linux-kernel - Re: [PATCH] time/timecounter: inline timecounter

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Message-ID: <CAPREpbYM+P6gD94ZTNZ+bHBT9=7vz1t8z+Ts_CS72-j3FeodNA@mail.gmail.com>
Date: Mon, 1 Dec 2025 13:16:38 -0500
From: Kevin Yang <yyd@...gle.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: Eric Dumazet <edumazet@...gle.com>, John Stultz <jstultz@...gle.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Stephen Boyd <sboyd@...nel.org>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, linux-kernel <linux-kernel@...r.kernel.org>, netdev@...r.kernel.org, 
	Eric Dumazet <eric.dumazet@...il.com>, Willem de Bruijn <willemb@...gle.com>, 
	Neal Cardwell <ncardwell@...gle.com>, Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH] time/timecounter: inline timecounter_cyc2time()

Reviewed-by: Kevin Yang <yyd@...gle.com>


On Sat, Nov 29, 2025 at 12:06 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> Eric Dumazet wrote:
> > New network transport protocols want NIC drivers to get hwtstamps
> > of all incoming packets, and possibly all outgoing packets.
> >
> > Swift congestion control is used by good old TCP transport and is
> > our primary need for timecounter_cyc2time(). This will be upstreamed soon.
> >
> > This means timecounter_cyc2time() can be called more than 100 million
> > times per second on a busy server.
> >
> > Inlining timecounter_cyc2time() brings a 12 % improvement on a
> > UDP receive stress test on a 100Gbit NIC.
> >
> > Note that FDO, LTO, PGO are unable to magically help for this
> > case, presumably because NIC drivers are almost exclusively shipped
> > as modules.
> >
> > Add an unlikely() around the cc_cyc2ns_backwards() case,
> > even if FDO (when used) is able to take care of this optimization.
> >
> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> > Link: https://research.google/pubs/swift-delay-is-simple-and-effective-for-congestion-control-in-the-datacenter/
> > Cc: Kevin Yang <yyd@...gle.com>
> > Cc: Willem de Bruijn <willemb@...gle.com>
> > Cc: Neal Cardwell <ncardwell@...gle.com>
> > Cc: Yuchung Cheng <ycheng@...gle.com>
>
> Reviewed-by: Willem de Bruijn <willemb@...gle.com>