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

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Date:   Mon, 13 Jun 2022 10:11:02 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        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 Tue, May 17, 2022 at 02:08:12PM +0200, Sebastian Andrzej Siewior wrote:
> On 2022-05-17 11:59:19 [+0200], Jason A. Donenfeld wrote:
> > On Tue, May 17, 2022 at 11:53:43AM +0200, Jason A. Donenfeld wrote:
> > > Hi Sebastian,
> 
> Hi Jason,
> 
> > > 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.
> 
> lockdep is special here. Haven't seen other explosions so far ;)
> 
> > > 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. 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...
> 
> it might work I guess. PeterZ? Would this_cpu_inc_return() work?

Probably. But sched_clock() is plenty fine enough. No need to waste
space on a variable.

> 
> > Also, what is that `1 +` doing there? If the intention is to make sure
> > this is non-zero, you might want the mask to be 0xfffe? Or you're
> > counting on the assigned type being a u32 so it all overflows into the
> > next zone the way you want it? Kinda weird.
> 
> hmm. It used to be 1 before prandom_u32() was introduced and the point
> is probably to have a cookie != 0. val and pin_count are both unsigned
> int/ 32bit so that overflow doesn't matter.

Right, must not be 0.