linux-kernel - Re: [PATCH 2/2] tracing: Fix for non-continuous cpu ids

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Date:	Tue, 2 Jun 2015 18:47:25 +0300
From:	Gilad Ben-Yossef <gilad@...yossef.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Gil Fruchter <gilf@...hip.com>, Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Gilad Ben-Yossef <giladb@...hip.com>,
	Chris Metcalf <cmetcalf@...hip.com>, noamc@...hip.com
Subject: Re: [PATCH 2/2] tracing: Fix for non-continuous cpu ids

On Tue, Jun 2, 2015 at 5:26 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Tue, 2 Jun 2015 16:26:29 +0300
> Gil Fruchter <gilf@...hip.com> wrote:
>
>> Currently exception occures due to access beyond buffer_iter
>> range while using index of cpu bigger than num_possible_cpus().
>> Below there is an example for such exception when we use
>> cpus 0,1,16,17.
>
> I'm curious, what broken boxes have non-continuous cpu ids?

The NPS-400 is a single SoC with 256 cores based on Synopsys Arc, each
one is capable to work as up to 16 SMT hardware threads or logical
cores. I gave a talk about it two years ago at Plumbers/LinuxCon.

Most of  the Linux support is in the process being up-streamed via the
Arc architecture maintainer (the patch set is at
https://github.com/EZchip/linux) but there are some none architecture
specific patches that we just happen to run into like this one.

As to the why, well, the CPU ids has a mapping to the topology - which
cluster (NUMA node), core and SMT thread you are looking at.

The thing is, the number of SMT threads per core is configurable, and
you can hot plug down a core, reconfigure it for a different number of
SMT thread and hot them back in. So the "empty slots" are possible
logical cores that don't happen to be active at the time, but could
hotplug up later on.

I hope this makes some sense.

Since the NPS-400 is not generally available yet I wouldn't worry too
much about stable kernel for now...

Thanks,

Gilad Ben-Yossef

-- 
Gilad Ben-Yossef
Chief Coffee Drinker
gilad@...yossef.com
Israel Cell: +972-52-8260388
US Cell: +1-973-8260388
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