linux-kernel - Re: [PATCH 2/6] sched/preempt: refactor sched_dynamic

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Message-ID: <20220203115218.GB471778@lothringen>
Date:   Thu, 3 Feb 2022 12:52:18 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, ardb@...nel.org,
        catalin.marinas@....com, juri.lelli@...hat.com,
        linux-kernel@...r.kernel.org, mingo@...hat.com,
        peterz@...radead.org, will@...nel.org
Subject: Re: [PATCH 2/6] sched/preempt: refactor sched_dynamic_update()

On Wed, Feb 02, 2022 at 06:08:27PM +0000, Mark Rutland wrote:
> On Wed, Feb 02, 2022 at 05:01:44PM +0100, Frederic Weisbecker wrote:
> > On Wed, Feb 02, 2022 at 03:13:57PM +0000, Mark Rutland wrote:
> > > > > +#define preempt_schedule_notrace_dynamic_enabled	preempt_schedule_notrace_thunk
> > > > > +#define preempt_schedule_notrace_dynamic_disabled	NULL
> > > > 
> > > > I'm worried about un-greppable macro definitions like this.
> > > I assume you mean that it's hard to go from:
> > > 
> > >   preempt_dynamic_enable(preempt_schedule_notrace);
> > > 
> > > ... to this, because the `_dynamic_enabled` or `_dynamic_disabled` part gets
> > > token-pasted on?
> > 
> > Right.
> 
> Looking at this some more, I'm probably going to need to do token-pasting at
> some level no matter what we do, so how big of a concern is this? Searching
> for 'foo_function' should also find 'foo_function_dynamic_enabled' and
> 'foo_function_dynamic_disabled', and searching for either of those will find
> their original definition.
> 
> If others aren't concerned, could we just live with that for now?

Sure, I don't have a better idea right now. I'll try to think of something
after the next iteration.

> > I was hoping to make a default backend based on static keys to implement these
> > toggeable static calls, but I had some issues on the way, although I can't
> > remember exactly which.
> > 
> > So eventually I don't know if this stuff will be useful for you....
> 
> Having had a play with this, since you need to generate a wrapper for the
> static_key case, you either need to match the prototype or have a generic
> macro (and you likely end up back in token-pasting hell again anyhow).
> 
> So as above, how much does this matter for now?
> 
> > Well, I guess this can still ease a wrapper like:
> > 
> > preempt_dynamic_enable(sym)
> > 	---> CONFIG_STATIC_CALL=y? -----> static_call_enable(sym)
> > 	else
> > 	---> CONFIG_STATIC_KEY=y? -----> static_key_enable(sym)
> 
> In this series I just define preempt_dynamic_enable() dependent on
> CONFIG_STATIC_CALL or CONFIG_STATIC_KEY, which is functionally equivalent.

You're right.

It's just that instead of:

#if defined(CONFIG_HAVE_PREEMPT_DYNAMIC_CALL)
 #ifndef preempt_schedule_notrace_dynamic_enabled
 #define preempt_schedule_notrace_dynamic_enabled       preempt_schedule_notrace
 #define preempt_schedule_notrace_dynamic_disabled      NULL
 #endif

#if defined(CONFIG_HAVE_PREEMPT_DYNAMIC_CALL)
 #define preempt_dynamic_enable(f)      static_call_update(f, f##_dynamic_enabled)
 #define preempt_dynamic_disable(f)     static_call_update(f, #f##_dynamic_disabled)

You have:

DECLARE_STATIC_CALL_TOGGLE(preempt_schedule_notrace, __preempt_schedule_notrace_func);

#define preempt_dynamic_enable(f)      static_call_enable(f)
#define preempt_dynamic_disable(f)     static_call_disable(f)

Thanks.