Hi Thomas,
* Thomas Gleixner <tglx@xxxxxxxxxxxxx>:
> 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;
}
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