linux-kernel - Re: [PATCH v3 2/3] livepatch,sched: Add livepatch task switching to cond

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Date:   Fri, 3 Mar 2023 15:00:13 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
        Seth Forshee <sforshee@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Song Liu <song@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Miroslav Benes <mbenes@...e.cz>,
        Jiri Kosina <jikos@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH v3 2/3] livepatch,sched: Add livepatch task switching to
 cond_resched()

On Tue 2023-02-28 08:56:08, Josh Poimboeuf wrote:
> On Mon, Feb 27, 2023 at 04:55:47PM +0100, Petr Mladek wrote:
> > On Fri 2023-02-24 08:50:00, Josh Poimboeuf wrote:
> > > There have been reports [1][2] of live patches failing to complete
> > > within a reasonable amount of time due to CPU-bound kthreads.
> > > 
> > > Fix it by patching tasks in cond_resched().
> > > 
> > > There are four different flavors of cond_resched(), depending on the
> > > kernel configuration.  Hook into all of them.
> > > 
> > > A more elegant solution might be to use a preempt notifier.  However,
> > > non-ORC unwinders can't unwind a preempted task reliably.
> > > 
> > > [1] https://lore.kernel.org/lkml/20220507174628.2086373-1-song@kernel.org/
> > > [2] https://lkml.kernel.org/lkml/20230120-vhost-klp-switching-v1-0-7c2b65519c43@kernel.org
> > > 
> > > Tested-by: Seth Forshee (DigitalOcean) <sforshee@...nel.org>
> > > Reviewed-by: Petr Mladek <pmladek@...e.com>
> > > Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
> > 
> > Just for record, I have double checked the changes against v2
> > and everything looks good to me.
> 
> Whoops, so I found another little surprise:
> 
> static int klp_check_stack(struct task_struct *task, const char **oldname)
> {
>         static unsigned long entries[MAX_STACK_ENTRIES];
> 	^^^^^^
> 
> That entries array is shared between the klp_mutex owner and all
> cond_resched() callers.

Huh, great catch!

> MAX_STACK_ENTRIES is 100, which seems excessive.  If we halved that, the
> array would be "only" 400 bytes, which is *almost* reasonable to
> allocate on the stack?

It is just for the stack in the process context. Right?

I think that I have never seen a stack with over 50 entries. And in
the worst case, a bigger amount of entries would "just" result in
a non-reliable stack which might be acceptable.

It looks acceptable to me.

> Alternatively we could have a percpu entries array... :-/

That said, percpu entries would be fine as well. It sounds like
a good price for the livepatching feature. I think that livepatching
is used on big systems anyway.

I slightly prefer the per-cpu solution.

Best Regards,
Petr