* timerfd_settime/timerfd_gettime issue ?
@ 2015-12-26 12:26 Helge Deller
2015-12-29 9:44 ` Thomas Gleixner
0 siblings, 1 reply; 7+ messages in thread
From: Helge Deller @ 2015-12-26 12:26 UTC (permalink / raw)
To: Thomas Gleixner; +Cc: Linux Kernel Development, linux-parisc
Hi Thomas,
I see a strange behavior on the parisc platform, for which I'm not sure if it's intended or
if there is a bug somewhere.
The program calls timerfd_settime() and sets a timer (e.g. sec=0, nsec=100000000).
Directly after setting the timer it calls timerfd_gettime() and receives (sec=0, nsec=103914413).
The second nsec is higher than the initial nsec value which was set.
Does timerfd_settime() maybe tries to add the initial time it takes to start the timer?
Any idea or hint?
Thanks,
Helge
Background:
I'm debugging the build-failure on debian for the liblinux-fd-perl package:
https://buildd.debian.org/status/fetch.php?pkg=liblinux-fd-perl&arch=hppa&ver=0.011-1&stamp=1443355593
Here is a log which I get from kernel after adding some printks.
The problematic line is #3.
[ 465.888000] timerfd_settime: interval (sec=0, nsec=0) it_value (sec=0, nsec=100000000)
[ 466.196000] timerfd_settime: interval (sec=0, nsec=100000000) it_value (sec=0, nsec=100000000)
[ 466.300000] timerfd_gettime: interval (sec=0, nsec=100000000) it_value (sec=0, nsec=103914413)
[ 466.404000] timerfd_gettime: interval (sec=0, nsec=100000000) it_value (sec=0, nsec=97444552)
[ 466.508000] timerfd_gettime: interval (sec=0, nsec=100000000) it_value (sec=0, nsec=92611704)
[ 466.616000] timerfd_gettime: interval (sec=0, nsec=100000000) it_value (sec=0, nsec=87376859)
[ 466.720000] timerfd_gettime: interval (sec=0, nsec=100000000) it_value (sec=0, nsec=82538534)
[ 466.824000] timerfd_gettime: interval (sec=0, nsec=100000000) it_value (sec=0, nsec=77293289)
[ 466.928000] timerfd_gettime: interval (sec=0, nsec=100000000) it_value (sec=0, nsec=72501584)
[ 467.036000] timerfd_gettime: interval (sec=0, nsec=100000000) it_value (sec=0, nsec=67377673)
[ 467.140000] timerfd_gettime: interval (sec=0, nsec=100000000) it_value (sec=0, nsec=62631601)
[ 467.244000] timerfd_gettime: interval (sec=0, nsec=100000000) it_value (sec=0, nsec=57401824)
^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: timerfd_settime/timerfd_gettime issue ?
2015-12-26 12:26 timerfd_settime/timerfd_gettime issue ? Helge Deller
@ 2015-12-29 9:44 ` Thomas Gleixner
2015-12-29 20:13 ` Helge Deller
0 siblings, 1 reply; 7+ messages in thread
From: Thomas Gleixner @ 2015-12-29 9:44 UTC (permalink / raw)
To: Helge Deller; +Cc: Linux Kernel Development, linux-parisc
Helge,
On Sat, 26 Dec 2015, Helge Deller wrote:
> I see a strange behavior on the parisc platform, for which I'm not sure if
> it's intended or if there is a bug somewhere.
> The program calls timerfd_settime() and sets a timer (e.g. sec=0, nsec=100000000).
> Directly after setting the timer it calls timerfd_gettime() and receives
> (sec=0, nsec=103914413).
> The second nsec is higher than the initial nsec value which was set.
>
> Does timerfd_settime() maybe tries to add the initial time it takes to start
> the timer?
>
> Any idea or hint?
Yes. This is a fallout from the power aware batching magic. Interesting that
nobody noticed this within 7 years.
Does the patch below fix your issue?
Thanks,
tglx
8<----------------
diff --git a/include/linux/hrtimer.h b/include/linux/hrtimer.h
index 76dd4f0da5ca..0f4a3e8734f1 100644
--- a/include/linux/hrtimer.h
+++ b/include/linux/hrtimer.h
@@ -268,7 +268,7 @@ static inline s64 hrtimer_get_expires_ns(const struct hrtimer *timer)
static inline ktime_t hrtimer_expires_remaining(const struct hrtimer *timer)
{
- return ktime_sub(timer->node.expires, timer->base->get_time());
+ return ktime_sub(timer->_softexpires, timer->base->get_time());
}
static inline ktime_t hrtimer_cb_get_time(struct hrtimer *timer)
^ permalink raw reply related [flat|nested] 7+ messages in thread
* Re: timerfd_settime/timerfd_gettime issue ?
2015-12-29 9:44 ` Thomas Gleixner
@ 2015-12-29 20:13 ` Helge Deller
2015-12-30 9:57 ` Thomas Gleixner
0 siblings, 1 reply; 7+ messages in thread
From: Helge Deller @ 2015-12-29 20:13 UTC (permalink / raw)
To: Thomas Gleixner; +Cc: Linux Kernel Development, linux-parisc
On 29.12.2015 10:44, Thomas Gleixner wrote:
> On Sat, 26 Dec 2015, Helge Deller wrote:
>> I see a strange behavior on the parisc platform, for which I'm not sure if
>> it's intended or if there is a bug somewhere.
>
>> The program calls timerfd_settime() and sets a timer (e.g. sec=0, nsec=100000000).
>> Directly after setting the timer it calls timerfd_gettime() and receives
>> (sec=0, nsec=103914413).
>> The second nsec is higher than the initial nsec value which was set.
>>
>> Does timerfd_settime() maybe tries to add the initial time it takes to start
>> the timer?
>>
>> Any idea or hint?
>
> Yes. This is a fallout from the power aware batching magic. Interesting that
> nobody noticed this within 7 years.
>
> Does the patch below fix your issue?
No, the patch below doesn't help.
I still see:
[ 644.916000] timerfd_settime: interval (sec=0, nsec=100000000) it_value (sec=0, nsec=100000000)
[ 645.024000] timerfd_gettime: interval (sec=0, nsec=100000000) it_value (sec=0, nsec=103029949)
Helge
> diff --git a/include/linux/hrtimer.h b/include/linux/hrtimer.h
> index 76dd4f0da5ca..0f4a3e8734f1 100644
> --- a/include/linux/hrtimer.h
> +++ b/include/linux/hrtimer.h
> @@ -268,7 +268,7 @@ static inline s64 hrtimer_get_expires_ns(const struct hrtimer *timer)
>
> static inline ktime_t hrtimer_expires_remaining(const struct hrtimer *timer)
> {
> - return ktime_sub(timer->node.expires, timer->base->get_time());
> + return ktime_sub(timer->_softexpires, timer->base->get_time());
> }
>
> static inline ktime_t hrtimer_cb_get_time(struct hrtimer *timer)
^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: timerfd_settime/timerfd_gettime issue ?
2015-12-29 20:13 ` Helge Deller
@ 2015-12-30 9:57 ` Thomas Gleixner
2016-01-04 22:27 ` Helge Deller
0 siblings, 1 reply; 7+ messages in thread
From: Thomas Gleixner @ 2015-12-30 9:57 UTC (permalink / raw)
To: Helge Deller; +Cc: Linux Kernel Development, linux-parisc
On Tue, 29 Dec 2015, Helge Deller wrote:
> No, the patch below doesn't help.
>
> I still see:
> [ 644.916000] timerfd_settime: interval (sec=0, nsec=100000000) it_value (sec=0, nsec=100000000)
> [ 645.024000] timerfd_gettime: interval (sec=0, nsec=100000000) it_value (sec=0, nsec=103029949)
>
Right. It can't help. Sorry for the distraction.
Looking deeper I found the issue. It's caused by CONFIG_TIME_LOW_RES. See the
comment in hrtimer_start_range_ns(). We round the expiry time to the next
jiffies period to avoid short timeouts. Assuming you are running with HZ=250
this is exactly 4ms. So that's where your extra time comes from.
Not sure what to do about that.
Thanks,
tglx
^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: timerfd_settime/timerfd_gettime issue ?
2015-12-30 9:57 ` Thomas Gleixner
@ 2016-01-04 22:27 ` Helge Deller
2016-01-10 20:57 ` Helge Deller
0 siblings, 1 reply; 7+ messages in thread
From: Helge Deller @ 2016-01-04 22:27 UTC (permalink / raw)
To: Thomas Gleixner, linux-parisc, James Bottomley, John David Anglin
Cc: Linux Kernel Development
Hi Thomas,
* Thomas Gleixner <tglx@linutronix.de>:
> On Tue, 29 Dec 2015, Helge Deller wrote:
> > No, the patch below doesn't help.
> >
> > I still see:
> > [ 644.916000] timerfd_settime: interval (sec=0, nsec=100000000) it_value (sec=0, nsec=100000000)
> > [ 645.024000] timerfd_gettime: interval (sec=0, nsec=100000000) it_value (sec=0, nsec=103029949)
> >
>
> Right. It can't help. Sorry for the distraction.
>
> Looking deeper I found the issue. It's caused by CONFIG_TIME_LOW_RES. See the
> comment in hrtimer_start_range_ns(). We round the expiry time to the next
> jiffies period to avoid short timeouts. Assuming you are running with HZ=250
> this is exactly 4ms. So that's where your extra time comes from.
Yes, that seems right.
> Not sure what to do about that.
The patch below seems to solve the issue for me. But I'm not sure if
there might be any side-effects. What do you think? Does the patch
seems correct? It just adjusts to values returned to userspace (and thus
hides the internal roundings).
Thanks,
Helge
diff --git a/fs/timerfd.c b/fs/timerfd.c
index b94fa6c..9b6cc73 100644
--- a/fs/timerfd.c
+++ b/fs/timerfd.c
@@ -152,8 +152,16 @@ static ktime_t timerfd_get_remaining(struct timerfd_ctx *ctx)
if (isalarm(ctx))
remaining = alarm_expires_remaining(&ctx->t.alarm);
- else
+ else {
remaining = hrtimer_expires_remaining(&ctx->t.tmr);
+#ifdef CONFIG_TIME_LOW_RES
+ /* Expiry time was rounded up in hrtimer_start_range_ns()
+ * to the next jiffies period to avoid short timeouts.
+ * Subtract it here again to avoid userspace seeing higher
+ * values than expected. */
+ remaining.tv64 -= hrtimer_resolution;
+#endif
+ }
return remaining.tv64 < 0 ? ktime_set(0, 0): remaining;
}
^ permalink raw reply related [flat|nested] 7+ messages in thread
* [PATCH parisc,frv,m68k] timerfd: Fix timeout values with CONFIG_TIME_LOW_RES=y
2016-01-04 22:27 ` Helge Deller
@ 2016-01-10 20:57 ` Helge Deller
0 siblings, 0 replies; 7+ messages in thread
From: Helge Deller @ 2016-01-10 20:57 UTC (permalink / raw)
To: linux-parisc, Thomas Gleixner, linux-m68k, dhowells, linux-kernel
On architectures where CONFIG_TIME_LOW_RES is set (currently parisc,
m68k, frv) calling timerfd_settime() to set a timeout and directly
afterwards calling timerfd_gettime() to get the remaining time shows a
behaviour that the remaining time can be higher than the originally set
timeout.
Here is an example showing the problem, that the nsec value of it_value
is higher than the set nsec value:
timerfd_settime: interval (sec=0, nsec=100000000) it_value (sec=0, nsec=100000000)
timerfd_gettime: interval (sec=0, nsec=100000000) it_value (sec=0, nsec=103029949)
This happens because in hrtimer_start_range_ns() the expiry time is
rounded to the next jiffies period to avoid short timeouts. When running
with HZ=250 this is 4ms which can be seen in the example above.
This behaviour confuses userspace programs. For example, the debian
liblinux-fd-perl and libnanomsg-raw-perl packges fail to build because
the timeout is higher than expected.
Fix this problem by subtracting the value added by
hrtimer_start_range_ns() before returning the timeout back to userspace.
Signed-off-by: Helge Deller <deller@gmx.de>
diff --git a/fs/timerfd.c b/fs/timerfd.c
index b94fa6c..098ac0a 100644
--- a/fs/timerfd.c
+++ b/fs/timerfd.c
@@ -152,8 +152,17 @@ static ktime_t timerfd_get_remaining(struct timerfd_ctx *ctx)
if (isalarm(ctx))
remaining = alarm_expires_remaining(&ctx->t.alarm);
- else
+ else {
remaining = hrtimer_expires_remaining(&ctx->t.tmr);
+#ifdef CONFIG_TIME_LOW_RES
+ /* Expiry time was rounded up in hrtimer_start_range_ns()
+ * to the next jiffies period to avoid short timeouts.
+ * Subtract it here again to avoid userspace seeing higher
+ * values than originally programmed. */
+ if (!(ctx->settime_flags & TFD_TIMER_ABSTIME))
+ remaining.tv64 -= hrtimer_resolution;
+#endif
+ }
return remaining.tv64 < 0 ? ktime_set(0, 0): remaining;
}
^ permalink raw reply related [flat|nested] 7+ messages in thread
* [PATCH parisc,frv,m68k] timerfd: Fix timeout values with CONFIG_TIME_LOW_RES=y
@ 2016-01-10 20:57 ` Helge Deller
0 siblings, 0 replies; 7+ messages in thread
From: Helge Deller @ 2016-01-10 20:57 UTC (permalink / raw)
To: linux-parisc, Thomas Gleixner, linux-m68k, dhowells, linux-kernel
On architectures where CONFIG_TIME_LOW_RES is set (currently parisc,
m68k, frv) calling timerfd_settime() to set a timeout and directly
afterwards calling timerfd_gettime() to get the remaining time shows a
behaviour that the remaining time can be higher than the originally set
timeout.
Here is an example showing the problem, that the nsec value of it_value
is higher than the set nsec value:
timerfd_settime: interval (sec=0, nsec=100000000) it_value (sec=0, nsec=100000000)
timerfd_gettime: interval (sec=0, nsec=100000000) it_value (sec=0, nsec=103029949)
This happens because in hrtimer_start_range_ns() the expiry time is
rounded to the next jiffies period to avoid short timeouts. When running
with HZ=250 this is 4ms which can be seen in the example above.
This behaviour confuses userspace programs. For example, the debian
liblinux-fd-perl and libnanomsg-raw-perl packges fail to build because
the timeout is higher than expected.
Fix this problem by subtracting the value added by
hrtimer_start_range_ns() before returning the timeout back to userspace.
Signed-off-by: Helge Deller <deller@gmx.de>
diff --git a/fs/timerfd.c b/fs/timerfd.c
index b94fa6c..098ac0a 100644
--- a/fs/timerfd.c
+++ b/fs/timerfd.c
@@ -152,8 +152,17 @@ static ktime_t timerfd_get_remaining(struct timerfd_ctx *ctx)
if (isalarm(ctx))
remaining = alarm_expires_remaining(&ctx->t.alarm);
- else
+ else {
remaining = hrtimer_expires_remaining(&ctx->t.tmr);
+#ifdef CONFIG_TIME_LOW_RES
+ /* Expiry time was rounded up in hrtimer_start_range_ns()
+ * to the next jiffies period to avoid short timeouts.
+ * Subtract it here again to avoid userspace seeing higher
+ * values than originally programmed. */
+ if (!(ctx->settime_flags & TFD_TIMER_ABSTIME))
+ remaining.tv64 -= hrtimer_resolution;
+#endif
+ }
return remaining.tv64 < 0 ? ktime_set(0, 0): remaining;
}
^ permalink raw reply related [flat|nested] 7+ messages in thread
end of thread, other threads:[~2016-01-10 20:57 UTC | newest]
Thread overview: 7+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2015-12-26 12:26 timerfd_settime/timerfd_gettime issue ? Helge Deller
2015-12-29 9:44 ` Thomas Gleixner
2015-12-29 20:13 ` Helge Deller
2015-12-30 9:57 ` Thomas Gleixner
2016-01-04 22:27 ` Helge Deller
2016-01-10 20:57 ` [PATCH parisc,frv,m68k] timerfd: Fix timeout values with CONFIG_TIME_LOW_RES=y Helge Deller
2016-01-10 20:57 ` Helge Deller
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