From mboxrd@z Thu Jan 1 00:00:00 1970 From: Josh Triplett Subject: Re: [RFC PATCH v2 1/3] getcpu_cache system call: cache CPU number of running thread Date: Wed, 27 Jan 2016 11:16:45 -0800 Message-ID: <20160127191645.GA8045@cloud> References: <1453913683-28915-1-git-send-email-mathieu.desnoyers@efficios.com> <1453913683-28915-2-git-send-email-mathieu.desnoyers@efficios.com> <20160127172044.GA7514@cloud> <2049061625.6140.1453916208296.JavaMail.zimbra@efficios.com> <20160127180353.GB7514@cloud> <604294939.6161.1453920216268.JavaMail.zimbra@efficios.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <604294939.6161.1453920216268.JavaMail.zimbra@efficios.com> Sender: linux-kernel-owner@vger.kernel.org To: Mathieu Desnoyers Cc: Thomas Gleixner , Paul Turner , Andrew Hunter , Peter Zijlstra , linux-kernel@vger.kernel.org, linux-api , Andy Lutomirski , Andi Kleen , Dave Watson , Chris Lameter , Ingo Molnar , Ben Maurer , rostedt , "Paul E. McKenney" , Linus Torvalds , Andrew Morton , Russell King , Catalin Marinas , Will Deacon , Michael Kerrisk List-Id: linux-api@vger.kernel.org On Wed, Jan 27, 2016 at 06:43:36PM +0000, Mathieu Desnoyers wrote: > ----- On Jan 27, 2016, at 1:03 PM, Josh Triplett josh@joshtriplett.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@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. > > 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.