linux-kernel - Re: [PATCH] sched/tracing: append prev

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Date:   Fri, 22 Apr 2022 13:09:03 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Delyan Kratunov <delyank@...com>
Cc:     "valentin.schneider@....com" <valentin.schneider@....com>,
        "bigeasy@...utronix.de" <bigeasy@...utronix.de>,
        "dietmar.eggemann@....com" <dietmar.eggemann@....com>,
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        "andrii@...nel.org" <andrii@...nel.org>,
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        "mingo@...nel.org" <mingo@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "rdunlap@...radead.org" <rdunlap@...radead.org>,
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        "juri.lelli@...hat.com" <juri.lelli@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH] sched/tracing: append prev_state to tp args instead

On Thu, Apr 21, 2022 at 10:12:25PM +0000, Delyan Kratunov wrote:
> Hi folks,
> 
> While working on bpf tooling, we noticed that the sched_switch tracepoint
> signature recently changed in an incompatible manner. This affects the
> runqslower tools in the kernel tree, as well as multiple libbpf tools in iovisor/bcc.
> 
> It would be a fair amount of churn to fix all these tools, not to mention any
> non-public tools people may be using.

So on the one hand, too bad, deal with it. None of this is ABI.

> If you are open to it, here's a
> description and a patch that moves the new argument to the end,
> so existing tools can continue working without change (the new argument
> just won't be extracted in existing programs):

And on the other hand; those users need to be fixed anyway, right?
Accessing prev->__state is equally broken.

Yes, the proposed hack is cute, but I have very little sympathy for any
of this. These are in-kernel interfaces, they change, there must not be
any impediment to change.

If bpf wants to ride on them, it needs to suffer the pain of doing so.