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

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Date:   Thu, 9 Nov 2023 09:39:33 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Steven Rostedt' <rostedt@...dmis.org>,
        Kees Cook <keescook@...omium.org>
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Subject: RE: [RFC PATCH 71/86] treewide: lib: remove cond_resched()

From: Steven Rostedt
> Sent: 08 November 2023 19:42
> 
> 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.

Delaying the reschedule that long seems like a regression.
I'm sure a lot of the cond_resched() calls were added to cause
pre-emption much earlier than 1 tick.

I doubt the distibutions will change from VOLUTARY any time soon.
So that is what most people will be using.

	David.

> 
> 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

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