linux-kernel - Re: [PATCH 1/5] Split wait_noreap

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Message-ID: <87zldjwemo.wl%vmayatsk@redhat.com>
Date:	Mon, 11 May 2009 22:47:43 +0200
From:	Vitaly Mayatskikh <v.mayatskih@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>,
	Vitaly Mayatskikh <v.mayatskih@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1/5] Split wait_noreap_copyout()

At Mon, 11 May 2009 14:17:08 +0200, Ingo Molnar wrote:

> > 	if (put_user(signal, &infop->si_signo) ||
> > 	    put_user(0, &infop->si_errno) ||
> > 	    put_user((short)why, &infop->si_code) ||
> > 	    put_user(pid, &infop->si_pid) ||
> > 	    put_user(uid, &infop->si_uid) ||
> > 	    put_user(status, &infop->si_status))
> > 		return -EFAULT;
> 
> For best assembly code this should generally be written as a series 
> of:
> 
>    __uaccess_err |= __put_user(x, ptr);
>    __uaccess_err |= __put_user(y, ptr);
>    __uaccess_err |= __put_user(z, ptr);
> 
> As this generates non-dependent, compressed, branch-less code.

Yeah, my first intention was to eliminate a lot of branches in one
place. It's terrible for CPU pipeline, I bet.

> See the (new) put_user_try / put_user_ex() / put_user_catch() 
> abstraction in arch/x86/include/asm/uaccess.h, and how all the x86 
> signal code makes use of that to optimize such patterns of per field 
> user copies.

So, there's catch block to handle GPF and the code inside of `try'
block is still branch-less, right? I was thinking of minimized version
of struct siginfo (up to si_uid) and copying it with single
copy_to_user(), but the idea with try/catch is definitely much
better.

-- 
wbr, Vitaly
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