From: Jonathan Austin <jonathan.austin@arm.com>
To: "André Hentschel" <nerv@dawncrow.de>
Cc: "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
Russell King - ARM Linux <linux@arm.linux.org.uk>,
"linux-arm-kernel@lists.infradead.org"
<linux-arm-kernel@lists.infradead.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Will Deacon <Will.Deacon@arm.com>
Subject: Re: arm: Only load TLS values when needed
Date: Wed, 17 Jul 2013 12:10:16 +0100 [thread overview]
Message-ID: <51E67B98.9040101@arm.com> (raw)
In-Reply-To: <51E59E8F.1060501@dawncrow.de>
Hi André,
On 16/07/13 20:27, André Hentschel wrote:
> Hi Jonathan, First, thank you for your review.
>
> Am 16.07.2013 19:31, schrieb Jonathan Austin:
>> Hi André,
>>
>> On 15/07/13 18:14, André Hentschel wrote:
>>> From: André Hentschel <nerv@dawncrow.de>
>>>
>>> This patch intents to reduce loading instructions when the
>>> resulting value is not used. It's a follow up on
>>> a4780adeefd042482f624f5e0d577bf9cdcbb760
>>>
>>
>> Have you done any benchmarking to see that this has any real
>> impact? Or tested on a !Vv6k system? It looks possible that the
>> only case where this will perform better is where we're using
>> switch_tls_none or switch_tls_software (both rare cases, I think)
>> and there's some change it will make things worse in other cases?
>
> I have to admit that i only tested it on v6k and did no benchmark.
>
Do you have access to anything v6-NOT-k-ish? If not I can try and test
this on something appropriate. How does your test-case access tpidrurw?
If it uses inline asm then it won't work on v6-not-k, as those
instructions aren't defined...
>> One of the reasons for Russell's suggestion of placing the ldrd
>> (which became the two ldr instructions you've removed from
>> __switch_to, in order to maintain building for older cores) where
>> it is was in order to reduce the chance of pipeline stalls.
>>
>> As I've pointed out below, there is some risk that changing that
>> has implications for the v6 only case below (and the v6k case is
>> now more prone to stalls with !CONFIG_CPU_USE_DOMAINS, but newer
>> cores should have more advanced scheduling to avoid such issues
>> anyway...)
>
> I'm not sure how this could make things worse on v6k, could you
> elaborate please? Besides of the ldr and str being too close to each
> other
Yea, that's the only issue, and in the !CONFIG_CPU_USE_DOMAINS case
things are slightly worse than they were before
> i thought this patch is a good idea, because it removes two ldr
> which are always executed. (Continuing below...)
Indeed, as long as it doesn't cause pipeline stalls then that's a gain
for some cases :)
[...]
>> Now we've only got one instruction between the store and the load
>> and risk stalling the pipeline...
>>
>> Dave M cautiously says "The ancient advice was that one instruction
>> was enough" but this is very core dependent... I wonder if anyone
>> has a good idea about whether this is an issue here...?
>
> We could use a ldrd at the top, that'd be nearly what we have right
> now, don't we?
Yea, that'd be good - as far as I can see from an 1136 TRM, the ldrd
*may* be two cycles (depending on alignment of the words) but the ldr
and ldrne will always be two cycles. Ahhh, the joys of modifying the
fast path ;)
Jonny
>
>
>
next prev parent reply other threads:[~2013-07-17 11:10 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-07-15 17:14 arm: Only load TLS values when needed André Hentschel
2013-07-16 17:31 ` Jonathan Austin
2013-07-16 19:27 ` André Hentschel
2013-07-17 11:10 ` Jonathan Austin [this message]
2013-07-17 19:49 ` André Hentschel
[not found] ` <520B8F37.4040609@dawncrow.de>
[not found] ` <520BAE58.3060600@arm.com>
2013-08-14 21:21 ` André Hentschel
2013-08-15 17:29 ` Jonathan Austin
2013-08-15 18:27 ` André Hentschel
2013-08-15 18:27 ` André Hentschel
2013-08-26 19:19 ` André Hentschel
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