linux-kernel - Re: [PATCH 6/7] sched: Clean up preempt_enable_no

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Date:	Wed, 20 Nov 2013 22:14:02 +0200
From:	Eliezer Tamir <eliezer.tamir@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Arjan van de Ven <arjan@...ux.intel.com>, lenb@...nel.org,
	rjw@...ysocki.net, Chris Leech <christopher.leech@...el.com>,
	David Miller <davem@...emloft.net>, rui.zhang@...el.com,
	jacob.jun.pan@...ux.intel.com,
	Mike Galbraith <bitbucket@...ine.de>,
	Ingo Molnar <mingo@...nel.org>, hpa@...or.com,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	Eliezer Tamir <eliezer@...ir.org.il>
Subject: Re: [PATCH 6/7] sched: Clean up preempt_enable_no_resched() abuse

On 20/11/2013 20:15, Peter Zijlstra wrote:
> There are options (CONFIG_PREEMPT_RCU) that allow scheduling while
> holding rcu_read_lock().
> 
> Also, preempt_enable() only schedules when its possible to schedule, so
> calling it when you know you cannot schedule is no issue.
> 

I have no issue with you changing busy_loop_us_clock() to use a regular
preempt enable.

I think that we still need to only do this if config preempt debug
is on. When it's off we should use the alternate implementation.
We are silencing a warning, but this is a performance critical path,
and we think we know what we are doing.

I tried to explain this in the comments. If you think my comments are
not clear enough, I'm open to suggestions.

Cheers,
Eliezer
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