From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id F13751FBC9B for ; Tue, 12 Nov 2024 13:56:11 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1731419772; cv=none; b=Ddm/s52ASGwE4D5QvqNW9fxlYjRhmAgnAwNi/rqQJIhhr1FuJZby3koeGSBHkLlldAwZIYsa6jdp91T6dr2eKPql/F27EbtPra25tfSlFbcT8sa3u2XJa6jcddlbE8mMO+GWg5J0AiIvEThukfipHODEJ5Q/QeFNj/sujwMmze8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1731419772; c=relaxed/simple; bh=T6YQnYADiOc+eB2ZCcn4o+pCA4TyC4vZ8TSHZ5+kOMU=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=duiKLblZANNUfrwFcg/hvzYn6moThGJzkzcwH3wMgh6V2bZ8hm4X+yuXhf1c6/Oc+ISRlci6PIzyoYluHPD/kCtuWSq0c0TXlfjD8xLWE9NrUxBLqF9AhtHQhL00SFiVQgLUUXKJ3GskMd7WIIYMfqrsdiMCDU75nyi+xnFeKEc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=hc24ecNC; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="hc24ecNC" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4EDD3C4CECD; Tue, 12 Nov 2024 13:56:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1731419771; bh=T6YQnYADiOc+eB2ZCcn4o+pCA4TyC4vZ8TSHZ5+kOMU=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=hc24ecNCVjkhpfrV8ycxiIWZhdPjJrw4TacSlWMX6JYcVnUXCfIfAGDEVyqVQ1Y0R GewWL+5dsT9Es6z5WK3wa070E3ixHzqnYYKEjACP4XL3ljYsfaZF9rJ33UMOs2WbGr aWAFwEyKQ52ScJcI9k+ybt6QeQ6UB1tckZomd6l3RJYShsHQbWRHMyJ/j5uBUbgFsh WM/7AH8ERdOv7XykYciWcu3PAL9dyIY4WI4bLjSLZydvaDN4Ilw++FsD+wlKp4Q3+F c7jwA+4d9HVor1nxvNgt0W1G8PQnb86JLID44Rdf4U//Oh3mfjmscW0cxsDz3cxSTp JlT+Q5Mpq8sig== Date: Tue, 12 Nov 2024 14:56:08 +0100 From: Frederic Weisbecker To: Thomas Gleixner Cc: "Joel Fernandes (Google)" , linux-kernel@vger.kernel.org, Anna-Maria Behnsen , Ingo Molnar Subject: Re: [RFC 1/3] tick-sched: Remove last_tick and calculate next tick from now Message-ID: References: <20241108174839.1016424-1-joel@joelfernandes.org> <20241108174839.1016424-2-joel@joelfernandes.org> <874j4co98w.ffs@tglx> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <874j4co98w.ffs@tglx> Le Tue, Nov 12, 2024 at 02:46:23PM +0100, Thomas Gleixner a écrit : > On Tue, Nov 12 2024 at 00:43, Frederic Weisbecker wrote: > > Le Fri, Nov 08, 2024 at 05:48:34PM +0000, Joel Fernandes (Google) a écrit : > > >> During tick restart, we use last_tick and forward it past now. > >> > >> Since we are forwarding past now, we can simply use now as a reference > >> instead of last_tick. This patch removes last_tick and does so. > >> > >> This patch potentially does more mul/imul than the existing code, > >> as sometimes forwarding past now need not be done if last_tick > now. > >> However, the patch is a cleanup which reduces LOC and reduces the size > >> of struct tick_sched. > > May I politely ask you to read and follow the Documentation > vs. changelogs? > > https://www.kernel.org/doc/html/latest/process/maintainer-tip.html#changelog > > Also > > git grep 'This patch' Documentation/process > > might give you a hint. > > >> - /* Forward the time to expire in the future */ > >> - hrtimer_forward(&ts->sched_timer, now, TICK_NSEC); > >> + hrtimer_set_expires(&ts->sched_timer, DIV_ROUND_UP_ULL(now, TICK_NSEC) * TICK_NSEC); > > How is a division and multiplication in this hotpath helpful? That's > awfully slow on 32-bit machines and pointless on 64-bit too. > > Using now is also wrong as it breaks the sched_skew_tick distribution by > aligning the tick on all CPUs again. > > IOW, this "cleanup" is making things worse. > > > We don't want to rewrite hrtimer_forward() but, after all, the current expiry is > > enough a relevant information. > > > > How about just this? It's worth it as it now forwards after the real last programmed > > tick, which should be close enough from @now with a delta below TICK_NSEC, or even > > better @now is below the expiry. Therefore it should resume as just a no-op > > or at worst an addition within hrtimer_forward(): > > > > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c > > index 753a184c7090..ffd0c026a248 100644 > > --- a/kernel/time/tick-sched.c > > +++ b/kernel/time/tick-sched.c > > @@ -838,7 +838,6 @@ EXPORT_SYMBOL_GPL(get_cpu_iowait_time_us); > > static void tick_nohz_restart(struct tick_sched *ts, ktime_t now) > > { > > hrtimer_cancel(&ts->sched_timer); > > - hrtimer_set_expires(&ts->sched_timer, ts->last_tick); > > > > /* Forward the time to expire in the future */ > > hrtimer_forward(&ts->sched_timer, now, TICK_NSEC); > > That's just wrong. ts->sched_timer.expires contains a tick in the > future. If tick_nohz_stop_tick() set it to 10 ticks in the future and > the CPU goes out of idle due to a device interrupt before the timer > expires, then hrtimer_forward() will do nothing because expires is ahead > of now. > > Which means the CPU is not idle and has no tick until the delayed tick > which was set by tick_nohz_stop_tick() expires. Not really correct. Bah! Yes of course... > > Thanks, > > tglx