From: Mathieu Desnoyers <mathieu.desnoyers-vg+e7yoeK/dWk0Htik3J/w@public.gmane.org>
To: Thomas Gleixner <tglx-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>
Cc: Josh Triplett <josh-iaAMLnmF4UmaiuxdJuQwMA@public.gmane.org>,
Paul Turner <pjt-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>,
Andrew Hunter <ahh-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>,
Peter Zijlstra <peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>,
linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
linux-api <linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org>,
Andi Kleen <andi-Vw/NltI1exuRpAAqCnN02g@public.gmane.org>,
Dave Watson <davejwatson-b10kYP2dOMg@public.gmane.org>,
Chris Lameter <cl-vYTEC60ixJUAvxtiuMwx3w@public.gmane.org>,
Ingo Molnar <mingo-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
Ben Maurer <bmaurer-b10kYP2dOMg@public.gmane.org>,
rostedt <rostedt-nx8X9YLhiw1AfugRpC6u6w@public.gmane.org>,
"Paul E. McKenney"
<paulmck-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>,
Linus Torvalds
<torvalds-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>,
Andrew Morton
<akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>,
Russell King <linux-lFZ/pmaqli7XmaaqVzeoHQ@public.gmane.org>,
Catalin Marinas <catalin.marinas-5wv7dgnIgG8@public.gmane.org>,
Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>,
Michael Kerrisk
<mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Subject: Re: [RFC PATCH v2 1/3] getcpu_cache system call: cache CPU number of running thread
Date: Wed, 27 Jan 2016 17:36:48 +0000 (UTC) [thread overview]
Message-ID: <2049061625.6140.1453916208296.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <alpine.DEB.2.11.1601271822190.3886@nanos>
----- On Jan 27, 2016, at 12:24 PM, Thomas Gleixner tglx-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org wrote:
> On Wed, 27 Jan 2016, Josh Triplett wrote:
>> With the dynamic allocation removed, this seems sensible to me. One
>> minor nit: s/int32_t/uint32_t/g, since a location intended to hold a CPU
>> number should never need to hold a negative number.
>
> You try to block the future of computing: https://lwn.net/Articles/638673/
Besides impossible architectures, there is actually a use-case for
signedness here. It makes it possible to initialize the cpu number
cache to a negative value, e.g. -1, in userspace. Then, a check for
value < 0 can be used to figure out cases where the getcpu_cache
system call is not implemented, and where a fallback (vdso or getcpu
syscall) needs to be used.
This is why I have chosen a signed type for the cpu cache so far.
Thoughts ?
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
WARNING: multiple messages have this Message-ID (diff)
From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Josh Triplett <josh@joshtriplett.org>,
Paul Turner <pjt@google.com>, Andrew Hunter <ahh@google.com>,
Peter Zijlstra <peterz@infradead.org>,
linux-kernel@vger.kernel.org,
linux-api <linux-api@vger.kernel.org>,
Andy Lutomirski <luto@amacapital.net>,
Andi Kleen <andi@firstfloor.org>,
Dave Watson <davejwatson@fb.com>, Chris Lameter <cl@linux.com>,
Ingo Molnar <mingo@redhat.com>, Ben Maurer <bmaurer@fb.com>,
rostedt <rostedt@goodmis.org>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
Andrew Morton <akpm@linux-foundation.org>,
Russell King <linux@arm.linux.org.uk>,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will.deacon@arm.com>,
Michael Kerrisk <mtk.manpages@gmail.com>
Subject: Re: [RFC PATCH v2 1/3] getcpu_cache system call: cache CPU number of running thread
Date: Wed, 27 Jan 2016 17:36:48 +0000 (UTC) [thread overview]
Message-ID: <2049061625.6140.1453916208296.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <alpine.DEB.2.11.1601271822190.3886@nanos>
----- On Jan 27, 2016, at 12:24 PM, Thomas Gleixner tglx@linutronix.de wrote:
> On Wed, 27 Jan 2016, Josh Triplett wrote:
>> With the dynamic allocation removed, this seems sensible to me. One
>> minor nit: s/int32_t/uint32_t/g, since a location intended to hold a CPU
>> number should never need to hold a negative number.
>
> You try to block the future of computing: https://lwn.net/Articles/638673/
Besides impossible architectures, there is actually a use-case for
signedness here. It makes it possible to initialize the cpu number
cache to a negative value, e.g. -1, in userspace. Then, a check for
value < 0 can be used to figure out cases where the getcpu_cache
system call is not implemented, and where a fallback (vdso or getcpu
syscall) needs to be used.
This is why I have chosen a signed type for the cpu cache so far.
Thoughts ?
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
next prev parent reply other threads:[~2016-01-27 17:36 UTC|newest]
Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-01-27 16:54 [RFC PATCH v2 0/3] getcpu_cache system call Mathieu Desnoyers
2016-01-27 16:54 ` [RFC PATCH v2 1/3] getcpu_cache system call: cache CPU number of running thread Mathieu Desnoyers
[not found] ` <1453913683-28915-2-git-send-email-mathieu.desnoyers-vg+e7yoeK/dWk0Htik3J/w@public.gmane.org>
2016-01-27 17:20 ` Josh Triplett
2016-01-27 17:20 ` Josh Triplett
2016-01-27 17:24 ` Thomas Gleixner
2016-01-27 17:24 ` Thomas Gleixner
2016-01-27 17:36 ` Mathieu Desnoyers [this message]
2016-01-27 17:36 ` Mathieu Desnoyers
[not found] ` <2049061625.6140.1453916208296.JavaMail.zimbra-vg+e7yoeK/dWk0Htik3J/w@public.gmane.org>
2016-01-27 18:02 ` Andrew Hunter
2016-01-27 18:02 ` Andrew Hunter
2016-01-27 18:03 ` Josh Triplett
2016-01-27 18:03 ` Josh Triplett
2016-01-27 18:43 ` Mathieu Desnoyers
2016-01-27 19:16 ` Josh Triplett
2016-01-27 21:02 ` Mathieu Desnoyers
[not found] ` <2037701859.6303.1453928564519.JavaMail.zimbra-vg+e7yoeK/dWk0Htik3J/w@public.gmane.org>
2016-01-27 21:30 ` Josh Triplett
2016-01-27 21:30 ` Josh Triplett
2016-01-27 17:22 ` Thomas Gleixner
2016-01-27 17:22 ` Thomas Gleixner
2016-01-27 17:31 ` Mathieu Desnoyers
2016-01-27 17:31 ` Mathieu Desnoyers
[not found] ` <671969438.6129.1453915918933.JavaMail.zimbra-vg+e7yoeK/dWk0Htik3J/w@public.gmane.org>
2016-01-27 17:34 ` Thomas Gleixner
2016-01-27 17:34 ` Thomas Gleixner
2016-01-27 17:37 ` Thomas Gleixner
2016-01-27 17:37 ` Thomas Gleixner
2016-01-27 21:34 ` Mathieu Desnoyers
2016-01-27 21:34 ` Mathieu Desnoyers
[not found] ` <974364259.6329.1453930475174.JavaMail.zimbra-vg+e7yoeK/dWk0Htik3J/w@public.gmane.org>
2016-01-27 22:11 ` Josh Triplett
2016-01-27 22:11 ` Josh Triplett
2016-01-27 22:47 ` Mathieu Desnoyers
2016-01-27 22:47 ` Mathieu Desnoyers
[not found] ` <75735238.6347.1453934857246.JavaMail.zimbra-vg+e7yoeK/dWk0Htik3J/w@public.gmane.org>
2016-01-28 11:12 ` Heiko Carstens
2016-01-28 11:12 ` Heiko Carstens
2016-01-28 13:33 ` Mathieu Desnoyers
2016-01-28 13:33 ` Mathieu Desnoyers
2016-01-28 3:12 ` Alexei Starovoitov
2016-01-28 3:12 ` Alexei Starovoitov
[not found] ` <20160128031213.GA55682-+o4/htvd0TDFYCXBM6kdu7fOX0fSgVTm@public.gmane.org>
2016-01-28 17:41 ` Mathieu Desnoyers
2016-01-28 17:41 ` Mathieu Desnoyers
2016-01-27 16:54 ` [RFC PATCH v2 2/3] getcpu_cache: wire up ARM system call Mathieu Desnoyers
[not found] ` <1453913683-28915-3-git-send-email-mathieu.desnoyers-vg+e7yoeK/dWk0Htik3J/w@public.gmane.org>
2016-01-27 18:19 ` Russell King - ARM Linux
2016-01-27 18:19 ` Russell King - ARM Linux
[not found] ` <20160127181915.GA10826-l+eeeJia6m9vn6HldHNs0ANdhmdF6hFW@public.gmane.org>
2016-01-27 18:46 ` Mathieu Desnoyers
2016-01-27 18:46 ` Mathieu Desnoyers
[not found] ` <1011987684.6168.1453920399905.JavaMail.zimbra-vg+e7yoeK/dWk0Htik3J/w@public.gmane.org>
2016-01-27 23:03 ` Mathieu Desnoyers
2016-01-27 23:03 ` Mathieu Desnoyers
2016-01-27 16:54 ` [RFC PATCH v2 3/3] getcpu_cache: wire up x86 32/64 " Mathieu Desnoyers
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