From: Peter Zijlstra <peterz@infradead•org>
To: Pan Xinhui <xinhui@linux•vnet.ibm.com>
Cc: linux-kernel@vger•kernel.org, linuxppc-dev@lists•ozlabs.org,
benh@kernel•crashing.org, paulus@samba•org, mpe@ellerman•id.au,
boqun.feng@gmail•com, paulmck@linux•vnet.ibm.com,
tglx@linutronix•de
Subject: Re: [PATCH V3] powerpc: Implement {cmp}xchg for u8 and u16
Date: Thu, 21 Apr 2016 18:13:54 +0200 [thread overview]
Message-ID: <20160421161354.GI3430@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <5718F32B.3050409@linux.vnet.ibm.com>
On Thu, Apr 21, 2016 at 11:35:07PM +0800, Pan Xinhui wrote:
> yes, you are right. more load/store will be done in C code.
> However such xchg_u8/u16 is just used by qspinlock now. and I did not see any performance regression.
> So just wrote in C, for simple. :)
Which is fine; but worthy of a note in your Changelog.
> Of course I have done xchg tests.
> we run code just like xchg((u8*)&v, j++); in several threads.
> and the result is,
> [ 768.374264] use time[1550072]ns in xchg_u8_asm
> [ 768.377102] use time[2826802]ns in xchg_u8_c
>
> I think this is because there is one more load in C.
> If possible, we can move such code in asm-generic/.
So I'm not actually _that_ familiar with the PPC LL/SC implementation;
but there are things a CPU can do to optimize these loops.
For example, a CPU might choose to not release the exclusive hold of the
line for a number of cycles, except when it passes SC or an interrupt
happens. This way there's a smaller chance the SC fails and inhibits
forward progress.
By doing the modification outside of the LL/SC you loose such
advantages.
And yes, doing a !exclusive load prior to the exclusive load leads to an
even bigger window where the data can get changed out from under you.
next prev parent reply other threads:[~2016-04-21 16:14 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-04-19 6:29 [PATCH V2] powerpc: Implement {cmp}xchg for u8 and u16 Pan Xinhui
2016-04-19 9:18 ` Boqun Feng
2016-04-20 3:39 ` Pan Xinhui
2016-04-20 13:24 ` [PATCH V3] " Pan Xinhui
2016-04-20 14:24 ` Peter Zijlstra
2016-04-21 15:35 ` Pan Xinhui
2016-04-21 15:52 ` Boqun Feng
2016-04-22 1:59 ` Pan Xinhui
2016-04-22 3:16 ` Boqun Feng
2016-04-21 16:13 ` Peter Zijlstra [this message]
2016-04-25 10:10 ` Pan Xinhui
2016-04-25 15:37 ` Peter Zijlstra
2016-04-26 11:35 ` Pan Xinhui
2016-04-27 9:16 ` [PATCH V4] " Pan Xinhui
2016-04-27 13:58 ` Boqun Feng
2016-04-27 14:16 ` Boqun Feng
2016-04-27 14:50 ` Boqun Feng
2016-04-27 14:59 ` Boqun Feng
2016-04-28 10:21 ` Pan Xinhui
2016-04-28 7:59 ` Peter Zijlstra
2016-04-28 10:21 ` Pan Xinhui
2016-11-25 0:04 ` [V4] " Michael Ellerman
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20160421161354.GI3430@twins.programming.kicks-ass.net \
--to=peterz@infradead$(echo .)org \
--cc=benh@kernel$(echo .)crashing.org \
--cc=boqun.feng@gmail$(echo .)com \
--cc=linux-kernel@vger$(echo .)kernel.org \
--cc=linuxppc-dev@lists$(echo .)ozlabs.org \
--cc=mpe@ellerman$(echo .)id.au \
--cc=paulmck@linux$(echo .)vnet.ibm.com \
--cc=paulus@samba$(echo .)org \
--cc=tglx@linutronix$(echo .)de \
--cc=xinhui@linux$(echo .)vnet.ibm.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox