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Date: Mon, 10 Jan 2022 04:33:34 +0200
From: Konstantin Belousov <kostikbel@gmail.com>
To: Alexander Motin <mav@freebsd.org>
Cc: src-committers@freebsd.org, dev-commits-src-all@freebsd.org,
dev-commits-src-main@freebsd.org
Subject: Re: git: cb1f5d11366d - main - Reduce minimum idle hardclock rate
from 2Hz to 1Hz.
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References: <202201100026.20A0Q1fs045878@gitrepo.freebsd.org>
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On Mon, Jan 10, 2022 at 12:26:01AM +0000, Alexander Motin wrote:
> The branch main has been updated by mav:
>
> URL: https://cgit.FreeBSD.org/src/commit/?id=cb1f5d11366dc9b803f2755d83fe02599830882a
>
> commit cb1f5d11366dc9b803f2755d83fe02599830882a
> Author: Alexander Motin <mav@FreeBSD.org>
> AuthorDate: 2022-01-10 00:17:43 +0000
> Commit: Alexander Motin <mav@FreeBSD.org>
> CommitDate: 2022-01-10 00:25:56 +0000
>
> Reduce minimum idle hardclock rate from 2Hz to 1Hz.
>
> On idle 80-thread system it allows to improve package-level idle state
> residency and so power consumption by several percent.
Are you sure that this is safe?
Our timecounters are tailored to wrap at 1sec, at least the most precise/
important TSC-low is. If idle system interrupts with 1Hz frequency,
it probably comes dangerously close to situations where tc_windup() could
be missed.
>
> MFC after: 2 weeks
> ---
> sys/kern/kern_clocksource.c | 5 +++--
> sys/kern/kern_timeout.c | 5 ++---
> 2 files changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/sys/kern/kern_clocksource.c b/sys/kern/kern_clocksource.c
> index dfc9081ba9f6..48e06ee082fc 100644
> --- a/sys/kern/kern_clocksource.c
> +++ b/sys/kern/kern_clocksource.c
> @@ -239,13 +239,14 @@ getnextcpuevent(int idle)
> /* Handle hardclock() events, skipping some if CPU is idle. */
> event = state->nexthard;
> if (idle) {
> - hardfreq = (u_int)hz / 2;
> - if (tc_min_ticktock_freq > 2
> + if (tc_min_ticktock_freq > 1
> #ifdef SMP
> && curcpu == CPU_FIRST()
> #endif
> )
> hardfreq = hz / tc_min_ticktock_freq;
> + else
> + hardfreq = hz;
> if (hardfreq > 1)
> event += tick_sbt * (hardfreq - 1);
> }
> diff --git a/sys/kern/kern_timeout.c b/sys/kern/kern_timeout.c
> index 91882ddb5fba..d0fb19661fa4 100644
> --- a/sys/kern/kern_timeout.c
> +++ b/sys/kern/kern_timeout.c
> @@ -434,8 +434,7 @@ callout_process(sbintime_t now)
> struct callout_cpu *cc;
> struct callout_list *sc;
> struct thread *td;
> - sbintime_t first, last, max, tmp_max;
> - uint32_t lookahead;
> + sbintime_t first, last, lookahead, max, tmp_max;
> u_int firstb, lastb, nowb;
> #ifdef CALLOUT_PROFILING
> int depth_dir = 0, mpcalls_dir = 0, lockcalls_dir = 0;
> @@ -455,7 +454,7 @@ callout_process(sbintime_t now)
> else if (nowb - firstb == 1)
> lookahead = (SBT_1S / 8);
> else
> - lookahead = (SBT_1S / 2);
> + lookahead = SBT_1S;
> first = last = now;
> first += (lookahead / 2);
> last += lookahead;