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

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Message-ID: <YoNwp+9ko89Tf1ep@zx2c4.com>
Date:   Tue, 17 May 2022 11:53:43 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.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.

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. 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...

Jason