linux-kernel - Re: [PATCH 00/30] PREEMPT_AUTO: support lazy rescheduling

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Date: Tue, 23 Apr 2024 09:13:34 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>
Cc: Ankur Arora <ankur.a.arora@...cle.com>, Thomas Gleixner <tglx@...utronix.de>, peterz@...radead.org, 
	paulmck@...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, jpoimboe@...nel.org, mark.rutland@....com, jgross@...e.com, 
	andrew.cooper3@...rix.com, 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, jon.grimm@....com, bharata@....com, 
	raghavendra.kt@....com, boris.ostrovsky@...cle.com, konrad.wilk@...cle.com, 
	LKML <linux-kernel@...r.kernel.org>, Michael Ellerman <mpe@...erman.id.au>, 
	Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH 00/30] PREEMPT_AUTO: support lazy rescheduling

On Tue, 23 Apr 2024 at 08:23, Shrikanth Hegde <sshegde@...ux.ibm.com> wrote:
>
> Tried this patch on PowerPC by defining LAZY similar to x86. The change is below.
> Kept it at PREEMPT=none for PREEMPT_AUTO.
>
> Running into soft lockup on large systems (40Cores, SMT8) and seeing close to 100%
> regression on small system ( 12 Cores, SMT8). More details are after the patch.
>
> Are these the only arch bits that need to be defined? am I missing something very
> basic here? will try to debug this further. Any inputs?

I don't think powerpc uses the generic *_exit_to_user_mode() helper
functions, so you'll need to also add that logic to the low-level
powerpc code.

IOW, on x86, with this patch series, patch 06/30 did this:

-               if (ti_work & _TIF_NEED_RESCHED)
+               if (ti_work & (_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY))
                        schedule();

in kernel/entry/common.c exit_to_user_mode_loop().

But that works on x86 because it uses the irqentry_exit_to_user_mode().

On PowerPC, I think you need to at least fix up

    interrupt_exit_user_prepare_main()

similarly (and any other paths like that - I used to know the powerpc
code, but that was long long LOOONG ago).

                Linus