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]

Message-ID: <f97b7506a84e43ea82eb0b947324e835@AcuMS.aculab.com>
Date:   Wed, 8 Nov 2023 09:43:10 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Ankur Arora' <ankur.a.arora@...cle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "paulmck@...nel.org" <paulmck@...nel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "luto@...nel.org" <luto@...nel.org>, "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "juri.lelli@...hat.com" <juri.lelli@...hat.com>,
        "vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
        "willy@...radead.org" <willy@...radead.org>,
        "mgorman@...e.de" <mgorman@...e.de>,
        "jon.grimm@....com" <jon.grimm@....com>,
        "bharata@....com" <bharata@....com>,
        "raghavendra.kt@....com" <raghavendra.kt@....com>,
        "boris.ostrovsky@...cle.com" <boris.ostrovsky@...cle.com>,
        "konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
        "jgross@...e.com" <jgross@...e.com>,
        "andrew.cooper3@...rix.com" <andrew.cooper3@...rix.com>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "bristot@...nel.org" <bristot@...nel.org>,
        "mathieu.desnoyers@...icios.com" <mathieu.desnoyers@...icios.com>,
        "geert@...ux-m68k.org" <geert@...ux-m68k.org>,
        "glaubitz@...sik.fu-berlin.de" <glaubitz@...sik.fu-berlin.de>,
        "anton.ivanov@...bridgegreys.com" <anton.ivanov@...bridgegreys.com>,
        "mattst88@...il.com" <mattst88@...il.com>,
        "krypton@...ich-teichert.org" <krypton@...ich-teichert.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "richard@....at" <richard@....at>,
        "mjguzik@...il.com" <mjguzik@...il.com>
Subject: RE: [RFC PATCH 00/86] Make the kernel preemptible

From: Ankur Arora
> Sent: 07 November 2023 21:57
...
> There are four main sets of preemption points in the kernel:
> 
>  1. return to user
>  2. explicit preemption points (cond_resched() and its ilk)
>  3. return to kernel (tick/IPI/irq at irqexit)
>  4. end of non-preemptible sections at (preempt_count() == preempt_offset)
> 
...
> Policies:
> 
> A - preemption=none: run to completion
> B - preemption=voluntary: run to completion, unless a task of higher
>     sched-class awaits
> C - preemption=full: optimized for low-latency. Preempt whenever a higher
>     priority task awaits.

If you remove cond_resched() then won't both B and C require an extra IPI.
That is probably OK for RT tasks but could get expensive for
normal tasks that aren't bound to a specific cpu.

I suspect C could also lead to tasks being pre-empted just before
they sleep (eg after waking another task).
There might already be mitigation for that, I'm not sure if
a voluntary sleep can be done in a non-pre-emptible section.

Certainly it should all help the scheduling of RT tasks - which
can currently get delayed by a non-RT task in a slow kernel path.
Although the worst one is the softint code...

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)