linux-kernel - Re: [PATCH 2/4] tile: support CONTEXT_TRACKING and thus NOHZ

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Date:	Tue, 24 Mar 2015 22:15:17 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	cmetcalf@...hip.com
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Gilad Ben-Yossef <gilad@...yossef.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	Li Zhong <zhong@...ux.vnet.ibm.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/4] tile: support CONTEXT_TRACKING and thus NOHZ_FULL

On Tue, Mar 24, 2015 at 03:21:33PM -0400, cmetcalf@...hip.com wrote:
> From: Chris Metcalf <cmetcalf@...hip.com>
> 
> Add the TIF_NOHZ flag appropriately.
> 
> Add call to user_exit() on entry to do_work_pending() and on entry
> to syscalls via do_syscall_trace_enter(), and also the top of
> do_syscall_trace_exit() just because it's done in x86.

You only need to protect do_syscall_trace_exit() if there is a risk
that something calls user_enter() before. x86 does it so because
schedule_user() can be called before although I think we've changed
schedule_user() to use exception_enter/exit. I should check if that
user_exit() in do_syscall_trace_exit() is still necessary in x86.

Anyway, calling user_exit() on context tracking kerne mode doesn't do
any harm.

> diff --git a/arch/tile/kernel/process.c b/arch/tile/kernel/process.c
> index 48e5773dd0b7..b403c2e3e263 100644
> --- a/arch/tile/kernel/process.c
> +++ b/arch/tile/kernel/process.c
> @@ -27,6 +27,7 @@
>  #include <linux/kernel.h>
>  #include <linux/tracehook.h>
>  #include <linux/signal.h>
> +#include <linux/context_tracking.h>
>  #include <asm/stack.h>
>  #include <asm/switch_to.h>
>  #include <asm/homecache.h>
> @@ -474,6 +475,8 @@ int do_work_pending(struct pt_regs *regs, u32 thread_info_flags)
>  	if (!user_mode(regs))
>  		return 0;
>  
> +	user_exit();
> +
>  	/* Enable interrupts; they are disabled again on return to caller. */
>  	local_irq_enable();
>  
> @@ -496,11 +499,12 @@ int do_work_pending(struct pt_regs *regs, u32 thread_info_flags)
>  		tracehook_notify_resume(regs);
>  		return 1;
>  	}
> -	if (thread_info_flags & _TIF_SINGLESTEP) {
> +	if (thread_info_flags & _TIF_SINGLESTEP)
>  		single_step_once(regs);
> -		return 0;
> -	}
> -	panic("work_pending: bad flags %#x\n", thread_info_flags);
> +
> +	user_enter();

So, do work pending is called from syscall exit only? Or does it concern
interrupts, exceptions as well?

Well if it's always followed by a return to userspace, it should be fine.
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