Re: [PATCH 08/12] time: Add warnings when overflows or underflows are observed
* John Stultz wrote: > Thanks Ingo for the very close review, and apologies for my poor > keyboardmanship (I hope I didn't burn much of your good will here). No problem. I usually fix typos up when the patch is otherwise good, except for Git pulls, where I cannot, so I'm pushing back ... > I'll work to get these trivial changes integrated along with the > more substantial feedback as well. It's all nice changes otherwise. I'm fairly sure the new sanity checks are going to show us interesting things in the future. Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [PATCH 08/12] time: Add warnings when overflows or underflows are observed
On Sat, Mar 7, 2015 at 1:40 AM, Ingo Molnar wrote: ... > > Typo. ... > > Typo. > ... > > Typo. > ... > > Typo... > ... > > Spurious space. I know they are cheap, but still. And a big D with a circle around it. Back to grade-school with me. :) Thanks Ingo for the very close review, and apologies for my poor keyboardmanship (I hope I didn't burn much of your good will here). I'll work to get these trivial changes integrated along with the more substantial feedback as well. thanks -john -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [PATCH 08/12] time: Add warnings when overflows or underflows are observed
* John Stultz 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
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Re: [PATCH 08/12] time: Add warnings when overflows or underflows are observed
On Thu, Jan 22, 2015 at 04:09:23PM -0800, John Stultz wrote:
> #ifdef CONFIG_DEBUG_TIMEKEEPING
> +#define WARNINGFREQ (HZ*300) /* 5 minute rate-limiting */
> +/*
> + * These simple flag variables are managed
> + * without locks, which is racy, but ok since
> + * we don't really care about being super
> + * precise about how many events were seen,
> + * just that a problem was observed.
> + */
> +static int timekeeping_underflow_seen;
> +static int timekeeping_overflow_seen;
> +
> static void timekeeping_check_update(struct timekeeper *tk, cycle_t offset)
> {
>
> 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 */
>
> 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");
> + last_warning = jiffies;
> + }
> + timekeeping_overflow_seen = 0;
> + }
> +
> }
Ah, ignore my last comment. Excellent!
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