linux-kernel - Re: [RFC PATCH 07/86] Revert "livepatch,sched: Add livepatch task switching to cond

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Message-ID: <87o7g2bkxj.fsf@oracle.com>
Date:   Thu, 09 Nov 2023 14:50:48 -0800
From:   Ankur Arora <ankur.a.arora@...cle.com>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Ankur Arora <ankur.a.arora@...cle.com>,
        linux-kernel@...r.kernel.org, tglx@...utronix.de,
        peterz@...radead.org, torvalds@...ux-foundation.org,
        paulmck@...nel.org, linux-mm@...ck.org, x86@...nel.org,
        akpm@...ux-foundation.org, luto@...nel.org, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        willy@...radead.org, mgorman@...e.de, jon.grimm@....com,
        bharata@....com, raghavendra.kt@....com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com,
        jgross@...e.com, andrew.cooper3@...rix.com, mingo@...nel.org,
        bristot@...nel.org, mathieu.desnoyers@...icios.com,
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        krypton@...ich-teichert.org, David.Laight@...LAB.COM,
        richard@....at, mjguzik@...il.com, Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Petr Mladek <pmladek@...e.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        live-patching@...r.kernel.org
Subject: Re: [RFC PATCH 07/86] Revert "livepatch,sched: Add livepatch task
 switching to cond_resched()"


Josh Poimboeuf <jpoimboe@...nel.org> writes:

> On Thu, Nov 09, 2023 at 12:31:47PM -0500, Steven Rostedt wrote:
>> On Thu, 9 Nov 2023 09:26:37 -0800
>> Josh Poimboeuf <jpoimboe@...nel.org> wrote:
>>
>> > On Tue, Nov 07, 2023 at 06:16:09PM -0500, Steven Rostedt wrote:
>> > > On Tue,  7 Nov 2023 13:56:53 -0800
>> > > Ankur Arora <ankur.a.arora@...cle.com> wrote:
>> > >
>> > > > This reverts commit e3ff7c609f39671d1aaff4fb4a8594e14f3e03f8.
>> > > >
>> > > > Note that removing this commit reintroduces "live patches failing to
>> > > > complete within a reasonable amount of time due to CPU-bound kthreads."
>> > > >
>> > > > Unfortunately this fix depends quite critically on PREEMPT_DYNAMIC and
>> > > > existence of cond_resched() so this will need an alternate fix.
>> >
>> > We definitely don't want to introduce a regression, something will need
>> > to be figured out before removing cond_resched().
>> >
>> > We could hook into preempt_schedule_irq(), but that wouldn't work for
>> > non-ORC.
>> >
>> > Another option would be to hook into schedule().  Then livepatch could
>> > set TIF_NEED_RESCHED on remaining unpatched tasks.  But again if they go
>> > through the preemption path then we have the same problem for non-ORC.
>> >
>> > Worst case we'll need to sprinkle cond_livepatch() everywhere :-/
>> >
>>
>> I guess I'm not fully understanding what the cond rescheds are for. But
>> would an IPI to all CPUs setting NEED_RESCHED, fix it?

Yeah. We could just temporarily toggle to full preemption, when
NEED_RESCHED_LAZY is always upgraded to NEED_RESCHED which will
then send IPIs.

> If all livepatch arches had the ORC unwinder, yes.
>
> The problem is that frame pointer (and similar) unwinders can't reliably
> unwind past an interrupt frame.

Ah, I wonder if we could just disable the preempt_schedule_irq() path
temporarily? Hooking into schedule() alongside something like this:

@@ -379,7 +379,7 @@ noinstr irqentry_state_t irqentry_enter(struct pt_regs *regs)

 void irqentry_exit_cond_resched(void)
 {
-       if (!preempt_count()) {
+       if (klp_cond_resched_disable() && !preempt_count()) {

The problem would be tasks that don't go through any preemptible
sections.

--
ankur