linux-kernel - Re: [PATCH] sched/core: Export pelt_thermal

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Message-ID: <20211125165616.2i24qr6wtuddl32q@e107158-lin.cambridge.arm.com>
Date:   Thu, 25 Nov 2021 16:56:16 +0000
From:   Qais Yousef <qais.yousef@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Thara Gopinath <thara.gopinath@...aro.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/core: Export pelt_thermal_tp

Hi Peter

On 10/28/21 18:50, Peter Zijlstra wrote:
> On Thu, Oct 28, 2021 at 09:22:05AM -0700, Christoph Hellwig wrote:
> > On Thu, Oct 28, 2021 at 06:18:55PM +0200, Peter Zijlstra wrote:
> > > > > @@ -36,6 +36,7 @@ EXPORT_TRACEPOINT_SYMBOL_GPL(pelt_rt_tp);
> > > > >  EXPORT_TRACEPOINT_SYMBOL_GPL(pelt_dl_tp);
> > > > >  EXPORT_TRACEPOINT_SYMBOL_GPL(pelt_irq_tp);
> > > > >  EXPORT_TRACEPOINT_SYMBOL_GPL(pelt_se_tp);
> > > > > +EXPORT_TRACEPOINT_SYMBOL_GPL(pelt_thermal_tp);
> > > > >  EXPORT_TRACEPOINT_SYMBOL_GPL(sched_cpu_capacity_tp);
> > > > >  EXPORT_TRACEPOINT_SYMBOL_GPL(sched_overutilized_tp);
> > > > >  EXPORT_TRACEPOINT_SYMBOL_GPL(sched_util_est_cfs_tp);
> > > > 
> > > > ... and while we're at it, all these exports are unused and should
> > > > be deleted as well.
> > > 
> > > This is my concession wrt tracepoints. Actual tracepoints are ABI,
> > > exports are in-kernel interfaces and are explicitly not ABI.
> > > 
> > > This way people can use an external module to get at the tracepoint data
> > > without having in-tree tracepoints.
> > 
> > All of this makes no sense at all.  These are entirely dead exports.
> > If you remove them nothing else changes.  Note taht the tracepoints
> > do have in-kernel callers, so if people thing of them as an ABI you've
> > got your ABI already with or without the exports.
> 
> These are not normal traceevents, these are tracepoints, the distinction
> is that these things do not show up in tracefs and there is no userspace
> visible representation of them. No userspace gives no ABI.
> 
> All they provide is the in-code hook and in-kernel registration
> interface. These EXPORTS export that registration interface, such that
> an out-of-tree module can make use of them.
> 
> And yes, unused exports are iffy, out-of-tree modules are iffy, but in
> this case I made an exception since ABI contraints are worse. We very
> clearly state there is no such thing is kabi, so breaking any user of
> these exports if fair game.
> 
> Breaking users of userspace trace-events gets kernel patches reverted
> (been there, done that, never want to ever be there again).
> 
> People want to trace this stuff, but I *REALLY* do not want to commit to
> ABI, this is the middle-ground that sucks least :/

AFAICS this wasn't picked up. Should I tweak the commit message to make things
clearer?

Thanks!

--
Qais Yousef