linux-kernel - Re: [PATCH bpf-next v1 12/22] rqspinlock: Add basic support for CONFIG

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Message-ID: <2eaf52fb-b7d4-4024-a671-02d5375fca22@redhat.com>
Date: Wed, 8 Jan 2025 11:27:12 -0500
From: Waiman Long <llong@...hat.com>
To: Kumar Kartikeya Dwivedi <memxor@...il.com>, bpf@...r.kernel.org,
 linux-kernel@...r.kernel.org
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
 Peter Zijlstra <peterz@...radead.org>, Waiman Long <llong@...hat.com>,
 Alexei Starovoitov <ast@...nel.org>, Andrii Nakryiko <andrii@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>,
 Martin KaFai Lau <martin.lau@...nel.org>,
 Eduard Zingerman <eddyz87@...il.com>, "Paul E. McKenney"
 <paulmck@...nel.org>, Tejun Heo <tj@...nel.org>,
 Barret Rhoden <brho@...gle.com>, Josh Don <joshdon@...gle.com>,
 Dohyun Kim <dohyunkim@...gle.com>, kernel-team@...a.com
Subject: Re: [PATCH bpf-next v1 12/22] rqspinlock: Add basic support for
 CONFIG_PARAVIRT

On 1/7/25 8:59 AM, Kumar Kartikeya Dwivedi wrote:
> We ripped out PV and virtualization related bits from rqspinlock in an
> earlier commit, however, a fair lock performs poorly within a virtual
> machine when the lock holder is preempted. As such, retain the
> virt_spin_lock fallback to test and set lock, but with timeout and
> deadlock detection.
>
> We don't integrate support for CONFIG_PARAVIRT_SPINLOCKS yet, as that
> requires more involved algorithmic changes and introduces more
> complexity. It can be done when the need arises in the future.

virt_spin_lock() doesn't scale well. It is for hypervisors that don't 
support PV qspinlock yet. Now rqspinlock() will be in this category.

I wonder if we should provide an option to disable rqspinlock and fall 
back to the regular qspinlock with strict BPF locking semantics.

Another question that I have is about PREEMPT_RT kernel which cannot 
tolerate any locking stall. That will probably require disabling 
rqspinlock if CONFIG_PREEMPT_RT is enabled.

Cheers,
Longman