linux-kernel - Re: [RFC PATCH 71/86] treewide: lib: remove cond

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Date:   Wed, 8 Nov 2023 14:41:44 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Ankur Arora <ankur.a.arora@...cle.com>,
        linux-kernel@...r.kernel.org, tglx@...utronix.de,
        peterz@...radead.org, torvalds@...ux-foundation.org,
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        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, Thomas Graf <tgraf@...g.ch>
Subject: Re: [RFC PATCH 71/86] treewide: lib: remove cond_resched()

On Wed, 8 Nov 2023 11:15:37 -0800
Kees Cook <keescook@...omium.org> wrote:

> FOr the memcpy_kunit.c cases, I don't think there are preemption
> locations in its loops. Perhaps I'm misunderstanding something? Why will
> the memcpy test no longer produce softlockup splats?

This patchset will switch over to a NEED_RESCHED_LAZY routine, so that
VOLUNTARY and NONE preemption models will be forced to preempt if its in
the kernel for too long.

Time slice is over: set NEED_RESCHED_LAZY

For VOLUNTARY and NONE, NEED_RESCHED_LAZY will not preempt the kernel (but
will preempt user space).

If in the kernel for over 1 tick (1ms for 1000Hz, 4ms for 250Hz, etc),
if NEED_RESCHED_LAZY is still set after one tick, then set NEED_RESCHED.

NEED_RESCHED will now schedule in the kernel once it is able to regardless
of preemption model. (PREEMPT_NONE will now use preempt_disable()).

This allows us to get rid of all cond_resched()s throughout the kernel as
this will be the new mechanism to keep from running inside the kernel for
too long. The watchdog is always longer than one tick.

-- Steve