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Message-ID: <CALCETrX4y2_cPdPHsr=iqcqS7k3GvBz7yBGb_d4A0wDUzbTWCg@mail.gmail.com>
Date: Tue, 26 May 2015 14:44:06 -0700
From: Andy Lutomirski <luto@...capital.net>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Andi Kleen <andi@...stfloor.org>, Borislav Petkov <bp@...en8.de>,
"H. Peter Anvin" <hpa@...or.com>,
Lai Jiangshan <laijs@...fujitsu.com>,
Ben Maurer <bmaurer@...com>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
Ingo Molnar <mingo@...hat.com>,
Josh Triplett <josh@...htriplett.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Michael Kerrisk <mtk.manpages@...il.com>,
Linux API <linux-api@...r.kernel.org>,
Linux Kernel <linux-kernel@...r.kernel.org>,
Paul Turner <pjt@...gle.com>,
Peter Zijlstra <peterz@...radead.org>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Steven Rostedt <rostedt@...dmis.org>,
Andrew Hunter <ahh@...gle.com>
Subject: Re: [RFC PATCH] percpu system call: fast userspace percpu critical sections
On Tue, May 26, 2015 at 2:18 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Tue, May 26, 2015 at 2:04 PM, Mathieu Desnoyers
>>
>>>
>>> It's too bad that not all architectures have a single-instruction
>>> unlocked compare-and-exchange.
>>
>> Based on my benchmarks, it's not clear that single-instruction
>> unlocked CAS is actually faster than doing the same with many
>> instructions.
>
> True, but with a single instruction the user can't get preempted in the middle.
>
> Looking at your code, it looks like percpu_user_sched_in has some
> potentially nasty issues with page faults. Avoiding touching user
> memory from the scheduler would be quite nice from an implementation
> POV, and the x86-specific gs hack wins in that regard.
ARM has "TLB lockdown entries" which could, I think, be used to
implement per-cpu or per-thread mappings. I'm actually rather
surprised that Linux doesn't already use a TLB lockdown entry for TLS.
(Hmm. Maybe it's because the interface to write the entries requires
actually touching the page. Maybe not -- the ARM docs, in general,
seem to be much less clear than the Intel and AMD docs.)
ARM doesn't seem to have any single-instruction compare-exchange or
similar instruction, though, so this might be all that useful. On the
other hand, ARM can probably do reasonably efficient per-cpu memory
allocation and such with a single ldrex/strex pair.
--Andy
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