Date: Fri, 3 Jun 2022 15:10:10 +0200 From: Sebastian Huber <sebastian.huber@embedded-brains.de> To: Poul-Henning Kamp <phk@phk.freebsd.dk> Cc: Konstantin Belousov <kostikbel@gmail.com>, hackers@freebsd.org Subject: Re: pps_capture() and pps_fetch() Message-ID: <35d4d55c-ff62-cff7-cdbf-ea2549dee86c@embedded-brains.de> In-Reply-To: <202206030828.2538ScDg080165@critter.freebsd.dk> References: <5b8310db-c94b-709f-8c57-bec2d413a80f@embedded-brains.de> <202206010725.2517PEfF036703@critter.freebsd.dk> <e1093df0-c719-0421-3e96-c6d7df861a51@embedded-brains.de> <YphDlMyWxDWvJo5Q@kib.kiev.ua> <c4fe6a08-24b6-522e-3963-cf03eb994496@embedded-brains.de> <202206030828.2538ScDg080165@critter.freebsd.dk>
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On 03.06.22 10:28, Poul-Henning Kamp wrote:
>> The expensive part in pps_event() is after the th_generation checks. I
>> think from a performance point of view, the checks can be reduced to
>> just one th_generation check. I am more concerned if this would
>> introduce a subtle functional change.
> Assuming that your timecounter hardware does not roll over fast enough
> to open any races, I think that is correct.
Would a timecounter overflow within a time interval from th_generation=20
to th_generation + 1 not be a bug in general?
static __inline void
bintime_off(struct bintime *bt, u_int off)
{
struct timehands *th;
struct bintime *btp;
uint64_t scale;
u_int delta, gen, large_delta;
do {
th =3D timehands;
gen =3D atomic_load_acq_int(&th->th_generation);
For example, if the thread gets interrupted here and the timecounter=20
overflows, then the measurement below would be wrong if th_generation=20
didn't change.
btp =3D (struct bintime *)((vm_offset_t)th + off);
*bt =3D *btp;
scale =3D th->th_scale;
delta =3D tc_delta(th);
large_delta =3D th->th_large_delta;
atomic_thread_fence_acq();
} while (gen =3D=3D 0 || gen !=3D th->th_generation);
bintime_add_tc_delta(bt, scale, large_delta, delta);
}
--=20
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