linux-kernel - Re: [RESEND][PATCH v2 1/3] sched/tp: Add new tracepoint to track uclamp set from user-space

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Date:   Tue, 4 Jul 2023 15:02:59 +0100
From:   Qais Yousef <qyousef@...alina.io>
To:     Lukasz Luba <lukasz.luba@....com>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
        rafael@...nel.org, linux-pm@...r.kernel.org, rostedt@...dmis.org,
        mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com,
        vschneid@...hat.com, delyank@...com, qyousef@...gle.com
Subject: Re: [RESEND][PATCH v2 1/3] sched/tp: Add new tracepoint to track
 uclamp set from user-space

On 07/04/23 08:49, Lukasz Luba wrote:
> Hi Masami, Qais,
> 
> On 6/30/23 12:49, Qais Yousef wrote:
> > On 06/21/23 12:25, Masami Hiramatsu wrote:
> > > On Wed, 31 May 2023 19:26:29 +0100
> > > Qais Yousef <qyousef@...alina.io> wrote:
> > > 
> > > > On 05/22/23 15:57, Lukasz Luba wrote:
> > > > > The user-space can set uclamp value for a given task. It impacts task
> > > > > placement decisions made by the scheduler. This is very useful information
> > > > > and helps to understand the system behavior or track improvements in
> > > > > middleware and applications which start using uclamp mechanisms and report
> > > > > better performance in tests.
> > > > 
> > > > Do you mind adding a generic one instead please? And explain why we can't just
> > > > attach to the syscall via kprobes? I think you want to bypass the permission
> > > > checks, so maybe a generic tracepoint after that might be justifiable?
> > > 
> > > Could you tell me more about this point? I would like to know what kind of
> > > permission checks can be bypassed with tracepoints.
> > 
> > Sorry bad usage of English from my end.
> > 
> > The syscall can fail if the caller doesn't have permission to change the
> > attribute (some of them are protected with CAP_NICE) or if the boundary check
> > fails. The desire here is to emit a tracepoint() when the user successfully
> > changes an attribute of a task.
> > 
> > Lukasz would like to have this tracepoint to help debug and analyse workloads.
> > We are not really bypassing anything. So to rephrase, emit the tracepointn if
> > the syscall is successfully changing an attribute.
> 
> Sorry, but no. As I said, I don't want to add more dependencies in our
> tooling and kernel configuration.

Fair enough. But as I said before a dedicate uclamp only tracepoint doesn't
make sense to me. If maintainers are convinced, then be it. My point of view is
that We want generic tracepoints that scale to other use cases and it makes
sense to go this way to accommodate all potential future users, not just you.


Cheers

--
Qais Yousef