linux-kernel - Re: [RFC PATCH 00/86] Make the kernel preemptible

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

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,
        paulmck@...nel.org, linux-mm@...ck.org, x86@...nel.org,
        akpm@...ux-foundation.org, luto@...nel.org, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        willy@...radead.org, mgorman@...e.de, jon.grimm@....com,
        bharata@....com, raghavendra.kt@....com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com,
        jgross@...e.com, andrew.cooper3@...rix.com, mingo@...nel.org,
        bristot@...nel.org, mathieu.desnoyers@...icios.com,
        geert@...ux-m68k.org, glaubitz@...sik.fu-berlin.de,
        anton.ivanov@...bridgegreys.com, mattst88@...il.com,
        krypton@...ich-teichert.org, rostedt@...dmis.org,
        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.