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Message-ID: <87plctwq7x.fsf@oracle.com>
Date: Sun, 17 Aug 2025 15:14:26 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: Catalin Marinas <catalin.marinas@....com>
Cc: Ankur Arora <ankur.a.arora@...cle.com>, linux-kernel@...r.kernel.org,
linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
bpf@...r.kernel.org, arnd@...db.de, will@...nel.org,
peterz@...radead.org, akpm@...ux-foundation.org, mark.rutland@....com,
harisokn@...zon.com, cl@...two.org, ast@...nel.org, memxor@...il.com,
zhenglifeng1@...wei.com, xueshuai@...ux.alibaba.com,
joao.m.martins@...cle.com, boris.ostrovsky@...cle.com,
konrad.wilk@...cle.com, rafael@...nel.org, daniel.lezcano@...aro.org
Subject: Re: [PATCH v3 1/5] asm-generic: barrier: Add
smp_cond_load_relaxed_timewait()
Catalin Marinas <catalin.marinas@....com> writes:
> On Thu, Aug 14, 2025 at 12:30:36AM -0700, Ankur Arora wrote:
>> Catalin Marinas <catalin.marinas@....com> writes:
>> > On Mon, Aug 11, 2025 at 02:15:56PM -0700, Ankur Arora wrote:
>> >> Catalin Marinas <catalin.marinas@....com> writes:
>> >> > Also I feel the spinning added to poll_idle() is more of an architecture
>> >> > choice as some CPUs could not cope with local_clock() being called too
>> >> > frequently.
>> >>
>> >> Just on the frequency point -- I think it might be a more general
>> >> problem that just on specific architectures.
>> >>
>> >> Architectures with GENERIC_SCHED_CLOCK could use a multitude of
>> >> clocksources and from a quick look some of them do iomem reads.
>> >> (AFAICT GENERIC_SCHED_CLOCK could also be selected by the clocksource
>> >> itself, so an architecture header might not need to be an arch choice
>> >> at all.)
>> >>
>> >> Even for something like x86 which doesn't use GENERIC_SCHED_CLOCK,
>> >> we might be using tsc or jiffies or paravirt-clock all of which would
>> >> have very different performance characteristics. Or, just using a
>> >> clock more expensive than local_clock(); rqspinlock uses
>> >> ktime_get_mono_fast_ns().
>> >>
>> >> So, I feel we do need a generic rate limiter.
>> >
>> > That's a good point but the rate limiting is highly dependent on the
>> > architecture, what a CPU does in the loop, how fast a loop iteration is.
>> >
>> > That's why I'd keep it hidden in the arch code.
>>
>> Yeah, this makes sense. However, I would like to keep as much of the
>> code that does this common.
>
> You can mimic what poll_idle() does for x86 in the generic
> implementation, maybe with some comment referring to the poll_idle() CPU
> usage of calling local_clock() in a loop. However, allow the arch code
> to override the whole implementation and get rid of the policy. If an
> arch wants to spin for some power reason, it can do it itself. The code
> duplication for a while loop is much more readable than a policy setting
> some spin/wait parameters just to have a single spin loop. If at some
> point we see some pattern, we could revisit the common code.
>
> For arm64, I doubt the extra spinning makes any difference. Our
> cpu_relax() doesn't do anything (almost), it's probably about the same
> cost as reading the monotonic clock. I also see a single definition
> close enough to the logic in __delay() on arm64. It would be more
> readable than a policy callback setting wait/spin with a separate call
> for actually waiting. Well, gut feel, let's see how that would look
> like.
So, I tried to pare back the code and the following (untested) is
what I came up with. Given the straight-forward rate-limiting, and the
current users not needing accurate timekeeping, this uses a
bool time_check_expr. Figured I'd keep it simple until someone actually
needs greater complexity as you suggested.
diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
index d4f581c1e21d..e8793347a395 100644
--- a/include/asm-generic/barrier.h
+++ b/include/asm-generic/barrier.h
@@ -273,6 +273,34 @@ do { \
})
#endif
+
+#ifndef SMP_TIMEWAIT_SPIN_COUNT
+#define SMP_TIMEWAIT_SPIN_COUNT 200
+#endif
+
+#ifndef smp_cond_load_relaxed_timewait
+#define smp_cond_load_relaxed_timewait(ptr, cond_expr, \
+ time_check_expr) \
+({ \
+ typeof(ptr) __PTR = (ptr); \
+ __unqual_scalar_typeof(*ptr) VAL; \
+ u32 __n = 0, __spin = SMP_TIMEWAIT_SPIN_COUNT; \
+ \
+ for (;;) { \
+ VAL = READ_ONCE(*__PTR); \
+ if (cond_expr) \
+ break; \
+ cpu_relax(); \
+ if (++__n < __spin) \
+ continue; \
+ if ((time_check_expr)) \
+ break; \
+ __n = 0; \
+ } \
+ (typeof(*ptr))VAL; \
+})
+#endif
diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
index f5801b0ba9e9..c9934ab68da2 100644
--- a/arch/arm64/include/asm/barrier.h
+++ b/arch/arm64/include/asm/barrier.h
@@ -219,6 +219,43 @@ do { \
(typeof(*ptr))VAL; \
})
+extern bool arch_timer_evtstrm_available(void);
+
+#ifndef SMP_TIMEWAIT_SPIN_COUNT
+#define SMP_TIMEWAIT_SPIN_COUNT 200
+#endif
+
+#define smp_cond_load_relaxed_timewait(ptr, cond_expr, \
+ time_check_expr) \
+({ \
+ typeof(ptr) __PTR = (ptr); \
+ __unqual_scalar_typeof(*ptr) VAL; \
+ u32 __n = 0, __spin = 0; \
+ bool __wfet = alternative_has_cap_unlikely(ARM64_HAS_WFXT); \
+ bool __wfe = arch_timer_evtstrm_available(); \
+ bool __wait = false; \
+ \
+ if (__wfet || __wfe) \
+ __wait = true; \
+ else \
+ __spin = SMP_TIMEWAIT_SPIN_COUNT; \
+ \
+ for (;;) { \
+ VAL = READ_ONCE(*__PTR); \
+ if (cond_expr) \
+ break; \
+ cpu_relax(); \
+ if (++__n < __spin) \
+ continue; \
+ if ((time_check_expr)) \
+ break; \
+ if (__wait) \
+ __cmpwait_relaxed(__PTR, VAL); \
+ __n = 0; \
+ } \
+ (typeof(*ptr))VAL; \
+})
+
#include <asm-generic/barrier.h>
__cmpwait_relaxed() will need adjustment to set a deadline for WFET.
AFAICT the rqspinlock code should be able to work by specifying something
like:
((ktime_get_mono_fast_ns() > tval)) || (deadlock_check(&lock_context)))
as the time_check_expr.
I think they also want to rate limit how often deadlock_check() is
called, so they can redefine SMP_TIMEWAIT_SPIN_COUNT to some large
value for arm64.
How does this look?
Thanks
--
ankur
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