm64/include/asm/barrier.h
> >> +++ b/arch/arm64/include/asm/barrier.h
> >> @@ -224,8 +224,8 @@ do {									\
> >>  extern bool arch_timer_evtstrm_available(void);
> >>
> >>  /*
> >> - * In the common case, cpu_poll_relax() sits waiting in __cmpwait_relaxed()
> >> - * for the ptr value to change.
> >> + * In the common case, cpu_poll_relax() sits waiting in __cmpwait_relaxed()/
> >> + * __cmpwait_relaxed_timeout() for the ptr value to change.
> >>   *
> >>   * Since this period is reasonably long, choose SMP_TIMEOUT_POLL_COUNT
> >>   * to be 1, so smp_cond_load_{relaxed,acquire}_timeout() does a
> >> @@ -234,7 +234,9 @@ extern bool arch_timer_evtstrm_available(void);
> >>  #define SMP_TIMEOUT_POLL_COUNT	1
> >>
> >>  #define cpu_poll_relax(ptr, val, timeout_ns) do {			\
> >> -	if (arch_timer_evtstrm_available())				\
> >> +	if (alternative_has_cap_unlikely(ARM64_HAS_WFXT))		\
> >> +		__cmpwait_relaxed_timeout(ptr, val, timeout_ns);	\
> >> +	else if (arch_timer_evtstrm_available())			\
> >>  		__cmpwait_relaxed(ptr, val);				\
> >
> > Don't you want to make sure that we have the event stream available for
> > __cmpwait_relaxed_timeout() too? Otherwise, a large timeout is going to
> > cause problems.
> 
> Would that help though? If called from smp_cond_load_relaxed_timeout()
> then we would wake up and just call __cmpwait_relaxed_timeout() again.

Fair enough, I can see that. Is it worth capping the maximum timeout
like we do for udelay()?

Will