linux-kernel - TIF_NOHZ can escape nonhz mask? (Was: [PATCH v3 6/8] x86: Split syscall_trace

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Date:	Mon, 28 Jul 2014 20:58:03 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
	Will Drewry <wad@...omium.org>, x86@...nel.org,
	linux-arm-kernel@...ts.infradead.org, linux-mips@...ux-mips.org,
	linux-arch@...r.kernel.org, linux-security-module@...r.kernel.org,
	Alexei Starovoitov <ast@...mgrid.com>, hpa@...or.com
Subject: TIF_NOHZ can escape nonhz mask? (Was: [PATCH v3 6/8] x86: Split
	syscall_trace_enter into two phases)

Off-topic, but...

On 07/28, Oleg Nesterov wrote:
>
> But we should always call user_exit() unconditionally?

Frederic, don't we need the patch below? In fact clear_() can be moved
under "if ()" too. and probably copy_process() should clear this flag...

Or. __context_tracking_task_switch() can simply do

	 if (context_tracking_cpu_is_enabled())
	 	set_tsk_thread_flag(next, TIF_NOHZ);
	 else
	 	clear_tsk_thread_flag(next, TIF_NOHZ);

and then we can forget about copy_process(). Or I am totally confused?


I am also wondering if we can extend user_return_notifier to handle
enter/exit and kill TIF_NOHZ.

Oleg.

--- x/kernel/context_tracking.c
+++ x/kernel/context_tracking.c
@@ -202,7 +202,8 @@ void __context_tracking_task_switch(stru
 				    struct task_struct *next)
 {
 	clear_tsk_thread_flag(prev, TIF_NOHZ);
-	set_tsk_thread_flag(next, TIF_NOHZ);
+	if (context_tracking_cpu_is_enabled())
+		set_tsk_thread_flag(next, TIF_NOHZ);
 }
 
 #ifdef CONFIG_CONTEXT_TRACKING_FORCE

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