From: swarren@wwwdotorg•org (Stephen Warren)
To: linux-arm-kernel@lists•infradead.org
Subject: CPU hotplug issue w/ 0647065 clocksource: Add generic dummy timer driver
Date: Mon, 08 Jul 2013 11:36:21 -0600 [thread overview]
Message-ID: <51DAF895.1020700@wwwdotorg.org> (raw)
CPU hotplug (replug) on Tegra HW seems to be occasionally broken due to
commit 0647065 "clocksource: Add generic dummy timer driver" in
linux-next. Reverting that commit solves the issue.
The symptom is that ~10% of the time, when re-plugging CPU1 (in a 2-core
system, after unplugging it about 1 second before), I'll see the
following WARN trigger in clockevents_program_event():
> int clockevents_program_event(struct clock_event_device *dev, ktime_t expires,
> bool force)
> {
> unsigned long long clc;
> int64_t delta;
> int rc;
>
> if (unlikely(expires.tv64 < 0)) {
> WARN_ON_ONCE(1);
> return -ETIME;
> }
This appears to be because in tick_handle_periodic_broadcast(),
dev->next_event == KTIME_MAX. The system then hangs; I think that loop
just keeps adding tick_period onto next_event, which doesn't manage to
get to an acceptable value for a long time, if ever!
Do you have any idea why this could happen? I assume that during
switching between the dummy timer added by that patch, and the real
Tegra timer (drivers/clocksource/tegra20_timer.c) the Tegra timer's
dev->next_event is temporarily set to KTIME_MAX, but somehow the timer
IRQ handling goes off while the device is in this temporary state? The
timer core seems to take steps to prevent this though, i.e. callilng
spin_lock_irqsave() in places.
If I modify tick_handle_periodic_broadcast() to check for a negative
dev->next_event and simply return in that case, the system seems to work
fine, and I do see tick_handle_periodic_broadcast() being called at a
later time, so obviously something is coming along later and programming
the HW to generate additional events. On this HW, I believe struct
clock_event_device.set_next_event is being used to emulate the periodic
broadcast using a one-shot timer, rather than using the HW's native
periodic capability, probably due to CONFIG_NO_HZ.
Any hints greatly appreciated!
next reply other threads:[~2013-07-08 17:36 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-07-08 17:36 Stephen Warren [this message]
2013-07-09 0:58 ` CPU hotplug issue w/ 0647065 clocksource: Add generic dummy timer driver Stephen Boyd
2013-07-09 16:05 ` Stephen Warren
2013-07-09 16:35 ` Stephen Boyd
2013-07-09 16:52 ` Stephen Warren
2013-07-09 23:05 ` Stephen Boyd
2013-07-10 16:09 ` Stephen Warren
2013-07-11 14:00 ` Stephen Boyd
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=51DAF895.1020700@wwwdotorg.org \
--to=swarren@wwwdotorg$(echo .)org \
--cc=linux-arm-kernel@lists$(echo .)infradead.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox