linux-kernel - Re: [RFC PATCH v2 0/5] allow overriding default preempt mode from command line

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Date:   Tue, 27 Oct 2020 13:28:41 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Michal Hocko <mhocko@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mel Gorman <mgorman@...e.de>,
        Frederic Weisbecker <fweisbecker@...e.de>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [RFC PATCH v2 0/5] allow overriding default preempt mode from
 command line

On Tue, Oct 27, 2020 at 01:22:41PM +0100, Frederic Weisbecker wrote:
> On Fri, Oct 09, 2020 at 07:45:54PM +0200, Peter Zijlstra wrote:
> > +DEFINE_STATIC_KEY_TRUE(irq_preemption_key);
> > +
> > +/*
> > + * SC:cond_resched
> > + * SC:might_resched
> > + * SC:preempt_schedule
> > + * SC:preempt_schedule_notrace
> > + * SB:irq_preemption_key
> > + *
> > + *
> > + * ZERO
> > + *   cond_resched             <- RET0
> > + *   might_resched            <- NOP
> > + *   preempt_schedule         <- NOP
> > + *   preempt_schedule_notrace <- NOP
> > + *   irq_preemption_key       <- false
> > + *
> > + * NONE:
> > + *   cond_resched             <- __cond_resched
> > + *   might_resched            <- NOP
> > + *   preempt_schedule         <- NOP
> > + *   preempt_schedule_notrace <- NOP
> > + *   irq_preemption_key       <- false
> > + *
> > + * VOLUNTARY:
> > + *   cond_resched             <- __cond_resched
> > + *   might_resched            <- __might_resched
> > + *   preempt_schedule         <- NOP
> > + *   preempt_schedule_notrace <- NOP
> > + *   irq_preemption_key       <- false
> > + *
> > + * FULL:
> > + *   cond_resched             <- RET0
> > + *   might_resched            <- NOP
> > + *   preempt_schedule         <- preempt_schedule
> > + *   preempt_schedule_notrace <- preempt_schedule_notrace
> > + *   irq_preemption_key       <- true
> > + */
> 
> That's cute! I'll try to end up to that result.

Beware, ZERO is 'broken', I included it because I was curious what, if
anything, would happen :-)

> > +static int __init setup_preempt_mode(char *str)
> > +{
> > +	if (!strcmp(str, "zero")) {
> > +		static_call_update(cond_resched, __static_call_return0);
> > +		static_call_update(might_resched, (void (*)(void))NULL);
> > +		static_call_update(preempt_schedule, (void (*)(void))NULL);
> > +		static_call_update(preempt_schedule_notrace, (void (*)(void))NULL);
> > +		static_branch_disable(&irq_preemption_key);
> > +		printk("XXX PREEMPT: %s\n", str);
> > +	} else if (!strcmp(str, "none")) {
> > +		static_call_update(cond_resched, __cond_resched);
> > +		static_call_update(might_resched, (void (*)(void))NULL);
> > +		static_call_update(preempt_schedule, (void (*)(void))NULL);
> > +		static_call_update(preempt_schedule_notrace, (void (*)(void))NULL);
> > +		static_branch_disable(&irq_preemption_key);
> > +		printk("XXX PREEMPT: %s\n", str);
> > +	} else if (!strcmp(str, "voluntary")) {
> > +		static_call_update(cond_resched, __cond_resched);
> > +		static_call_update(might_resched, __might_resched);
> > +		static_call_update(preempt_schedule, (void (*)(void))NULL);
> > +		static_call_update(preempt_schedule_notrace, (void (*)(void))NULL);
> > +		static_branch_disable(&irq_preemption_key);
> > +		printk("XXX PREEMPT: %s\n", str);
> > +	} else if (!strcmp(str, "ponies")) {
> > +		static_call_update(cond_resched, __cond_resched);
> > +		static_call_update(might_resched, (void (*)(void))NULL);
> > +		static_call_update(preempt_schedule, preempt_schedule_thunk);
> > +		static_call_update(preempt_schedule_notrace, preempt_schedule_notrace_thunk);
> > +		static_branch_enable(&irq_preemption_key);
> > +		printk("XXX PREEMPT: %s\n", str);
> 
> Why would we need that ponies version?

We don't, but it was a missing combination (like ZERO), and I wanted to
test it worked (as expected, it does).

We'll only encounter it as an intermediate state when flipping states at
runtime, but unlike zero, it should work just fine.