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

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Message-ID: <20231109175118.olggitpaltz47n3b@treble>
Date:   Thu, 9 Nov 2023 09:51:18 -0800
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     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,
        geert@...ux-m68k.org, glaubitz@...sik.fu-berlin.de,
        anton.ivanov@...bridgegreys.com, mattst88@...il.com,
        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()"

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?

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.

-- 
Josh