linux-kernel - Re: [PATCH 1/3] x86/sched/context_tracking: Call new schedule_preempt_user() from entry

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Date:	Mon, 13 May 2013 11:03:39 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Li Zhong <zhong@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 1/3] x86/sched/context_tracking: Call new
 schedule_preempt_user() from entry_64.S

On Mon, 2013-05-13 at 17:56 +0800, Li Zhong wrote:

> > All this before schedule_user() was able to call user_exit() and take us
> > out of dynamic tick user context.
> 
> Maybe we could just disable function trace for schedule_user()? 
> 
> It seems that function trace might use RCU, at least in __schedule()
> when calling preempt_enable(), and if preemption is really enabled. 

There is a small use of rcu scheduling (preempt disable/enable) in
function tracing. One is for perf and dynamic call traces and some hash
code (multiple users of the function tracer). But those are the unique
cases, and I really do not want to remove this function just for the non
common case. Function tracing has proven extremely useful in debugging
RCU and context tracking. By adding notrace, it's a lost cause.

Not to mention, adding trace_printk() there will break it too, and that
does not use any RCU but still disables preemption because the writing
to the ring buffer requires staying on the same CPU.

> 
> user_exit() is used to allow RCU usage after that (with rcu_user_exit).
> RCU usage is not allowed before that because cpu is in user eqs. And if
> function trace needs use RCU, then it seems user_exit() itself or its
> callers couldn't be function traced.

And it can't be debugged either.

I can probably black list those functions manually, such that only the
main function tracer can trace it, all others will not. In otherwords, I
can have it such that function tracer will not trace those functions for
perf, dynamic function tracers, or anything that requires changing the
function list.

I could probably also add a heavier weight synchronize_sched() that even
synchronizes cpus in userspace.

> 
> If __schedule() in preempt_enable() is the only place function trace
> uses RCU, then after converting to schedule_preempt_user(), it is safe
> as irqs are disabled for __schedule() to happen. But user_exit() itself
> and some other callers might still need function trace be disabled.

The above makes no sense to me. function tracing didn't break, but the
user_exit() did because of a preemption in the wrong place, as there was
no protection there to prevent an unnecessary preemption.

-- Steve

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