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

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Date:	Tue, 14 Jul 2015 13:00:59 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, Chris Mason <clm@...com>,
	"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>
Subject: Re: [Ksummit-discuss] [CORE TOPIC] lightweight per-cpu locks /
 restartable sequences

On Mon, Jul 13, 2015 at 7:01 AM, Christoph Lameter <cl@...ux.com> wrote:
> On Mon, 13 Jul 2015, Peter Zijlstra wrote:
>
>> 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.
>
> Well one could move the entire functions that are using these ops into the
> special sections. That is certainly an area requiring much more thought.

Hmm.

>
>> > 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.
>
> We may be able to avoid RMV sequences which allows the processor to better
> schedule operations.

True, but cmpxchg is, surprisingly, pretty fast.

Crazy thought: At the risk of proposing something ridiculous, what if
we had per-cpu memory mappings?  We could do this at the cost of up to
2kB of memcpy whenever we switch mms.  Expensive but maybe not a
showstopper.

--Andy
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