linux-kernel - Re: [RFC PATCH] arm64: cpuinfo: reduce cache contention on update_{feature}

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Date:	Wed, 16 Sep 2015 15:49:14 +0100
From:	"Suzuki K. Poulose" <Suzuki.Poulose@....com>
To:	Yury Norov <ynorov@...iumnetworks.com>
CC:	Mark Rutland <Mark.Rutland@....com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"ard.biesheuvel@...aro.org" <ard.biesheuvel@...aro.org>,
	Will Deacon <Will.Deacon@....com>,
	"klimov.linux@...il.com" <klimov.linux@...il.com>,
	"ddaney.cavm@...il.com" <ddaney.cavm@...il.com>,
	"yury.norov@...il.com" <yury.norov@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC PATCH] arm64: cpuinfo: reduce cache contention on update_{feature}_support

On 04/09/15 20:52, Yury Norov wrote:
> On Fri, Sep 04, 2015 at 05:40:57PM +0100, Suzuki K. Poulose wrote:
>> On 04/09/15 17:04, Yury Norov wrote:
>>> This patch is on top of https://lkml.org/lkml/2015/9/2/413
>>>
>>> In master, there's only a single function -
>>> 	update_mixed_endian_el0_support
>>> And similar function is on review mentioned above.
>>>
>>> The algorithm for them is like this:
>>>   - there's system-wide boolean marker for the feature that is
>>>     initially enabled;
>>>   - there's also updater for the feature that may disable it
>>>     system-widely if feature is not supported on current CPU.
>>>   - updater is called for each CPU on bootup.
>>>
>>> The problem is the way updater does its work. On each CPU, it
>>> unconditionally updates system-wide marker. For multi-core
>>> system it makes CPU issue invalidate message for a cache
>>> line containing marker. This invalidate increases cache
>>> contention for nothing, because there's a single marker reset
>>> that is really needed, and the others are useless.
>>>
>>> If the number of system-wide markers of this sort will grow,
>>> it may become a trouble on large-scale SOCs. The fix is trivial,
>>> though: do system-wide marker update conditionally, and preserve
>>> corresponding cache line in shared state for all update() calls,
>>> except, probably, one.
>>
>> As I have mentioned already, this patch (and the per feature functions)
>> won't be needed once we merge my series (which is waiting for the merge
>> window to see the public lights)
>>
>
> OK. Than waiting for your patchset.

http://lists.infradead.org/pipermail/linux-arm-kernel/2015-September/370386.html

Cheers
Suzuki

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