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Subject: Re: [RFC PATCH v2 2/3] thread_local_abi: wire up x86 32/64 system call
From: Josh Triplett <josh@joshtriplett.org>
Date: Wed, 23 Dec 2015 19:42:14 -0800
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Paul Turner <pjt@google.com>, Andrew Hunter <ahh@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Russell King <linux@arm.linux.org.uk>
CC: linux-kernel@vger.kernel.org, Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Peter Zijlstra <peterz@infradead.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>, Steven Rostedt <rostedt@goodmis.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>, linux-api@vger.kernel.org
Message-ID: <1ECDFB6F-7C34-4BBB-A45F-E0D9A4B70737@joshtriplett.org>
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On December 22, 2015 10:02:12 AM PST, Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
>Wire up the thread local ABI on x86 32/64. Call the
>thread_local_abi_handle_notify_resume() function on return to
>userspace if TIF_NOTIFY_RESUME thread flag is set.
>
>This provides an ABI improving the speed of a getcpu operation
>on x86 by removing the need to perform a function call, "lsl"
>instruction, or system call on the fast path.
>
>Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>CC: Russell King <linux@arm.linux.org.uk>
>CC: Catalin Marinas <catalin.marinas@arm.com>
>CC: Will Deacon <will.deacon@arm.com>
>CC: Thomas Gleixner <tglx@linutronix.de>
>CC: Paul Turner <pjt@google.com>
>CC: Andrew Hunter <ahh@google.com>
>CC: Peter Zijlstra <peterz@infradead.org>
>CC: Andy Lutomirski <luto@amacapital.net>
>CC: Andi Kleen <andi@firstfloor.org>
>CC: Dave Watson <davejwatson@fb.com>
>CC: Chris Lameter <cl@linux.com>
>CC: Ingo Molnar <mingo@redhat.com>
>CC: Ben Maurer <bmaurer@fb.com>
>CC: Steven Rostedt <rostedt@goodmis.org>
>CC: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
>CC: Josh Triplett <josh@joshtriplett.org>
>CC: Linus Torvalds <torvalds@linux-foundation.org>
>CC: Andrew Morton <akpm@linux-foundation.org>
>CC: Thomas Gleixner <tglx@linutronix.de>
>CC: linux-api@vger.kernel.org
>---
> arch/x86/entry/common.c                | 2 ++
> arch/x86/entry/syscalls/syscall_32.tbl | 1 +
> arch/x86/entry/syscalls/syscall_64.tbl | 1 +
> 3 files changed, 4 insertions(+)
>
>diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
>index a89fdbc..222cacf 100644
>--- a/arch/x86/entry/common.c
>+++ b/arch/x86/entry/common.c
>@@ -249,6 +249,8 @@ static void exit_to_usermode_loop(struct pt_regs
>*regs, u32 cached_flags)
> 		if (cached_flags & _TIF_NOTIFY_RESUME) {
> 			clear_thread_flag(TIF_NOTIFY_RESUME);
> 			tracehook_notify_resume(regs);
>+			if (thread_local_abi_active(current))
>+				thread_local_abi_handle_notify_resume(current);

Every caller seems likely to duplicate this pattern; why not make the call itself a static inline containing this check and call (or no-op if compiled out)?

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