From: Miroslav Lichvar <mlichvar@redhat.com>
To: Rodolfo Giometti <giometti@enneenne.com>
Cc: David Woodhouse <dwmw2@infradead.org>,
Richard Cochran <richardcochran@gmail.com>,
Andrew Lunn <andrew+netdev@lunn.ch>,
"David S. Miller" <davem@davemloft.net>,
Eric Dumazet <edumazet@google.com>,
Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
John Stultz <jstultz@google.com>,
Thomas Gleixner <tglx@kernel.org>,
Stephen Boyd <sboyd@kernel.org>,
linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
Alexander Gordeev <agordeev@linux.ibm.com>
Subject: Re: [PATCH v4 3/4] pps: Always use ktime_get_snapshot_id() for pps_get_ts()
Date: Thu, 1 Oct 2026 15:14:19 +0200 [thread overview]
Message-ID: <ar5cq0vslvQ00Pek@localhost> (raw)
In-Reply-To: <4afec82e-9e3d-4615-9657-7d467d8dc3e7@enneenne.com>
On Mon, Sep 28, 2026 at 09:58:02AM +0200, Rodolfo Giometti wrote:
> On Sat, 2026-09-26 at 21:38 +0100, David Woodhouse wrote:
> > So yes, the specific code path you're looking at *does* get slightly
> > longer (50ns to the counter read instead of 30ns).
> [...]
> > However, they are *entirely* in the noise, as there's about 600 ns of
> > hardware and 2-4 *microseconds* of software latency before we even get
> > there.
>
> Thanks for measuring it, and on real hardware with a real edge. That
> answers my concern: ~20 ns of constant cost against microseconds of
> latency upstream of the handler is not something PPS can see, and
> trading it for the removal of a non-constant error is the right
> trade.
FWIW, there are some polling versions of the pps-gpio driver (out of
tree), which provide much more stable measurements by trading
interrupts for higher CPU use and where this additional delay might be
visible.
--
Miroslav Lichvar
next prev parent reply other threads:[~2026-10-01 13:14 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-08-29 20:56 [PATCH v4 0/4] Add ntp_error to clock snapshot, enable NTP_PPS on tickless kernel David Woodhouse
2026-08-29 20:56 ` [PATCH v4 1/4] timekeeping: Apply extrapolated ntp_error to clock snapshots David Woodhouse
2026-08-29 20:57 ` [PATCH v4 2/4] pps: Drop the !NO_HZ_COMMON dependency from NTP_PPS David Woodhouse
2026-09-01 15:35 ` Rodolfo Giometti
2026-09-02 0:13 ` David Woodhouse
2026-09-28 13:37 ` David Woodhouse
2026-09-28 16:41 ` Rodolfo Giometti
2026-09-28 19:28 ` David Woodhouse
2026-09-29 6:33 ` Rodolfo Giometti
2026-09-29 9:32 ` David Woodhouse
2026-09-29 11:48 ` Rodolfo Giometti
2026-09-29 12:02 ` David Woodhouse
2026-09-30 1:28 ` David Woodhouse
2026-09-30 12:57 ` Rodolfo Giometti
2026-09-30 10:37 ` David Woodhouse
2026-09-30 12:57 ` Rodolfo Giometti
2026-09-30 14:05 ` David Woodhouse
2026-09-30 18:24 ` David Woodhouse
2026-10-01 8:20 ` Rodolfo Giometti
2026-10-01 9:08 ` David Woodhouse
2026-08-29 20:57 ` [PATCH v4 3/4] pps: Always use ktime_get_snapshot_id() for pps_get_ts() David Woodhouse
2026-09-01 15:35 ` Rodolfo Giometti
2026-09-01 23:56 ` David Woodhouse
2026-09-26 20:38 ` David Woodhouse
2026-09-28 7:58 ` Rodolfo Giometti
2026-09-28 12:59 ` David Woodhouse
2026-09-28 16:41 ` Rodolfo Giometti
2026-10-01 13:14 ` Miroslav Lichvar [this message]
2026-10-01 15:38 ` David Woodhouse
2026-10-02 7:04 ` Rodolfo Giometti
2026-10-02 9:07 ` David Woodhouse
2026-10-02 12:29 ` David Woodhouse
2026-10-02 13:44 ` David Woodhouse
2026-08-29 20:57 ` [PATCH v4 4/4] [DO NOT MERGE] ptp: ptp_vmclock: Add simulated 1PPS support David Woodhouse
2026-09-01 15:35 ` [PATCH v4 0/4] Add ntp_error to clock snapshot, enable NTP_PPS on tickless kernel Rodolfo Giometti
2026-09-01 23:37 ` David Woodhouse
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