linux-kernel - Re: [Ksummit-discuss] [CORE TOPIC] lightweight per-cpu locks / restartable sequences

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Date:	Wed, 22 Jul 2015 22:22:02 +0800
From:	Lai Jiangshan <jiangshanlai+lkml@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Christoph Lameter <cl@...ux.com>, Chris Mason <clm@...com>,
	Andy Lutomirski <luto@...capital.net>,
	"ksummit-discuss@...ts.linuxfoundation.org" 
	<ksummit-discuss@...ts.linuxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jens Axboe <axboe@...com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Shaohua Li <shli@...com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [Ksummit-discuss] [CORE TOPIC] lightweight per-cpu locks /
 restartable sequences

On Mon, Jul 13, 2015 at 5:57 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Fri, Jul 10, 2015 at 12:26:21PM -0500, Christoph Lameter wrote:
>> On Thu, 9 Jul 2015, Chris Mason wrote:
>>
>> > I think the topic is really interesting and we'll be able to get numbers
>> > from production workloads to help justify and compare different
>> > approaches.
>>
>> Ok that would be important. I also think that the approach may be used
>> in kernel to reduce the overhead of CONFIG_PREEMPT and also to implement
>> fast versions of this_cpu_ops for non x86 architectures and maybe even
>
>
> Also, I don't think we need a schedule check for the in-kernel usage,
> pure interrupt should be good enough, nobody should (want to) call
> schedule() while inside such a critical section, which leaves us with
> involuntary preemption, and those are purely interrupt driven.
>
> Now the 'problem' is finding these special regions fast, the easy
> solution is the same as the one proposed for userspace, one big section.
> That way the interrupt only has to check if the IP is inside this
> section which is minimal effort.
>
> The down side is that all percpu ops would then end up being full
> function calls. Which on some archs is indeed faster than disabling
> interrupts, but not by much I'm afraid.

Anther down site is that all percpu ops can't call any function outside
the section.  Otherwise we would fail to detect whether it is a special
region  or be hard to detect it.

If we disallow the percpu ops calling any function, I think we can
insert some special instructions to the generated code along with
a notation in a table (like exception table for copy_to_user()).
So thus the interrupt only has to check the special instructions
near the IP and confirm it by check it on the table.

>
>> optimize the x86 variants if interrupts also can detect critical sections
>> and restart at defined points.
>
> I really don't see how we can beat %GS prefixes with any such scheme.
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