linux-kernel - Re: [RFC PATCH] Add split

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Message-ID: <YG7YWiaEZ1VKqFmb@hirez.programming.kicks-ass.net>
Date:   Thu, 8 Apr 2021 12:18:02 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [RFC PATCH] Add split_lock

On Thu, Apr 08, 2021 at 06:23:38AM +0100, Matthew Wilcox (Oracle) wrote:
> bit_spinlocks are horrible on RT because there's absolutely nowhere
> to put the mutex to sleep on.  They also do not participate in lockdep
> because there's nowhere to put the map.
> 
> Most (all?) bit spinlocks are actually a split lock; logically they
> could be treated as a single spinlock, but for performance, we want to
> split the lock over many objects.  Introduce the split_lock as somewhere
> to store the lockdep map and as somewhere that the RT kernel can put
> a mutex.  It may also let us store a ticket lock for better performance
> on non-RT kernels in the future, but I have left the current cpu_relax()
> implementation intact for now.

I think I like it, but I'm not sure it'll work for RT as is. It's a bit
like qrwlock in that it only uses the internal (split) lock for
contention, but that doesn't work for PI.

I've not recently looked at RT, but I think they simply used to bloat a
number of the data structures with a real lock. Sebastian and Thomas
will know better.