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

[Christophe Leroy]

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