Received-SPF: pass (google.com: domain of linux-kernel+bounces-71319-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45e3:2400::1 as permitted sender) client-ip=2604:1380:45e3:2400::1;
Date: Mon, 19 Feb 2024 12:32:18 +0000
From: Mark Rutland <mark.rutland@arm.com>
To: Ankur Arora <ankur.a.arora@oracle.com>
Cc: linux-kernel@vger.kernel.org, tglx@linutronix.de, peterz@infradead.org,
	torvalds@linux-foundation.org, paulmck@kernel.org,
	akpm@linux-foundation.org, luto@kernel.org, bp@alien8.de,
	dave.hansen@linux.intel.com, hpa@zytor.com, mingo@redhat.com,
	juri.lelli@redhat.com, vincent.guittot@linaro.org,
	willy@infradead.org, mgorman@suse.de, jpoimboe@kernel.org,
	jgross@suse.com, andrew.cooper3@citrix.com, bristot@kernel.org,
	mathieu.desnoyers@efficios.com, geert@linux-m68k.org,
	glaubitz@physik.fu-berlin.de, anton.ivanov@cambridgegreys.com,
	mattst88@gmail.com, krypton@ulrich-teichert.org,
	rostedt@goodmis.org, David.Laight@aculab.com, richard@nod.at,
	mjguzik@gmail.com, jon.grimm@amd.com, bharata@amd.com,
	raghavendra.kt@amd.com, boris.ostrovsky@oracle.com,
	konrad.wilk@oracle.com
Subject: Re: [PATCH 17/30] x86/thread_info: define TIF_NEED_RESCHED_LAZY
Message-ID: <ZdNKUlYzQG4fk4YF@FVFF77S0Q05N>
References: <20240213055554.1802415-1-ankur.a.arora@oracle.com>
 <20240213055554.1802415-18-ankur.a.arora@oracle.com>
 <Zcy_TJt9L0EXcsVM@FVFF77S0Q05N>
 <874jeardni.fsf@oracle.com>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <874jeardni.fsf@oracle.com>

On Wed, Feb 14, 2024 at 12:31:29PM -0800, Ankur Arora wrote:
> Mark Rutland <mark.rutland@arm.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.