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Date:   Tue, 7 Nov 2023 20:08:03 -0800 (PST)
From:   Christoph Lameter <cl@...ux.com>
To:     Ankur Arora <ankur.a.arora@...cle.com>
cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de,
        peterz@...radead.org, torvalds@...ux-foundation.org,
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        bharata@....com, raghavendra.kt@....com,
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        David.Laight@...LAB.COM, richard@....at, mjguzik@...il.com
Subject: Re: [RFC PATCH 00/86] Make the kernel preemptible

The kernel is not preemptible???? What are you smoking?

On Tue, 7 Nov 2023, Ankur Arora wrote:

> In voluntary models, the scheduler's job is to match the demand
> side of preemption points (a task that needs to be scheduled) with
> the supply side (a task which calls cond_resched().)

Voluntary preemption models are important for code optimization because 
the code can rely on the scheduler not changing the cpu we are running on. 
This allows removing code for preempt_enable/disable to be removed from 
the code and allows better code generation. The best performing code is 
generated with defined preemption points when we have a guarantee that the 
code is not being rescheduled on a different processor. This is f.e. 
important for consistent access to PER CPU areas.

> To do this add a new flag, TIF_NEED_RESCHED_LAZY which allows the
> scheduler to mark that a reschedule is needed, but is deferred until
> the task finishes executing in the kernel -- voluntary preemption
> as it were.

That is different from the current no preemption model? Seems to be 
the same.

> There's just one remaining issue: now that explicit preemption points are
> gone, processes that spread a long time in the kernel have no way to give
> up the CPU.

These are needed to avoid adding preempt_enable/disable to a lot of 
primitives that are used for synchronization. You cannot remove those 
without changing a lot of synchronization primitives to always have to 
consider being preempted while operating.

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