From linux-kernel  Tue Apr 23 16:13:34 2024
From: Linus Torvalds <torvalds () linux-foundation ! org>
Date: Tue, 23 Apr 2024 16:13:34 +0000
To: linux-kernel
Subject: Re: [PATCH 00/30] PREEMPT_AUTO: support lazy rescheduling
Message-Id: <CAHk-=whrTE30xzuq4mcXidT4M0z_HRYJW9i8M9gD9VG7d29Qpg () mail ! gmail ! com>
X-MARC-Message: https://marc.info/?l=linux-kernel&m=171388871619115

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