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

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Message-ID: <52944A9A.6090906@linux.intel.com>
Date:	Tue, 26 Nov 2013 09:15:38 +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, Eliezer Tamir <eliezer@...ir.org.il>,
	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
Subject: Re: [PATCH 6/7] sched: Clean up preempt_enable_no_resched() abuse

On 22/11/2013 13:30, Peter Zijlstra wrote:
> On Fri, Nov 22, 2013 at 08:56:00AM +0200, Eliezer Tamir wrote:
>> On 21/11/2013 15:39, Peter Zijlstra wrote:
>>> On Thu, Nov 21, 2013 at 03:26:17PM +0200, Eliezer Tamir wrote:
> 
> Please use local_clock(), yes its slightly more expensive, but I doubt
> you can actually measure the effects on sane hardware.

If we limit the discussion to sane hardware, I should mention that on
current Intel CPUs TSC is guaranteed to be monotonic for anything up to
8 sockets. Even on slightly older HS TSC skew is very small and should
not be an issue for this use case.

So:
Modern sane HW does not have this issue.
The people that do busy polling typically pin tasks to cores anyway.
You need cap_net_admin to use this setting.
There is no real damage if the issue happens.
This is fast-low-latency-path so we are very sensitive to adding even
a small cost.
Linus really didn't like adding to the cost of poll/select when busy
polling is not being used.

Having said that, since we need to fix the timeout issue you pointed
out, we will test the use of local_clock() and see if it matters or
not.

Again, I have no objection to changing the use of
preempt_enable_no_resched() to a plain preempt_enable().

Cheers,
Eliezer
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