linux-kernel - RE: [PATCH] x86/split_lock: Restore warn mode (and add a new one) to avoid userspace regression

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Message-ID: <SJ1PR11MB60839A08E01742321F8446BCFC579@SJ1PR11MB6083.namprd11.prod.outlook.com>
Date:   Thu, 29 Sep 2022 15:37:55 +0000
From:   "Luck, Tony" <tony.luck@...el.com>
To:     "Hansen, Dave" <dave.hansen@...el.com>,
        "Guilherme G. Piccoli" <gpiccoli@...lia.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>
CC:     "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "Lutomirski, Andy" <luto@...nel.org>,
        "kernel-dev@...lia.com" <kernel-dev@...lia.com>,
        "kernel@...ccoli.net" <kernel@...ccoli.net>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        Joshua Ashton <joshua@...ggi.es>,
        Paul Gofman <pgofman@...eweavers.com>,
        Pavel Machek <pavel@...x.de>,
        Pierre-Loup Griffais <pgriffais@...vesoftware.com>,
        "Melissa Wen" <mwen@...lia.com>
Subject: RE: [PATCH] x86/split_lock: Restore warn mode (and add a new one) to
 avoid userspace regression

>> I have a revert removing the misery ready and tested, let me know if I
>> should submit it.
>
> I'm a bit of a late arrival to the split lock party, so I'm a bit
> hesitant to merge any changes immediately.
>
> How about we give it a few weeks and see if the current behavior impacts
> anyone else?  Maybe the best route will be more clear then.

Applying "misery" to the processes that are executing split-lock flows saves
the rest of the system from a different level of misery (for the duration of the
split lock other logical CPUs and I/O devices have access to memory blocked).

So the "misery" serves a very useful purpose on multi-user systems.

Maybe the decision of which mode to use could be dynamic based on
number of online CPUs? Laptops/desktops with low counts (<50???)
could just "warn", while servers could default to the "seq" mode.

Or perhaps there is some other heuristic to distinguish single-user
systems where the split-locks are not causing pain to other users?

-Tony