From: Juri Lelli <juri.lelli@redhat.com>
To: luca abeni <luca.abeni@santannapisa.it>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Juri Lelli <juri.lelli@gmail.com>,
Peter Zijlstra <peterz@infradead.org>,
syzbot <syzbot+385468161961cee80c31@syzkaller.appspotmail.com>,
Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>,
LKML <linux-kernel@vger.kernel.org>,
mingo@redhat.com, nstange@suse.de,
syzkaller-bugs@googlegroups.com, henrik@austad.us,
Tommaso Cucinotta <tommaso.cucinotta@santannapisa.it>,
Claudio Scordino <claudio@evidence.eu.com>,
Daniel Bristot de Oliveira <bristot@redhat.com>
Subject: Re: INFO: rcu detected stall in do_idle
Date: Thu, 18 Oct 2018 14:21:42 +0200 [thread overview]
Message-ID: <20181018122142.GF21611@localhost.localdomain> (raw)
In-Reply-To: <20181018130811.61337932@luca64>
On 18/10/18 13:08, luca abeni wrote:
> On Thu, 18 Oct 2018 12:47:13 +0200
> Juri Lelli <juri.lelli@redhat.com> wrote:
>
> > Hi,
> >
> > On 18/10/18 12:23, luca abeni wrote:
> > > Hi Juri,
> > >
> > > On Thu, 18 Oct 2018 10:28:38 +0200
> > > Juri Lelli <juri.lelli@redhat.com> wrote:
> > > [...]
> > > > struct sched_attr {
> > > > .size = 0,
> > > > .policy = 6,
> > > > .flags = 0,
> > > > .nice = 0,
> > > > .priority = 0,
> > > > .runtime = 0x9917,
> > > > .deadline = 0xffff,
> > > > .period = 0,
> > > > }
> > > >
> > > > So, we seem to be correctly (in theory, see below) accepting the
> > > > task.
> > > >
> > > > What seems to generate the problem here is that CONFIG_HZ=100 and
> > > > reproducer task has "tiny" runtime (~40us) and deadline (~66us)
> > > > parameters, combination that "bypasses" the enforcing mechanism
> > > > (performed at each tick).
> > >
> > > Ok, so the task can execute for at most 1 tick before being
> > > throttled... Which does not look too bad.
> > >
> > > I missed the original emails, but maybe the issue is that the task
> > > blocks before the tick, and when it wakes up again something goes
> > > wrong with the deadline and runtime assignment? (maybe because the
> > > deadline is in the past?)
> >
> > No, the problem is that the task won't be throttled at all, because
> > its replenishing instant is always way in the past when tick
> > occurs. :-/
>
> Ok, I see the issue now: the problem is that the "while (dl_se->runtime
> <= 0)" loop is executed at replenishment time, but the deadline should
> be postponed at enforcement time.
>
> I mean: in update_curr_dl() we do:
> dl_se->runtime -= scaled_delta_exec;
> if (dl_runtime_exceeded(dl_se) || dl_se->dl_yielded) {
> ...
> enqueue replenishment timer at dl_next_period(dl_se)
> But dl_next_period() is based on a "wrong" deadline!
>
>
> I think that inserting a
> while (dl_se->runtime <= -pi_se->dl_runtime) {
> dl_se->deadline += pi_se->dl_period;
> dl_se->runtime += pi_se->dl_runtime;
> }
> immediately after "dl_se->runtime -= scaled_delta_exec;" would fix the
> problem, no?
Mmm, I also thought of letting the task "pay back" its overrunning. But,
doesn't this get us quite far from what one would expect. I mean,
enforcement granularity will be way different from task period, no?
next prev parent reply other threads:[~2018-10-18 12:21 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-13 7:31 INFO: rcu detected stall in do_idle syzbot
2018-10-16 13:24 ` Thomas Gleixner
2018-10-16 14:03 ` Peter Zijlstra
2018-10-16 14:41 ` Juri Lelli
2018-10-16 14:45 ` Thomas Gleixner
2018-10-16 15:36 ` Juri Lelli
2018-10-18 8:28 ` Juri Lelli
2018-10-18 9:48 ` Peter Zijlstra
2018-10-18 10:10 ` Juri Lelli
2018-10-18 10:38 ` luca abeni
2018-10-18 10:33 ` luca abeni
2018-10-19 13:14 ` Peter Zijlstra
2018-10-18 10:23 ` luca abeni
2018-10-18 10:47 ` Juri Lelli
2018-10-18 11:08 ` luca abeni
2018-10-18 12:21 ` Juri Lelli [this message]
2018-10-18 12:36 ` luca abeni
2018-10-19 11:39 ` Peter Zijlstra
2018-10-19 20:50 ` luca abeni
2018-10-24 12:03 ` Juri Lelli
2018-10-27 11:16 ` Dmitry Vyukov
2018-10-28 8:33 ` Juri Lelli
2018-10-30 10:45 ` Peter Zijlstra
2018-10-30 11:08 ` luca abeni
2018-10-31 16:18 ` Daniel Bristot de Oliveira
2018-10-31 16:40 ` Juri Lelli
2018-10-31 17:39 ` Peter Zijlstra
2018-10-31 17:58 ` Daniel Bristot de Oliveira
2018-11-01 5:55 ` Juri Lelli
2018-11-02 10:00 ` Daniel Bristot de Oliveira
2018-11-05 10:55 ` Juri Lelli
2018-11-07 10:12 ` Daniel Bristot de Oliveira
2018-10-31 17:38 ` Peter Zijlstra
2018-10-30 11:12 ` Juri Lelli
2018-11-06 11:44 ` Juri Lelli
2021-10-27 0:59 Hao Sun
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