linux-kernel - Re: [PATCH] livepatch: Fix idle cpu's tasks transition

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Message-ID: <alpine.LSU.2.21.2109101130560.19415@pobox.suse.cz>
Date:   Fri, 10 Sep 2021 11:32:19 +0200 (CEST)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Vasily Gorbik <gor@...ux.ibm.com>
cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Jiri Kosina <jikos@...nel.org>, Petr Mladek <pmladek@...e.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Sumanth Korikkar <sumanthk@...ux.ibm.com>,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] livepatch: Fix idle cpu's tasks transition

On Thu, 9 Sep 2021, Vasily Gorbik wrote:

> On an idle system with large amount of cpus it might happen that
> klp_update_patch_state() is not reached in do_idle() for a long periods
> of time. With debug messages enabled log is filled with:
> [  499.442643] livepatch: klp_try_switch_task: swapper/63:0 is running
> 
> without any signs of progress. Ending up with "failed to complete
> transition".
> 
> On s390 LPAR with 128 cpus not a single transition is able to complete
> and livepatch kselftests fail. Tests on idling x86 kvm instance with 128
> cpus demonstrate similar symptoms with and without CONFIG_NO_HZ.
> 
> To deal with that, since runqueue is already locked in
> klp_try_switch_task() identify idling cpus and trigger rescheduling
> potentially waking them up and making sure idle tasks break out of
> do_idle() inner loop and reach klp_update_patch_state(). This helps to
> speed up transition time while avoiding unnecessary extra system load.
> 
> Signed-off-by: Vasily Gorbik <gor@...ux.ibm.com>

Seems reasonable to me.

Acked-by: Miroslav Benes <mbenes@...e.cz>

M