linux-kernel - Re: [patch V5 00/20] sched: Rewrite MM CID management

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Message-ID: <20260128084616.GD3372621@noisy.programming.kicks-ass.net>
Date: Wed, 28 Jan 2026 09:46:16 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Ihor Solodrai <ihor.solodrai@...ux.dev>
Cc: Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Gabriele Monaco <gmonaco@...hat.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Michael Jeanson <mjeanson@...icios.com>,
	Jens Axboe <axboe@...nel.dk>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	"Gautham R. Shenoy" <gautham.shenoy@....com>,
	Florian Weimer <fweimer@...hat.com>,
	Tim Chen <tim.c.chen@...el.com>, Yury Norov <yury.norov@...il.com>,
	Shrikanth Hegde <sshegde@...ux.ibm.com>, bpf <bpf@...r.kernel.org>,
	sched-ext@...ts.linux.dev, Kernel Team <kernel-team@...a.com>,
	Alexei Starovoitov <ast@...nel.org>,
	Andrii Nakryiko <andrii@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Puranjay Mohan <puranjay@...nel.org>, Tejun Heo <tj@...nel.org>
Subject: Re: [patch V5 00/20] sched: Rewrite MM CID management

On Tue, Jan 27, 2026 at 04:01:11PM -0800, Ihor Solodrai wrote:
> On 11/19/25 9:26 AM, Thomas Gleixner wrote:
> > This is a follow up on the V4 series which can be found here:
> > 
> >     https://lore.kernel.org/20251104075053.700034556@linutronix.de
> > 
> > The V1 cover letter contains a detailed analyisis of the issues:
> > 
> >     https://lore.kernel.org/20251015164952.694882104@linutronix.de
> > 
> > TLDR: The CID management is way to complex and adds significant overhead
> > into scheduler hotpaths.
> > 
> > The series rewrites MM CID management in a more simplistic way which
> > focusses on low overhead in the scheduler while maintaining per task CIDs
> > as long as the number of threads is not exceeding the number of possible
> > CPUs.
> 
> Hello Thomas, everyone.
> 
> BPF CI caught a deadlock on current bpf-next tip (35538dba51b4).
> Job: https://github.com/kernel-patches/bpf/actions/runs/21417415035/job/61670254640
> 
> It appears to be related to this series. Pasting a splat below.
> 
> Any ideas what might be going on?

That splat is only CPU2, that's not typically very useful in a lockup
scenario.