linux-kernel - Re: Re-tune x86 uaccess code for PREEMPT

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Message-ID: <20130814182745.GB19640@redhat.com>
Date:	Wed, 14 Aug 2013 21:27:45 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org, mingo@...nel.org,
	torvalds@...ux-foundation.org
Subject: Re: Re-tune x86 uaccess code for PREEMPT_VOLUNTARY

On Fri, Aug 09, 2013 at 04:04:07PM -0700, Andi Kleen wrote:
> The x86 user access functions (*_user) were originally very well tuned,
> with partial inline code and other optimizations.
> 
> Then over time various new checks -- particularly the sleep checks for
> a voluntary preempt kernel -- destroyed a lot of the tunings
> 
> A typical user access operation is now doing multiple useless
> function calls. Also the without force inline gcc's inlining
> policy makes it even worse, with adding more unnecessary calls.
> 
> Here's a typical example from ftrace:
> 
>      10)               |    might_fault() {
>      10)               |      _cond_resched() {
>      10)               |        should_resched() {
>      10)               |          need_resched() {
>      10)   0.063 us    |            test_ti_thread_flag();
>      10)   0.643 us    |          }
>      10)   1.238 us    |        }
>      10)   1.845 us    |      }
>      10)   2.438 us    |    }
> 
> So we spent 2.5us doing nothing (ok it's a bit less without
> ftrace, but still pretty bad)

Hmm, which kernel version is this?

I thought I fixed this for good in
	commit 114276ac0a3beb9c391a410349bd770653e185ce
	Author: Michael S. Tsirkin <mst@...hat.com>
	Date:   Sun May 26 17:32:13 2013 +0300
	    mm, sched: Drop voluntary schedule from might_fault()

might_fault shouldn't be calling cond_resched anymore.

Did this get reverted at some point?
I hope not, there's more code relying on this now
(see e.g. 662bbcb2747c2422cf98d3d97619509379eee466)

> Then in other cases we would have an out of line function,
> but would actually do the might_sleep() checks in the inlined
> caller. This doesn't make any sense at all.
> 
> There were also a few other problems, for example the x86-64 uaccess
> code regularly falls back to string functions, even though a simple
> mov would be enough. For example every futex access to the lock
> variable would actually use string instructions, even though 
> it's just 4 bytes.
> 
> This patch kit is an attempt to get us back to sane code, 
> mostly by doing proper inlining and doing sleep checks in the right
> place. Unfortunately I had to add one tree sweep to avoid an nasty
> include loop.
> 
> It costs a bit of text space, but I think it's worth it
> (if only to keep my blood pressure down while reading ftrace logs...)
> 
> I haven't done any particular benchmarks, but important low level
> functions just ought to be fast.
> 
> 64bit:
> 13249492        1881328 1159168 16289988         f890c4 vmlinux-before-uaccess
> 13260877        1877232 1159168 16297277         f8ad3d vmlinux-uaccess
> + 11k, +0.08%
> 
> 32bit:
> 11223248         899512 1916928 14039688         d63a88 vmlinux-before-uaccess
> 11230358         895416 1916928 14042702         d6464e vmlinux-uaccess
> + 7k, +0.06%
> 
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