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Date:   Wed, 28 Sep 2022 17:56:56 -0300
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Subject: Re: [PATCH] x86/split_lock: Restore warn mode (and add a new one) to
 avoid userspace regression
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To:     "Luck, Tony" <tony.luck@intel.com>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "x86@kernel.org" <x86@kernel.org>
Cc:     "mingo@redhat.com" <mingo@redhat.com>,
        "bp@alien8.de" <bp@alien8.de>,
        "dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>,
        "hpa@zytor.com" <hpa@zytor.com>,
        "Lutomirski, Andy" <luto@kernel.org>,
        "kernel-dev@igalia.com" <kernel-dev@igalia.com>,
        "kernel@gpiccoli.net" <kernel@gpiccoli.net>,
        "Yu, Fenghua" <fenghua.yu@intel.com>,
        Joshua Ashton <joshua@froggi.es>,
        Paul Gofman <pgofman@codeweavers.com>,
        Pavel Machek <pavel@denx.de>,
        Pierre-Loup Griffais <pgriffais@valvesoftware.com>,
        Melissa Wen <mwen@igalia.com>
References: <20220928142109.150263-1-gpiccoli@igalia.com>
 <SJ1PR11MB6083113884DD0B3031FE372CFC549@SJ1PR11MB6083.namprd11.prod.outlook.com>
From:   "Guilherme G. Piccoli" <gpiccoli@igalia.com>
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On 28/09/2022 17:24, Luck, Tony wrote:
> [...] 
> Why not just use the workaround suggested in that bug report:
> 
>    "so manual switching from default setting to split_lock_detect=off helps as workaround here"
> 
> If you add this extra mode, I'm going to argue that the kernel default
> should be "seq" rather than "warn". So these game players will need
> to add a split_lock_detect=off (or warn) option.
> 

Hi Tony, thanks for your response. The workaround is the way to
circumvent the issue for now, but not all users want (or know how) to
deal with the kernel parameters. If a distro wants to default to show a
warning only, but don't break performance so hard, this would be
currently impossible.

I understand that maybe the kernel default could be seq, I could
re-submit - not a deal breaker, at least distros are given a way to
collect split lock reports without breaking!

The main/big issues here are two: defaulting to the disruptive behavior
(with no way of building a kernel not defaulting to that without
patching), and not having a way to warn about split locking without
breaking the performance, hence the new mode "seq".

> Has a bug report been filed against the God Of War game? Probably worth fixing,
> the performance penalty for split lock is only going to get worse as numbers of
> cores keeps increasing.
> 

Definitely worth fixing! And the plan is always to report it -
meanwhile, we'd like to have all users of old/proprietary software/games
to continue using the kernel the way it was before :-)

Regarding God of War specifically, I'm not sure but I can verify - the
devels should be made aware, for sure.

Cheers,


Guilherme