From: Ingo Molnar <mingo@kernel.org>
To: John Stultz <john.stultz@linaro.org>
Cc: lkml <linux-kernel@vger.kernel.org>,
Dave Jones <davej@codemonkey.org.uk>,
Linus Torvalds <torvalds@linux-foundation.org>,
Thomas Gleixner <tglx@linutronix.de>,
Richard Cochran <richardcochran@gmail.com>,
Prarit Bhargava <prarit@redhat.com>,
Stephen Boyd <sboyd@codeaurora.org>,
Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 08/12] time: Add warnings when overflows or underflows are observed
Date: Sat, 7 Mar 2015 10:40:17 +0100 [thread overview]
Message-ID: <20150307094017.GG30888@gmail.com> (raw)
In-Reply-To: <1425696603-16878-9-git-send-email-john.stultz@linaro.org>
* John Stultz <john.stultz@linaro.org> wrote:
> It was suggested that the underflow/overflow protection
> should probably throw some sort of warning out, rather
> then just silently fixing the issue.
Typo.
> So this patch adds some warnings here. The flag variables
> used are not protected by locks, but since we can't print
> from the reading functions, just being able to say we
> saw an issue in the update interval is useful enough,
> and can be slightly racy without real consequnece.
Typo.
> The big complication is that we're only under a read
> seqlock, so the data could shift under us during
> our calcualtion to see if there was a problem. This
Typo.
> patch avoids this issue by nesting another seqlock
> which allows us to snapshot the just required values
> atomically. So we shouldn't see false positives.
>
> I also added some basic ratelimiting here, since
> on one build machine w/ skewed TSCs it was fairly
> noisy at bootup.
> +#define WARNINGFREQ (HZ*300) /* 5 minute rate-limiting */
Nit: so in general wereallytrytokeepwordsapart, so I'd suggest a
name of WARNING_FREQ or so?
> cycle_t max_cycles = tk->tkr.clock->max_cycles;
> const char *name = tk->tkr.clock->name;
> + static long last_warning; /* we always hold write on timekeeper lock */
So I'm not sure I ever heard the phrase 'to hold write', this doesn't
parse for me.
Also, static global variables should really, really not be immersed
amongst on-stack variables, they are so easy to overlook. Just put
them in front of the function.
>
> if (offset > max_cycles)
> printk_deferred("ERROR: cycle offset (%lld) is larger then"
> @@ -133,28 +145,60 @@ static void timekeeping_check_update(struct timekeeper *tk, cycle_t offset)
> printk_deferred("WARNING: cycle offset (%lld) is past"
> " the %s 50%% safety margin (%lld)\n",
> offset, name, max_cycles>>1);
> +
> + if (timekeeping_underflow_seen) {
> + if (jiffies - last_warning > WARNINGFREQ) {
> + printk_deferred("WARNING: Clocksource underflow observed\n");
> + last_warning = jiffies;
> + }
> + timekeeping_underflow_seen = 0;
> + }
> + if (timekeeping_overflow_seen) {
> + if (jiffies - last_warning > WARNINGFREQ) {
> + printk_deferred("WARNING: Clocksource overflow observed\n");
I think the warning should be more informative. If a distro turns this
on and a user sees this value, what will he think? Is the kernel still
OK? What can he do about it?
> + last_warning = jiffies;
> + }
> + timekeeping_overflow_seen = 0;
> + }
> +
> }
>
> static inline cycle_t timekeeping_get_delta(struct tk_read_base *tkr)
> {
> - cycle_t cycle_now, delta;
> + cycle_t now, last, mask, max, delta;
> + unsigned int seq;
>
> - /* read clocksource */
> - cycle_now = tkr->read(tkr->clock);
> + /*
> + * Since we're called holding a seqlock, the data may shift
> + * under us while we're doign the calculation. This can cause
Typo...
> + * false positives, since we'd note a problem but throw the
> + * results away. So nest another seqlock here to atomically
Spurious space. I know they are cheap, but still.
Thanks,
Ingo
next prev parent reply other threads:[~2015-03-07 9:41 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-03-07 2:49 [PATCH 00/12] Increased clocksource validation and cleanups (v3) John Stultz
2015-03-07 2:49 ` [PATCH 01/12] clocksource: Simplify clocks_calc_max_nsecs logic John Stultz
2015-03-07 9:20 ` Ingo Molnar
2015-03-07 2:49 ` [PATCH 02/12] clocksource: Simplify logic around clocksource wrapping saftey margins John Stultz
2015-03-07 9:22 ` Ingo Molnar
2015-03-07 2:49 ` [PATCH 03/12] clocksource: Remove clocksource_max_deferment() John Stultz
2015-03-07 9:18 ` Ingo Molnar
2015-03-07 2:49 ` [PATCH 04/12] clocksource: Add max_cycles to clocksource structure John Stultz
2015-03-07 9:54 ` Ingo Molnar
2015-03-07 2:49 ` [PATCH 05/12] time: Add debugging checks to warn if we see delays John Stultz
2015-03-07 9:22 ` Paul Bolle
2015-03-07 9:29 ` Ingo Molnar
2015-03-07 2:49 ` [PATCH 06/12] time: Add infrastructure to cap clocksource reads to the max_cycles value John Stultz
2015-03-07 9:32 ` Ingo Molnar
2015-03-07 2:49 ` [PATCH 07/12] time: Try to catch clocksource delta underflows John Stultz
2015-03-07 9:34 ` Ingo Molnar
2015-03-07 2:49 ` [PATCH 08/12] time: Add warnings when overflows or underflows are observed John Stultz
2015-03-07 9:40 ` Ingo Molnar [this message]
2015-03-09 16:50 ` John Stultz
2015-03-10 5:05 ` Ingo Molnar
2015-03-07 2:50 ` [PATCH 09/12] clocksource: Improve clocksource watchdog reporting John Stultz
2015-03-07 2:50 ` [PATCH 10/12] clocksource: Mostly kill clocksource_register() John Stultz
2015-03-07 9:45 ` Ingo Molnar
2015-03-07 2:50 ` [PATCH 11/12] sparc: Convert to using clocksource_register_hz() John Stultz
2015-03-07 9:46 ` Ingo Molnar
2015-03-07 2:50 ` [PATCH 12/12] clocksource: Add some debug info about clocksources being registered John Stultz
2015-03-07 9:50 ` Ingo Molnar
2015-03-12 3:16 ` John Stultz
-- strict thread matches above, loose matches on Subject: below --
2015-01-23 0:09 [PATCH 00/12][RFC] Increased clocksource validation and cleanups (v2) John Stultz
2015-01-23 0:09 ` [PATCH 08/12] time: Add warnings when overflows or underflows are observed John Stultz
2015-01-23 14:30 ` Peter Zijlstra
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=20150307094017.GG30888@gmail.com \
--to=mingo@kernel.org \
--cc=davej@codemonkey.org.uk \
--cc=john.stultz@linaro.org \
--cc=linux-kernel@vger.kernel.org \
--cc=peterz@infradead.org \
--cc=prarit@redhat.com \
--cc=richardcochran@gmail.com \
--cc=sboyd@codeaurora.org \
--cc=tglx@linutronix.de \
--cc=torvalds@linux-foundation.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
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;
as well as URLs for NNTP newsgroup(s).