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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Tue, 23 Apr 2024 09:13:34 -0700
Message-ID: <CAHk-=whrTE30xzuq4mcXidT4M0z_HRYJW9i8M9gD9VG7d29Qpg@mail.gmail.com>
Subject: Re: [PATCH 00/30] PREEMPT_AUTO: support lazy rescheduling
To: Shrikanth Hegde <sshegde@linux.ibm.com>
Cc: Ankur Arora <ankur.a.arora@oracle.com>, Thomas Gleixner <tglx@linutronix.de>, peterz@infradead.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, mark.rutland@arm.com, jgross@suse.com, 
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	raghavendra.kt@amd.com, boris.ostrovsky@oracle.com, konrad.wilk@oracle.com, 
	LKML <linux-kernel@vger.kernel.org>, Michael Ellerman <mpe@ellerman.id.au>, 
	Nicholas Piggin <npiggin@gmail.com>
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On Tue, 23 Apr 2024 at 08:23, Shrikanth Hegde <sshegde@linux.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