linux-kernel - [PATCH 1/5] x86/entry: Remove _TIF_NOHZ from _TIF_WORK_SYSCALL

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Date:   Fri, 14 Feb 2020 16:26:11 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Paul Burton <paulburton@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "David S . Miller" <davem@...emloft.net>,
        Borislav Petkov <bp@...en8.de>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Andy Lutomirski <luto@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Russell King <linux@...linux.org.uk>
Subject: [PATCH 1/5] x86/entry: Remove _TIF_NOHZ from _TIF_WORK_SYSCALL_ENTRY

From: Thomas Gleixner <tglx@...utronix.de>

Evaluating _TIF_NOHZ to decide whether to use the slow syscall entry path
is not only pointless, it's actually counterproductive:

 1) Context tracking code is invoked unconditionally before that flag is
    evaluated.

 2) If the flag is set the slow path is invoked for nothing due to #1

Remove it.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Andy Lutomirski <luto@...nel.org>
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
---
 arch/x86/include/asm/thread_info.h | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
index cf4327986e98..6cb9d1b0d1e6 100644
--- a/arch/x86/include/asm/thread_info.h
+++ b/arch/x86/include/asm/thread_info.h
@@ -133,14 +133,10 @@ struct thread_info {
 #define _TIF_X32		(1 << TIF_X32)
 #define _TIF_FSCHECK		(1 << TIF_FSCHECK)
 
-/*
- * work to do in syscall_trace_enter().  Also includes TIF_NOHZ for
- * enter_from_user_mode()
- */
+/* Work to do before invoking the actual syscall. */
 #define _TIF_WORK_SYSCALL_ENTRY	\
 	(_TIF_SYSCALL_TRACE | _TIF_SYSCALL_EMU | _TIF_SYSCALL_AUDIT |	\
-	 _TIF_SECCOMP | _TIF_SYSCALL_TRACEPOINT |	\
-	 _TIF_NOHZ)
+	 _TIF_SECCOMP | _TIF_SYSCALL_TRACEPOINT)
 
 /* flags to check in __switch_to() */
 #define _TIF_WORK_CTXSW_BASE					\
-- 
2.25.0