linux-kernel - Re: [RFC PATCH v2 1/3] getcpu_cache system call: cache CPU number of running thread

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Date:	Wed, 27 Jan 2016 11:16:45 -0800
From:	Josh Triplett <josh@...htriplett.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>, Paul Turner <pjt@...gle.com>,
	Andrew Hunter <ahh@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org,
	linux-api <linux-api@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Andi Kleen <andi@...stfloor.org>,
	Dave Watson <davejwatson@...com>, Chris Lameter <cl@...ux.com>,
	Ingo Molnar <mingo@...hat.com>, Ben Maurer <bmaurer@...com>,
	rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Russell King <linux@....linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [RFC PATCH v2 1/3] getcpu_cache system call: cache CPU number of
 running thread

On Wed, Jan 27, 2016 at 06:43:36PM +0000, Mathieu Desnoyers wrote:
> ----- On Jan 27, 2016, at 1:03 PM, Josh Triplett josh@...htriplett.org wrote:
> 
> > On Wed, Jan 27, 2016 at 05:36:48PM +0000, Mathieu Desnoyers wrote:
> >> ----- On Jan 27, 2016, at 12:24 PM, Thomas Gleixner tglx@...utronix.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.
> 
> I'm referring to the code path that read the content of the cache.
> This code don't call the getcpu_cache system call each time (this
> would defeat the entire purpose of this cache), but still has to
> know whether it can rely on the cache content to contain the current
> CPU number. Seeing a "-1" there is a nice way to tell the fast path
> that it needs to go through a fallback.
> 
> Or perhaps you have another mechanism in mind for that ? How do
> you intend to communicate the ENOSYS from the kernel to all
> eventual readers of the cache, without adding extra function
> call overhead on the fast path ?

Have the fast path assume the cache, without even checking for -1; only
use that fast path if getcpu_cache exists.  If you don't have
getcpu_cache, don't even attempt to use the fast path; substitute in a
fallback implementation.  Don't have a conditional in either version;
just decide which version to use based on system capabilities.

Alternatively, use the implementation you have with a placeholder value,
and just use 0xFFFFFFFF as the placeholder; that seems no more or
less valid.