linux-kernel - Re: [RFC] sched,livepatch: call klp_try_switch_task in __cond

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Message-ID: <20220513133408.GI76023@worktop.programming.kicks-ass.net>
Date:   Fri, 13 May 2022 15:34:08 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Song Liu <songliubraving@...com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Rik van Riel <riel@...com>,
        "song@...nel.org" <song@...nel.org>,
        "joe.lawrence@...hat.com" <joe.lawrence@...hat.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
        "live-patching@...r.kernel.org" <live-patching@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>
Subject: Re: [RFC] sched,livepatch: call klp_try_switch_task in __cond_resched

On Fri, May 13, 2022 at 02:33:34PM +0200, Petr Mladek wrote:

> My concern is that klp_try_complete_transition() checks all processes
> under read_lock(&tasklist_lock). It might create some contention
> on this lock. I am not sure if this lock is fair. It might slow down
> block writers (creating/deleting tasks).

rwlock_t is sorta fair. Is it fair for process contect usage, but
in-interrupt use will have a reader bias.

That said; contention on tasklist_lock is a real scalability concern.

Can't you use RCU here? With RCU traversal of the tasklist you can miss
new entry (which should already run the new code) or miss exits (which
will stop running code).