linux-kernel - Re: [PATCH v4 14/17] x86/asm/entry/64: Migrate error and interrupt exit work to C

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Date:	Fri, 3 Jul 2015 09:29:11 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Borislav Petkov <bp@...en8.de>, Andy Lutomirski <luto@...nel.org>,
	X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Rik van Riel <riel@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Kees Cook <keescook@...omium.org>,
	Brian Gerst <brgerst@...il.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v4 14/17] x86/asm/entry/64: Migrate error and interrupt
 exit work to C

On Fri, Jul 3, 2015 at 9:27 AM, Andy Lutomirski <luto@...capital.net> wrote:
>
> I haven't benchmarked very carefully.  IIRC syscall timing was
> unaffected by this series.  Exceptions ought to be unaffected unless
> something very weird is going on with the trace cache or branch
> predictor.  IRQs will slow down by a couple cycles (I'm guessing 5,
> since it's more or less the same change I benchmarked when I was
> playing with the next pile of patches).

One clarification: I bet that context tracking performance changes
with these patches, but it sucked before and it'll suck afterwards
regardless.  I think that these patches are a good step in making it
not suck, though -- they will allow us to only change ct state once on
entry and once on exit, and we used to get it wrong.  They also allow
us to do context tracking with IRQs off, which Rik van Riel wanted to
help his speedup project.

So, in the long run, I expect a huge context tracking improvement, and
these patches will help.  By themselves, they won't do much.

--Andy
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