linux-kernel - Re: [POC][RFC][PATCH v2] sched: Extended Scheduler Time Slice

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Date:   Mon, 30 Oct 2023 14:39:04 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ankur Arora <ankur.a.arora@...cle.com>, linux-mm@...ck.org,
        x86@...nel.org, akpm@...ux-foundation.org, luto@...nel.org,
        bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com,
        mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, willy@...radead.org, mgorman@...e.de,
        jon.grimm@....com, bharata@....com, raghavendra.kt@....com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com,
        jgross@...e.com, andrew.cooper3@...rix.com,
        Joel Fernandes <joel@...lfernandes.org>,
        Youssef Esmat <youssefesmat@...omium.org>,
        Vineeth Pillai <vineethrp@...gle.com>,
        Suleiman Souhlal <suleiman@...gle.com>,
        Ingo Molnar <mingo@...nel.org>,
        Daniel Bristot de Oliveira <bristot@...nel.org>
Subject: Re: [POC][RFC][PATCH v2] sched: Extended Scheduler Time Slice

On Mon, 30 Oct 2023 14:27:10 -0400
Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:

> > So I just made every unlock disable the extended time slot. I need to go
> > back and enable both a counter and an on/off as I now realize that the spin
> > locks (called within the lwlock) will disable the extend time before the
> > lwlock is released. This should work if I have the spinlocks inc and dec
> > (they are straight forward and all locks are associated with an easily
> > found unlock), and have the lwlock use bit 31 as an on/off switch.  
> 
> This extra on/off switch appears to be working around userspace issues.

Yep!

But that doesn't mean there's not a legitimate use case for it. I don't
want to limit the feature for that. It's unlikely bit 31 would ever be hit
by a counter anyway, for which it could be used as an on/off switch the
same way the NEED_RESCHED bit is used as an on/off switch for preempt_count
in the kernel.

> 
> > Anyway, I would let user space decide what it wants to do, and giving it 31
> > bits to say "I'm extended" and let user space come up with how it handles
> > those 31 bits.  
> 
> If this makes it into the RSEQ uapi, RSEQ should state how userspace
> should collaborate wrt those bits (e.g. nesting counter protocol), even
> though it's not a kernel ABI per se. Otherwise we'll just push this to
> libc to specify this, which is odd.

I agree that user space should have the usage specified. Hell, that bit
could just be used for testing purposes. I think having it reserved is a
good thing than not specifying it and limiting its usage later.

-- Steve