From: Josh Poimboeuf <jpoimboe@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Petr Mladek <pmladek@suse.com>,
Joe Lawrence <joe.lawrence@redhat.com>,
kvm@vger.kernel.org, "Michael S. Tsirkin" <mst@redhat.com>,
netdev@vger.kernel.org, Jiri Kosina <jikos@kernel.org>,
linux-kernel@vger.kernel.org,
virtualization@lists.linux-foundation.org,
"Seth Forshee (DigitalOcean)" <sforshee@digitalocean.com>,
live-patching@vger.kernel.org, Miroslav Benes <mbenes@suse.cz>
Subject: Re: [PATCH 0/2] vhost: improve livepatch switching for heavily loaded vhost worker kthreads
Date: Mon, 30 Jan 2023 10:18:53 -0800 [thread overview]
Message-ID: <20230130181853.irl3iqs7e23gw2kr@treble> (raw)
In-Reply-To: <Y9e6ssSHUt+MUvum@hirez.programming.kicks-ass.net>
On Mon, Jan 30, 2023 at 01:40:18PM +0100, Peter Zijlstra wrote:
> On Fri, Jan 27, 2023 at 02:11:31PM -0800, Josh Poimboeuf wrote:
>
>
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index 4df2b3e76b30..fbcd3acca25c 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -36,6 +36,7 @@
> > #include <linux/seqlock.h>
> > #include <linux/kcsan.h>
> > #include <linux/rv.h>
> > +#include <linux/livepatch_sched.h>
> > #include <asm/kmap_size.h>
> >
> > /* task_struct member predeclarations (sorted alphabetically): */
> > @@ -2074,6 +2075,9 @@ DECLARE_STATIC_CALL(cond_resched, __cond_resched);
> >
> > static __always_inline int _cond_resched(void)
> > {
> > + //FIXME this is a bit redundant with preemption disabled
> > + klp_sched_try_switch();
> > +
> > return static_call_mod(cond_resched)();
> > }
>
> Right, I was thinking you'd do something like:
>
> static_call_update(cond_resched, klp_cond_resched);
>
> With:
>
> static int klp_cond_resched(void)
> {
> klp_try_switch_task(current);
> return __cond_resched();
> }
>
> That would force cond_resched() into doing the transition thing,
> irrespective of the preemption mode at hand. And then, when KLP be done,
> re-run sched_dynamic_update() to reset it to whatever it ought to be.
Ok, makes sense.
>
> > @@ -401,8 +421,10 @@ void klp_try_complete_transition(void)
> > */
> > read_lock(&tasklist_lock);
> > for_each_process_thread(g, task)
> > - if (!klp_try_switch_task(task))
> > + if (!klp_try_switch_task(task)) {
> > + set_tsk_need_resched(task);
> > complete = false;
> > + }
>
> Yeah, no, that's broken -- preemption state live in more than just the
> TIF bit.
Oops.
> > read_unlock(&tasklist_lock);
> >
> > /*
> > @@ -413,6 +435,7 @@ void klp_try_complete_transition(void)
> > task = idle_task(cpu);
> > if (cpu_online(cpu)) {
> > if (!klp_try_switch_task(task)) {
> > + set_tsk_need_resched(task);
> > complete = false;
> > /* Make idle task go through the main loop. */
> > wake_up_if_idle(cpu);
>
> Idem.
>
> Also, I don't see the point of this and the __schedule() hook here:
The (poorly executed) idea was to catch kthreads which do
if (need_resched())
schedule();
but I guess we can just replace those usages with cond_resched()?
> > @@ -8500,8 +8502,10 @@ EXPORT_STATIC_CALL_TRAMP(might_resched);
> > static DEFINE_STATIC_KEY_FALSE(sk_dynamic_cond_resched);
> > int __sched dynamic_cond_resched(void)
> > {
> > - if (!static_branch_unlikely(&sk_dynamic_cond_resched))
> > + if (!static_branch_unlikely(&sk_dynamic_cond_resched)) {
> > + klp_sched_try_switch();
> > return 0;
> > + }
> > return __cond_resched();
> > }
> > EXPORT_SYMBOL(dynamic_cond_resched);
>
> I would make the klp_sched_try_switch() not depend on
> sk_dynamic_cond_resched, because __cond_resched() is not a guaranteed
> pass through __schedule().
>
> But you'll probably want to check with Mark here, this all might
> generate crap code on arm64.
>
> Both ways this seems to make KLP 'depend' (or at least work lots better)
> when PREEMPT_DYNAMIC=y. Do we want a PREEMPT_DYNAMIC=n fallback for
> _cond_resched() too?
That was the intent but I obviously failed. Let me go rework it a bit.
--
Josh
next prev parent reply other threads:[~2023-01-30 18:19 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-01-20 22:12 [PATCH 0/2] vhost: improve livepatch switching for heavily loaded vhost worker kthreads Seth Forshee (DigitalOcean)
2023-01-20 22:12 ` [PATCH 1/2] livepatch: add an interface for safely switching kthreads Seth Forshee (DigitalOcean)
2023-01-20 22:12 ` [PATCH 2/2] vhost: check for pending livepatches from vhost worker kthreads Seth Forshee (DigitalOcean)
2023-01-24 14:17 ` Petr Mladek
2023-01-24 17:21 ` Seth Forshee
2023-01-25 11:34 ` Petr Mladek
2023-01-25 16:57 ` Seth Forshee
2023-01-26 11:16 ` Petr Mladek
2023-01-26 11:49 ` Petr Mladek
2023-01-22 8:34 ` [PATCH 0/2] vhost: improve livepatch switching for heavily loaded " Michael S. Tsirkin
2023-01-26 17:03 ` Petr Mladek
2023-01-26 21:12 ` Seth Forshee (DigitalOcean)
2023-01-27 4:43 ` Josh Poimboeuf
2023-01-27 10:37 ` Peter Zijlstra
2023-01-27 12:09 ` Petr Mladek
2023-01-27 14:37 ` Seth Forshee
2023-01-27 16:52 ` Josh Poimboeuf
2023-01-27 17:09 ` Josh Poimboeuf
2023-01-27 22:11 ` Josh Poimboeuf
2023-01-30 12:40 ` Peter Zijlstra
2023-01-30 17:50 ` Seth Forshee
2023-01-30 18:18 ` Josh Poimboeuf [this message]
2023-01-30 18:36 ` Mark Rutland
2023-01-30 19:48 ` Josh Poimboeuf
2023-01-31 1:53 ` Song Liu
2023-01-31 10:22 ` Mark Rutland
2023-01-31 16:38 ` Josh Poimboeuf
2023-02-01 11:10 ` Mark Rutland
2023-02-01 16:57 ` Josh Poimboeuf
2023-02-01 17:11 ` Mark Rutland
2023-01-30 19:59 ` Josh Poimboeuf
2023-01-31 10:02 ` Peter Zijlstra
2023-01-27 20:02 ` Seth Forshee
2023-01-27 11:19 ` Petr Mladek
2023-01-27 14:57 ` Seth Forshee
2023-01-30 9:55 ` Petr Mladek
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