linux-kernel - Re: [PATCH 17/30] x86/thread_info: define TIF_NEED_RESCHED

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Date: Mon, 19 Feb 2024 12:32:18 +0000
From: Mark Rutland <mark.rutland@....com>
To: Ankur Arora <ankur.a.arora@...cle.com>
Cc: linux-kernel@...r.kernel.org, tglx@...utronix.de, peterz@...radead.org,
	torvalds@...ux-foundation.org, paulmck@...nel.org,
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	raghavendra.kt@....com, boris.ostrovsky@...cle.com,
	konrad.wilk@...cle.com
Subject: Re: [PATCH 17/30] x86/thread_info: define TIF_NEED_RESCHED_LAZY

On Wed, Feb 14, 2024 at 12:31:29PM -0800, Ankur Arora wrote:
> Mark Rutland <mark.rutland@....com> writes:
> > On Mon, Feb 12, 2024 at 09:55:41PM -0800, Ankur Arora wrote:

> >> diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
> >> index d63b02940747..88c1802185fc 100644
> >> --- a/arch/x86/include/asm/thread_info.h
> >> +++ b/arch/x86/include/asm/thread_info.h
> >> @@ -81,8 +81,11 @@ struct thread_info {
> >>  #define TIF_NOTIFY_RESUME	1	/* callback before returning to user */
> >>  #define TIF_SIGPENDING		2	/* signal pending */
> >>  #define TIF_NEED_RESCHED	3	/* rescheduling necessary */
> >> -#define TIF_SINGLESTEP		4	/* reenable singlestep on user return*/
> >> -#define TIF_SSBD		5	/* Speculative store bypass disable */
> >> +#ifdef CONFIG_PREEMPT_AUTO
> >> +#define TIF_NEED_RESCHED_LAZY	4	/* Lazy rescheduling */
> >> +#endif
> >> +#define TIF_SINGLESTEP		5	/* reenable singlestep on user return*/
> >> +#define TIF_SSBD		6	/* Speculative store bypass disable */
> >
> > It's a bit awkward/ugly to conditionally define the TIF_* bits in arch code,
> > and we don't do that for other bits that are only used in some configurations
> > (e.g. TIF_UPROBE). That's not just for aesthetics -- for example, on arm64 we
> > try to keep the TIF_WORK_MASK bits contiguous, which is difficult if a bit in
> > the middle doesn't exist in some configurations.
> 
> That's useful to know. And, I think you are right about the
> ugliness of this.
> 
> > Is it painful to organise the common code so that arch code can define
> > TIF_NEED_RESCHED_LAZY regardless of whether CONFIG_PREEMPT_AUTO is selected?
> 
> So, the original reason I did it this way was because I wanted to have
> zero performance impact on !CONFIG_PREEMPT_AUTO configurations whether
> TIF_NEED_RESCHED_LAZY was defined or not.
> (I was doing some computation with TIF_NEED_RESCHED_LAZY at that point.)
> 
> Eventually I changed that part of code but this stayed.
> 
> Anyway, this should be easy enough to fix with done #ifdefry.
> 
> Thanks for reviewing.

Great!

BTW, the series overall looks to be in very good shape; thanks a lot for
working on this!

Mark.