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

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Message-ID: <1bd15361edfd4db9fc9271d35e7bbe5edad1b87a.camel@fb.com>
Date:   Tue, 10 May 2022 18:07:00 +0000
From:   Rik van Riel <riel@...com>
To:     "jpoimboe@...nel.org" <jpoimboe@...nel.org>
CC:     "song@...nel.org" <song@...nel.org>,
        "joe.lawrence@...hat.com" <joe.lawrence@...hat.com>,
        Song Liu <songliubraving@...com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
        "pmladek@...e.com" <pmladek@...e.com>,
        "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 Tue, 2022-05-10 at 09:52 -0700, Josh Poimboeuf wrote:
> On Tue, May 10, 2022 at 04:07:42PM +0000, Rik van Riel wrote:
> > > 
> > Now I wonder if we could just hook up a preempt notifier
> > for kernel live patches. All the distro kernels already
> > need the preempt notifier for KVM, anyway...
> > 
> 
> I wouldn't be opposed to that, but how does it solve this problem? 
> If
> as Peter said cond_resched() can be a NOP, then preemption would have
> to
> be from an interrupt, in which case frame pointers aren't reliable.
> 
The systems where we are seeing problems do not, as far
as I know, throw softlockup errors, so the kworker
threads that fail to transition to the new KLP version
are sleeping and getting scheduled out at times.

A KLP transition preempt notifier would help those
kernel threads transition to the new KLP version at
any time they reschedule.

How much it will help is hard to predict, but I should
be able to get results from a fairly large sample size
of systems within a few weeks :)