From: Josh Triplett <josh@joshtriplett.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
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 10:03:53 -0800 [thread overview]
Message-ID: <20160127180353.GB7514@cloud> (raw)
In-Reply-To: <2049061625.6140.1453916208296.JavaMail.zimbra@efficios.com>
On Wed, Jan 27, 2016 at 05:36:48PM +0000, Mathieu Desnoyers wrote:
> ----- 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.
If getcpu_cache doesn't exist, you'll get ENOSYS. If getcpu_cache
returns 0, then you can assume the kernel will give you a valid CPU
number.
- Josh Triplett
next prev parent reply other threads:[~2016-01-27 18:04 UTC|newest]
Thread overview: 27+ 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
2016-01-27 17:20 ` Josh Triplett
2016-01-27 17:24 ` Thomas Gleixner
2016-01-27 17:36 ` Mathieu Desnoyers
2016-01-27 18:02 ` Andrew Hunter
2016-01-27 18:03 ` Josh Triplett [this message]
2016-01-27 18:43 ` Mathieu Desnoyers
2016-01-27 19:16 ` Josh Triplett
2016-01-27 21:02 ` Mathieu Desnoyers
2016-01-27 21:30 ` Josh Triplett
2016-01-27 17:22 ` Thomas Gleixner
2016-01-27 17:31 ` Mathieu Desnoyers
2016-01-27 17:34 ` Thomas Gleixner
2016-01-27 17:37 ` Thomas Gleixner
2016-01-27 21:34 ` Mathieu Desnoyers
2016-01-27 22:11 ` Josh Triplett
2016-01-27 22:47 ` Mathieu Desnoyers
2016-01-28 11:12 ` Heiko Carstens
2016-01-28 13:33 ` Mathieu Desnoyers
2016-01-28 3:12 ` Alexei Starovoitov
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
2016-01-27 18:19 ` Russell King - ARM Linux
2016-01-27 18:46 ` 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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