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

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Date:	Tue, 24 Mar 2015 17:49:43 -0400
From:	Chris Metcalf <cmetcalf@...hip.com>
To:	Frederic Weisbecker <fweisbec@...il.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 3/24/2015 5:15 PM, Frederic Weisbecker wrote:
> 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.

Yes, I see the exception_enter/exit in schedule_user().  I guess I will plan to
leave the user_exit() in for now, but if you conclude it's not necessary for x86,
I can remove it for tile as well at that point.

>> 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?

It's called on every return to userspace if the TIF flags require it.

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

Let me know if you'd like me to put your Acked-by on the commit. Thanks!

-- 
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com

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