From: Peter Zijlstra <peterz@infradead.org>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: "K Prateek Nayak" <kprateek.nayak@amd.com>,
"Arnd Bergmann" <arnd@arndb.de>,
"Thomas Gleixner" <tglx@kernel.org>,
"Ingo Molnar" <mingo@redhat.com>,
"Borislav Petkov" <bp@alien8.de>,
"Dave Hansen" <dave.hansen@linux.intel.com>,
x86@kernel.org, "Catalin Marinas" <catalin.marinas@arm.com>,
"Will Deacon" <will@kernel.org>, "Paul Walmsley" <pjw@kernel.org>,
"Palmer Dabbelt" <palmer@dabbelt.com>,
"Albert Ou" <aou@eecs.berkeley.edu>,
"Heiko Carstens" <hca@linux.ibm.com>,
"Vasily Gorbik" <gor@linux.ibm.com>,
"Alexander Gordeev" <agordeev@linux.ibm.com>,
"Darren Hart" <dvhart@infradead.org>,
"Davidlohr Bueso" <dave@stgolabs.net>,
"André Almeida" <andrealmeid@igalia.com>,
linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org,
"Samuel Holland" <samuel.holland@sifive.com>,
"Charlie Jenkins" <thecharlesjenkins@gmail.com>,
linux-arm-kernel@lists.infradead.org,
linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org,
"H. Peter Anvin" <hpa@zytor.com>,
"Thomas Huth" <thuth@redhat.com>,
"Sean Christopherson" <seanjc@google.com>,
"Jisheng Zhang" <jszhang@kernel.org>,
"Alexandre Ghiti" <alex@ghiti.fr>,
"Christian Borntraeger" <borntraeger@linux.ibm.com>,
"Sven Schnelle" <svens@linux.ibm.com>
Subject: Re: [PATCH] futex: Optimise the size check get_futex_key()
Date: Thu, 2 Jul 2026 13:18:07 +0200 [thread overview]
Message-ID: <20260702111807.GI751831@noisy.programming.kicks-ass.net> (raw)
In-Reply-To: <20260702105615.PiYhQ9Rt@linutronix.de>
On Thu, Jul 02, 2026 at 12:56:15PM +0200, Sebastian Andrzej Siewior wrote:
> On 2026-07-02 10:59:21 [+0200], Peter Zijlstra wrote:
> > > Could someone verify this, please? The 5% look a bit high. This is on
> > > top of the series (but not worsen by the series).
> >
> > Bah, I tried to reproduce and couldn't. Then I noticed I did a clang
> > build and that is in fact clever enough to do this optimization itself.
> >
> > /me tries again with a GCC build.
> >
> > pre: [thread 0] futex: 0x561f14430680 [ 9021408 ops/sec ]
> > post: [thread 0] futex: 0x55feadbbb680 [ 8977527 ops/sec ]
> >
> > (and this seems to be well inside the error threshold of this test).
> >
> > So I see the GCC build do the DIV, and no longer with his patch applied,
> > but for some reason I cannot get the runtime performance to actually
> > improve much of anything on my system.
>
> I did open https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126078 for the
> div.
>
> My .config is the debian distro on the 4 node big iron. [ in case it
> it has so much overhead elsewhere that this place a bigger role].
>
> "perf top" showed this as 6% or something and red in the function. After
> the removal it did not show up.
Right, I build whatever random config I had on the SPR test box. But I
can't argue with the patch, it is sane and GCC does generate better code
with it. For $raisins it just didn't translate into actual performance
for me.
WARNING: multiple messages have this Message-ID (diff)
From: Peter Zijlstra <peterz@infradead.org>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: "K Prateek Nayak" <kprateek.nayak@amd.com>,
"Arnd Bergmann" <arnd@arndb.de>,
"Thomas Gleixner" <tglx@kernel.org>,
"Ingo Molnar" <mingo@redhat.com>,
"Borislav Petkov" <bp@alien8.de>,
"Dave Hansen" <dave.hansen@linux.intel.com>,
x86@kernel.org, "Catalin Marinas" <catalin.marinas@arm.com>,
"Will Deacon" <will@kernel.org>, "Paul Walmsley" <pjw@kernel.org>,
"Palmer Dabbelt" <palmer@dabbelt.com>,
"Albert Ou" <aou@eecs.berkeley.edu>,
"Heiko Carstens" <hca@linux.ibm.com>,
"Vasily Gorbik" <gor@linux.ibm.com>,
"Alexander Gordeev" <agordeev@linux.ibm.com>,
"Darren Hart" <dvhart@infradead.org>,
"Davidlohr Bueso" <dave@stgolabs.net>,
"André Almeida" <andrealmeid@igalia.com>,
linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org,
"Samuel Holland" <samuel.holland@sifive.com>,
"Charlie Jenkins" <thecharlesjenkins@gmail.com>,
linux-arm-kernel@lists.infradead.org,
linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org,
"H. Peter Anvin" <hpa@zytor.com>,
"Thomas Huth" <thuth@redhat.com>,
"Sean Christopherson" <seanjc@google.com>,
"Jisheng Zhang" <jszhang@kernel.org>,
"Alexandre Ghiti" <alex@ghiti.fr>,
"Christian Borntraeger" <borntraeger@linux.ibm.com>,
"Sven Schnelle" <svens@linux.ibm.com>
Subject: Re: [PATCH] futex: Optimise the size check get_futex_key()
Date: Thu, 2 Jul 2026 13:18:07 +0200 [thread overview]
Message-ID: <20260702111807.GI751831@noisy.programming.kicks-ass.net> (raw)
In-Reply-To: <20260702105615.PiYhQ9Rt@linutronix.de>
On Thu, Jul 02, 2026 at 12:56:15PM +0200, Sebastian Andrzej Siewior wrote:
> On 2026-07-02 10:59:21 [+0200], Peter Zijlstra wrote:
> > > Could someone verify this, please? The 5% look a bit high. This is on
> > > top of the series (but not worsen by the series).
> >
> > Bah, I tried to reproduce and couldn't. Then I noticed I did a clang
> > build and that is in fact clever enough to do this optimization itself.
> >
> > /me tries again with a GCC build.
> >
> > pre: [thread 0] futex: 0x561f14430680 [ 9021408 ops/sec ]
> > post: [thread 0] futex: 0x55feadbbb680 [ 8977527 ops/sec ]
> >
> > (and this seems to be well inside the error threshold of this test).
> >
> > So I see the GCC build do the DIV, and no longer with his patch applied,
> > but for some reason I cannot get the runtime performance to actually
> > improve much of anything on my system.
>
> I did open https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126078 for the
> div.
>
> My .config is the debian distro on the 4 node big iron. [ in case it
> it has so much overhead elsewhere that this place a bigger role].
>
> "perf top" showed this as 6% or something and red in the function. After
> the removal it did not show up.
Right, I build whatever random config I had on the SPR test box. But I
can't argue with the patch, it is sane and GCC does generate better code
with it. For $raisins it just didn't translate into actual performance
for me.
_______________________________________________
linux-riscv mailing list
linux-riscv@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-riscv
next prev parent reply other threads:[~2026-07-02 11:18 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-06-30 4:55 [PATCH v5 0/8] futex: Use runtime constants for futex_hash computation K Prateek Nayak
2026-06-30 4:55 ` K Prateek Nayak
2026-06-30 4:55 ` [PATCH v5 1/8] x86/runtime-const: Introduce runtime_const_mask_32() K Prateek Nayak
2026-06-30 4:55 ` K Prateek Nayak
2026-06-30 4:55 ` [PATCH v5 2/8] arm64/runtime-const: Use aarch64_insn_patch_text_nosync() for patching K Prateek Nayak
2026-06-30 4:55 ` K Prateek Nayak
2026-06-30 4:55 ` [PATCH v5 3/8] arm64/runtime-const: Introduce runtime_const_mask_32() K Prateek Nayak
2026-06-30 4:55 ` K Prateek Nayak
2026-06-30 5:07 ` sashiko-bot
2026-06-30 8:16 ` K Prateek Nayak
2026-06-30 4:55 ` [PATCH v5 4/8] riscv/runtime-const: Replace open-coded placeholder with RUNTIME_MAGIC K Prateek Nayak
2026-06-30 4:55 ` K Prateek Nayak
2026-06-30 6:47 ` Guo Ren
2026-06-30 6:47 ` Guo Ren
2026-06-30 4:55 ` [PATCH v5 5/8] riscv/runtime-const: Introduce runtime_const_mask_32() K Prateek Nayak
2026-06-30 4:55 ` K Prateek Nayak
2026-06-30 4:55 ` [PATCH v5 6/8] s390/runtime-const: " K Prateek Nayak
2026-06-30 4:55 ` K Prateek Nayak
2026-06-30 4:55 ` [PATCH v5 7/8] asm-generic/runtime-const: Add dummy runtime_const_mask_32() K Prateek Nayak
2026-06-30 4:55 ` K Prateek Nayak
2026-06-30 4:55 ` [PATCH v5 8/8] futex: Use runtime constants for __futex_hash() hot path K Prateek Nayak
2026-06-30 4:55 ` K Prateek Nayak
2026-06-30 5:20 ` sashiko-bot
2026-07-01 7:57 ` Peter Zijlstra
2026-07-01 7:57 ` Peter Zijlstra
2026-07-01 8:41 ` Sebastian Andrzej Siewior
2026-07-01 8:41 ` Sebastian Andrzej Siewior
2026-07-01 9:07 ` K Prateek Nayak
2026-07-01 9:07 ` K Prateek Nayak
2026-07-01 16:17 ` [PATCH] futex: Optimise the size check get_futex_key() Sebastian Andrzej Siewior
2026-07-01 16:17 ` Sebastian Andrzej Siewior
2026-07-02 8:59 ` Peter Zijlstra
2026-07-02 8:59 ` Peter Zijlstra
2026-07-02 10:56 ` Sebastian Andrzej Siewior
2026-07-02 10:56 ` Sebastian Andrzej Siewior
2026-07-02 11:18 ` Peter Zijlstra [this message]
2026-07-02 11:18 ` Peter Zijlstra
2026-07-01 11:01 ` [PATCH v5 8/8] futex: Use runtime constants for __futex_hash() hot path Sebastian Andrzej Siewior
2026-07-01 11:01 ` Sebastian Andrzej Siewior
2026-07-01 19:58 ` Sebastian Andrzej Siewior
2026-07-01 19:58 ` Sebastian Andrzej Siewior
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