From mboxrd@z Thu Jan 1 00:00:00 1970 From: Helge Deller Subject: Re: timerfd_settime/timerfd_gettime issue ? Date: Mon, 4 Jan 2016 23:27:57 +0100 Message-ID: <20160104222757.GA969@p100.box> References: <567E8766.6020707@gmx.de> <5682E96A.5040008@gmx.de> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Linux Kernel Development To: Thomas Gleixner , linux-parisc@vger.kernel.org, James Bottomley , John David Anglin Return-path: In-Reply-To: List-ID: List-Id: linux-parisc.vger.kernel.org Hi Thomas, * Thomas Gleixner : > 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; }