linux-kernel - Re: [PATCH v2 05/35] thread_info: selector for TIF_NEED_RESCHED[

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Date: Thu, 30 May 2024 02:07:42 -0700
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,
        rostedt@...dmis.org, mark.rutland@....com, juri.lelli@...hat.com,
        joel@...lfernandes.org, raghavendra.kt@....com, sshegde@...ux.ibm.com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com
Subject: Re: [PATCH v2 05/35] thread_info: selector for TIF_NEED_RESCHED[_LAZY]


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

> On Mon, May 27, 2024 at 05:34:51PM -0700, Ankur Arora wrote:
>> Define tif_resched() to serve as selector for the specific
>> need-resched flag: with tif_resched() mapping to TIF_NEED_RESCHED
>> or to TIF_NEED_RESCHED_LAZY.
>>
>> For !CONFIG_PREEMPT_AUTO, tif_resched() always evaluates
>> to TIF_NEED_RESCHED, preserving existing scheduling behaviour.
>>
>> Cc: Peter Ziljstra <peterz@...radead.org>
>> Originally-by: Thomas Gleixner <tglx@...utronix.de>
>> Link: https://lore.kernel.org/lkml/87jzshhexi.ffs@tglx/
>> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
>> ---
>>  include/linux/thread_info.h | 25 +++++++++++++++++++++++++
>>  1 file changed, 25 insertions(+)
>>
>> diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
>> index 06e13e7acbe2..65e5beedc915 100644
>> --- a/include/linux/thread_info.h
>> +++ b/include/linux/thread_info.h
>> @@ -71,6 +71,31 @@ enum syscall_work_bit {
>>  #define _TIF_NEED_RESCHED_LAZY _TIF_NEED_RESCHED
>>  #endif
>>
>> +typedef enum {
>> +	RESCHED_NOW = 0,
>> +	RESCHED_LAZY = 1,
>> +} resched_t;
>> +
>> +/*
>> + * tif_resched(r) maps to TIF_NEED_RESCHED[_LAZY] with CONFIG_PREEMPT_AUTO.
>> + *
>> + * For !CONFIG_PREEMPT_AUTO, both tif_resched(RESCHED_NOW) and
>> + * tif_resched(RESCHED_LAZY) reduce to the same value (TIF_NEED_RESCHED)
>> + * leaving any scheduling behaviour unchanged.
>> + */
>> +static __always_inline int tif_resched(resched_t rs)
>> +{
>> +	if (IS_ENABLED(CONFIG_PREEMPT_AUTO))
>> +		return (rs == RESCHED_NOW) ? TIF_NEED_RESCHED : TIF_NEED_RESCHED_LAZY;
>> +	else
>> +		return TIF_NEED_RESCHED;
>> +}
>
> Perhaps:
>
> 	if (IS_ENABLED(CONFIG_PREEMPT_AUTO) && rs == RESCHED_LAZY)
> 		return TIF_NEED_RESCHED_LAZY;
>
> 	return TIF_NEED_RESCHED;

This and similar other interface changes make it much clearer.
Thanks. Will fix.


--
ankur