linux-kernel - Re: [RFC PATCH 85/86] treewide: drivers: remove cond

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Message-ID: <20231109184151.52a9aa33@gandalf.local.home>
Date:   Thu, 9 Nov 2023 18:41:51 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:     Ankur Arora <ankur.a.arora@...cle.com>,
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Subject: Re: [RFC PATCH 85/86] treewide: drivers: remove cond_resched()

On Thu, 9 Nov 2023 15:25:54 -0800
Dmitry Torokhov <dmitry.torokhov@...il.com> wrote:

> Hi Anhur,
> 
> On Tue, Nov 07, 2023 at 03:08:21PM -0800, Ankur Arora wrote:
> > There are broadly three sets of uses of cond_resched():
> > 
> > 1.  Calls to cond_resched() out of the goodness of our heart,
> >     otherwise known as avoiding lockup splats.  
> 
> ...
> 
> What about RCU stalls? The calls to cond_resched() in evdev.c and
> mousedev.c were added specifically to allow RCU to run in cases when
> userspace passes a large buffer and the kernel is not fully preemptable.
> 

First, this patch is being sent out premature as it depends on acceptance
of the previous patches.

When the previous patches are finished, then we don't need cond_resched()
to protect against RCU stalls, because even "PREEMPT_NONE" will allow
preemption inside the kernel.

What the earlier patches do is introduce a concept of NEED_RESCHED_LAZY.
Then when the scheduler wants to resched the task, it will set that bit
instead of NEED_RESCHED (for the old PREEMPT_NONE version). For VOLUNTARY,
it sets the LAZY bit of SCHED_OTHER but NEED_RESCHED for RT/DL tasks. For
PREEMPT, it will always set NEED_RESCHED.

NEED_RESCHED will always schedule, but NEED_RESCHED_LAZY only schedules
when going to user space.

Now after on tick (depending on HZ it can be 1ms, 3.3ms, 4ms 10ms) if
NEED_RESCHED_LAZY is set, then it will set NEED_RESCHED, forcing a
preemption at the next available moment (when preempt count is zero).

This will be done even with the old PREEMPT_NONE configuration.

That way we will no longer play whack-a-mole to get rid of all he long
running kernel paths by inserting cond_resched() in them.

-- Steve