linux-kernel - Re: s390 && user_enable_single_step() (Was: odd utrace testing results on s390x)

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Message-ID: <20100121203207.GA20050@redhat.com>
Date:	Thu, 21 Jan 2010 21:32:07 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:	Roland McGrath <roland@...hat.com>, caiqian@...hat.com,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Jan Kratochvil <jkratoch@...hat.com>,
	linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
	utrace-devel@...hat.com
Subject: Re: s390 && user_enable_single_step() (Was: odd utrace testing
	results on s390x)

On 01/07, Martin Schwidefsky wrote:
>
> On Wed,  6 Jan 2010 12:56:33 -0800 (PST)
> Roland McGrath <roland@...hat.com> wrote:
>
> > In other circumstances with utrace, it is very possible to wind up with
> > user_disable_single_step being called superfluously when there was no
> > stop (and so not necessarily any context switch or other high overhead).
> > On other machines, user_disable_single_step is pretty cheap even where
> > user_enable_single_step is quite costly.  Given how simple and cheap it
> > is to short-circuit the excess work on s390, I think it is worthwhile.
>
> We could use the same compare of the control registers as the code in
> __switch_to. See below.

FYI, I tested your c3311c13adc1021e986fef12609ceb395ffc5014 commit which
does this optimization (compared to the patch you sent previously), it
works fine.

But please see another email I am going to send...

Oleg.

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