linux-kernel - Re: [RFC PATCH 34/86] thread_info: accessors for TIF_NEED

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Date:   Mon, 20 Nov 2023 21:59:09 -0800
From:   Ankur Arora <ankur.a.arora@...cle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ankur Arora <ankur.a.arora@...cle.com>,
        linux-kernel@...r.kernel.org, tglx@...utronix.de,
        torvalds@...ux-foundation.org, paulmck@...nel.org,
        linux-mm@...ck.org, x86@...nel.org, akpm@...ux-foundation.org,
        luto@...nel.org, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com, mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, willy@...radead.org, mgorman@...e.de,
        jon.grimm@....com, bharata@....com, raghavendra.kt@....com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com,
        jgross@...e.com, andrew.cooper3@...rix.com, mingo@...nel.org,
        bristot@...nel.org, mathieu.desnoyers@...icios.com,
        geert@...ux-m68k.org, glaubitz@...sik.fu-berlin.de,
        anton.ivanov@...bridgegreys.com, mattst88@...il.com,
        krypton@...ich-teichert.org, rostedt@...dmis.org,
        David.Laight@...lab.com, richard@....at, mjguzik@...il.com
Subject: Re: [RFC PATCH 34/86] thread_info: accessors for TIF_NEED_RESCHED*


Peter Zijlstra <peterz@...radead.org> writes:

> On Tue, Nov 07, 2023 at 01:57:20PM -0800, Ankur Arora wrote:
>> Add tif_resched() which will be used as an accessor for TIF_NEED_RESCHED
>> and TIF_NEED_RESCHED_LAZY. The intent is to force the caller to make an
>> explicit choice of how eagerly they want a reschedule.
>>
>> This interface will be used almost entirely from core kernel code, so
>> forcing a choice shouldn't be too onerous.
>>
>> Originally-by: Thomas Gleixner <tglx@...utronix.de>
>> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
>
>> ---
>>  include/linux/thread_info.h | 21 +++++++++++++++++++++
>>  1 file changed, 21 insertions(+)
>>
>> diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
>> index 9ea0b28068f4..4eb22b13bf64 100644
>> --- a/include/linux/thread_info.h
>> +++ b/include/linux/thread_info.h
>> @@ -59,6 +59,27 @@ enum syscall_work_bit {
>>
>>  #include <asm/thread_info.h>
>>
>> +#ifndef TIF_NEED_RESCHED_LAZY
>> +#error "Arch needs to define TIF_NEED_RESCHED_LAZY"
>> +#endif
>> +
>> +#define TIF_NEED_RESCHED_LAZY_OFFSET	(TIF_NEED_RESCHED_LAZY - TIF_NEED_RESCHED)
>> +
>> +typedef enum {
>> +	RESCHED_eager = 0,
>> +	RESCHED_lazy = TIF_NEED_RESCHED_LAZY_OFFSET,
>> +} resched_t;
>> +
>> +static inline int tif_resched(resched_t r)
>> +{
>> +	return TIF_NEED_RESCHED + r;
>> +}
>> +
>> +static inline int _tif_resched(resched_t r)
>> +{
>> +	return 1 << tif_resched(r);
>> +}
>
> So either I'm confused or I'm thinking this is wrong. If you want to
> preempt eagerly you want to preempt more than when you're not eager to
> preempt, right?
>
> So an eager preemption site wants to include the LAZY bit.
>
> Whereas a site that wants to lazily preempt would prefer to not preempt
> until forced, and hence would not include LAZY bit.

This wasn't meant to be quite that sophisticated.
tif_resched(RESCHED_eager) means you preempt immediately/eagerly and
tif_resched(RESCHED_lazy) means you want deferred preemption.

I changed it to:

typedef enum {
	NR_now = 0,
	NR_lazy = TIF_NEED_RESCHED_LAZY_OFFSET,
} resched_t;

So, to get the respective bit we would have: tif_resched(NR_now) or
tif_resched(NR_lazy).

And the immediate preemption checks would be...

	if (tif_need_resched(NR_now))
		preempt_schedule_irq();

Does this read better?

--
ankur