linux-kernel - Re: [RFC PATCH 0/3] Implement getcpu

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Date:	Tue, 12 Jan 2016 16:51:53 -0800
From:	Josh Triplett <josh@...htriplett.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Ben Maurer <bmaurer@...com>
CC:	Shane M Seymour <shane.seymour@....com>,
	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>, 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 0/3] Implement getcpu_cache system call

On January 12, 2016 4:22:29 PM PST, Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
>----- On Jan 12, 2016, at 4:02 PM, Ben Maurer bmaurer@...com wrote:
>
>>> One idea I have would be to let the kernel reserve some space either
>after the
>>> first stack address (for a stack growing down) or at the beginning
>of the
>>> allocated TLS area for each thread in copy_thread_tls() by fiddling
>with
>>> sp or the tls base address when creating a thread.
>> 
>> Could this be implemented by having glibc use a well known symbol
>name to define
>> the per-thread TLS area? If an high performance application wants to
>avoid any
>> relocations in accessing this variable it would define it and that
>definition
>> would override glibc's. This is how things work with malloc. glibc
>has a
>> default malloc implementation but we link jemalloc directly into our
>binaries.
>> in addition to changing the malloc implementation this means that
>calls to
>> malloc don't go through the PLT.
>
>Just to make sure I understand your proposal: defining a well known
>symbol
>with a weak attribute in glibc (or bionic...), e.g.:
>
>int32_t __thread __attribute__((weak)) __getcpu_cache;
>
>so that applications which care about bypassing the PLT can override it
>with:
>
>int32_t __thread __getcpu_cache;
>
>glibc/bionic would be responsible for calling the getcpu_cache() system
>call
>to register/unregister this TLS variable for each thread.
>
>One thing I would like to figure out is whether we can use this in a
>way that
>would allow introducing getcpu_cache() into applications and libraries
>(e.g. lttng-ust tracer) before it gets implemented into glibc, in a way
>that
>would keep forward compatibility for whenever it gets introduced in
>glibc.
>
>We can declare __getcpu_cache as a weak symbol in arbitrary libraries,
>and
>make them register/unregister the cache through the getcpu_cache
>syscall.
>The main thing that I would need to tweak at the kernel level within
>the
>system call would be to keep a refcount of the number of times the
>__getcpu_cache is registered per thread. This would allow multiple
>registrations,
>one per library (e.g. lttng-ust) and one for glibc, but we would
>validate
>that they all register the exact same address for a given thread.
>
>The reference counting trick should also work for cases where
>applications
>define a non-weak __getcpu_cache, and want to call the getcpu_cache
>system call to register it themselves (before glibc adds support for
>it).

This seems like something better done in a tiny common library, rather than the kernel or by playing symbol resolution games.