linux-kernel - Re: [PATCH v1 2/2] powerpc/debug: hook to user return notifier infrastructure

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Message-ID: <3fbdc957-2db4-4148-b325-263384f9a196@csgroup.eu>
Date: Wed, 28 Aug 2024 09:27:23 +0200
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Luming Yu <luming.yu@...ngroup.cn>
Cc: "shenghui.qu@...ngroup.cn" <shenghui.qu@...ngroup.cn>,
 npiggin <npiggin@...il.com>, "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
 mpe <mpe@...erman.id.au>, "luming.yu" <luming.yu@...il.com>,
 杨佳龙 <jialong.yang@...ngroup.cn>,
 linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
 linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 2/2] powerpc/debug: hook to user return notifier
 infrastructure



Le 28/08/2024 à 08:50, Luming Yu a écrit :
> On Wed, Aug 28, 2024 at 07:46:52AM +0200, Christophe Leroy wrote:
>> Hi,
>>
>> Le 28/08/2024 à 05:17, 虞陆铭 a écrit :
>>> Hi,
>>>
>>> it appears the little feature might require a little bit more work to find its value of the patch.
>>>
>>> Using the following debug module ,  some debugging shows the TIF_USER_RETURN_NOTIFY
>>> bit is propagated in __switch_to among tasks , but USER_RETURN_NOTIFY call back seems to
>>> be dropped somewhere on somone who carries the bit return to user space.
>>> side notes:
>>> there is an issue that the module symbols is not appended to /sys/kernel/debug/tracing/available_filter_functions
>>> which should be sovled first to make it easier for further debuggig.
>>
>> As far as I can see, user return notifier infrastructure was implemented in
>> 2009 for KVM on x86, see
>> https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Fall%2F1253105134-8862-1-git-send-email-avi%40redhat.com%2F&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C260e5ecf10764312459c08dcc72dc2f5%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C638604246584044745%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C0%7C%7C%7C&sdata=3hjAzcVu3xOq0QNK5WINQ8trLd9Xp7XCiQjw2htabpQ%3D&reserved=0
>>
>> Can you explain what is your usage of that infrastructure with your patch ?
>> You are talking about debug, what's the added value, what is it used for ?
> one example: I was thinking to live patch kernel at the moment that all cpus are
> either returning to user space or going into idle. But I'm not sure if it is truly
> valuable. secondly, it can help us get more accurate user/system time
> accounting via tracing rather than through sampling technique.
> The third: it could have similar usages in kvm for ppc as x86 for tsc_aux.
> etc :-)

Thanks.

Don't we already have a very accurate user/system time accounting with 
CONFIG_VIRT_CPU_ACCOUNTING_NATIVE ?

Christophe