linux-kernel - Re: context tracking vs. syscall_trace_leave & do_notify

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Message-ID: <CALCETrUhmZjF1NbYHHE=CBswvrwNOyZja7zxpXeEjbFv-mRq5Q@mail.gmail.com>
Date:	Fri, 1 May 2015 08:55:19 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Rik van Riel <riel@...hat.com>
Cc:	Fr馘駻ic Weisbecker <fweisbec@...il.com>,
	X86 <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Denys Vlasenko <dvlasenk@...hat.com>
Subject: Re: context tracking vs. syscall_trace_leave & do_notify_resume loop

On Thu, Apr 30, 2015 at 6:30 PM, Rik van Riel <riel@...hat.com> wrote:
> Andy pointed out to me something I should have seen earlier: both
> syscall_trace_leave and do_notify_resume call both user_exit()
> and user_enter(), which has the potential to greatly increase the
> cost of context tracking.
>
> I believe (though it is hard to know for sure) there are legitimate
> reasons why there is a loop around syscall_trace_leave and
> do_notify_resume, but I strongly suspect the context tracking code
> does not need to be in that loop.
>
> I suspect it would be possible to stick a call to a new function
> (return_to_user ?) right after the DISABLE_INTERRUPTS below, which
> could be used to do the context tracking user_enter just once, and
> later on also to load the user FPU context (patches I have sitting
> around).
>
> syscall_return:
>         /* The IRETQ could re-enable interrupts: */
>         DISABLE_INTERRUPTS(CLBR_ANY)
>         TRACE_IRQS_IRETQ
>
> Andy, Denys, do you guys see any issues with that idea?

Ick.  Let's make the mess better before we make it worse.  Now that
Denys disentangled the syscall exit path from the interrupt exit path,
let me see if I can just rewrite the syscall exit path entirely later
this week.
'
--Andy
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