linux-kernel - Re: [PATCH] locking/lockdep: Use sched

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Message-ID: <YqbyUfp8e4CZoFBT@zx2c4.com>
Date:   Mon, 13 Jun 2022 10:16:17 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Theodore Ts'o <tytso@....edu>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] locking/lockdep: Use sched_clock() for random numbers.

On Mon, Jun 13, 2022 at 10:05:43AM +0200, Peter Zijlstra wrote:
> On Tue, May 17, 2022 at 11:53:43AM +0200, Jason A. Donenfeld wrote:
> > Hi Sebastian,
> > 
> > Interesting RT consideration. I hope there aren't too many of these
> > special cases that would necessitate a general mechanism. Fingers
> > crossed this is the only one.
> > 
> > On Tue, May 17, 2022 at 11:16:14AM +0200, Sebastian Andrzej Siewior wrote:
> > > -			cookie.val = 1 + (prandom_u32() >> 16);
> > > +			cookie.val = 1 + (sched_clock() & 0xffff);
> > >  			hlock->pin_count += cookie.val;
> >  
> > I have no idea what the requirements here are.
> 
> Mostly nothing. It's debug code, and if someone wants to circumvent they
> can, but then their code is ugly and stands out like a sort thumb which
> then serves its goal as it won't pass review etc..
> 
> > What would happen if you
> > just did atomic_inc_return(&some_global) instead? That'd be faster
> > anyhow, and it's not like 16 bits gives you much variance anyway...
> 
> That would in fact be slower, sched_clock() will, on any sane hardware,
> be a rdtsc, mul and shr, which are all local.

Fine by me. Given Sasha's comment, we should probably get this queued up
in somebody's tree asap?

Jason