From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from galois.linutronix.de (Galois.linutronix.de [193.142.43.55]) (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 177652E11CB; Sat, 14 Jun 2025 18:13:13 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=193.142.43.55 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1749924795; cv=none; b=k4W4mM+9JkqXjqO1KWzuraYMFD0iABEEmrv6whs8QEy3H+aO41TPQRGT1TrxwvoiqzRII2pE7RNVmOlXKCIe/FEQfzuObWs8vl3cDSmhFGKXGcMK1bpJGXhOsZ41OeLgRU62K+fwAuw+Lw1O90N2vSKbE2CJx7pI1bn5h5rotRM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1749924795; c=relaxed/simple; bh=rlmbF4zigqLq6m1FmysOfquTnG8xp6m9P7s7mkUUx8o=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=tVYQsXzCoZRt/jkyRm4a7eg5/0OYY/5KjRyF8hIwGcfYvwRqTDIJ1tFobwQFOpcRV1HErTwE3RQcMttKThCBBWdh/1LnKWeGGBCWXw4C4RXfR/ouW/vsQ4Jz2QjobU2YigyCnjM96BKhOh0xUHWTFZP8X/awhTS+h1I61vueoHI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linutronix.de; spf=pass smtp.mailfrom=linutronix.de; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b=m5dftfq4; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b=J0pKpUa4; arc=none smtp.client-ip=193.142.43.55 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linutronix.de Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linutronix.de Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="m5dftfq4"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="J0pKpUa4" Date: Sat, 14 Jun 2025 20:13:10 +0200 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1749924792; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=rlmbF4zigqLq6m1FmysOfquTnG8xp6m9P7s7mkUUx8o=; b=m5dftfq4HEtKgx+K919pvAHVnLGK1Lcxv7bu0rVqUD7P9eLCtqR4IfIZygyL4ghqBzXPmp 51YXYE/WwVi1t3RMRft7CADmxsVvAZYai8wGEDZxPZbz5hcWXICJbLyqq3ojONE6CuJYZK XfaU4wjxHdtWowog0Lrj4tOVbA6In1wQSCejaNrPC0UGpLX2vLLbEo8DWpmijJmyY4l7wS bVAdIWnX/7ydikgKZeO25+bnAee/Fke1HVfEPM8oTmzEOah75nu9yYStQgIsttt4MRoEVD 7uLcp9WllmcL1hoCwBzAx3Xs0udVg85uVDoNv6tKxU509RjjY5ld6UAwn+/vEw== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1749924792; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=rlmbF4zigqLq6m1FmysOfquTnG8xp6m9P7s7mkUUx8o=; b=J0pKpUa47IxJcLnVy/GIcFx4ZphfRJEEZVrA8f1PqJqm5HaLAEU1tszcYHvTjTQU97F+AB ZhOLOngoQQXonaBA== From: Sebastian Andrzej Siewior To: Marc =?utf-8?Q?Str=C3=A4mke?= Cc: linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org Subject: Re: Latency spikes on V6.15.1 Preempt RT and maybe related to intel IGB Message-ID: <20250614181310.08nm0KnL@linutronix.de> References: <20250613145434.T2x2ML8_@linutronix.de> <522704b0-5a35-42c3-9bcc-627b487b740c@eltropuls.de> 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=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable In-Reply-To: <522704b0-5a35-42c3-9bcc-627b487b740c@eltropuls.de> On 2025-06-14 14:59:43 [+0200], Marc Str=C3=A4mke wrote: > Hello Sebastian, Hi, > Am 13.06.2025 um 16:54 schrieb Sebastian Andrzej Siewior: > > > I have attached my config for reference. > > There is nothing wrong with it. You might want to disable NO_HZ (and use > > PERIODIC) and use HZ=3D250 (or less) >=20 > I am a bit puzzled by that. Most advise I have read suggests disabling the > periodic ticks to decrease jitter in an RT Application. What is the trade= off > here? TICK =3D periodic does HZ timer interrupts a second. Each timer interrupt serves possible timer_list timers within the kernel (not user space timers such as clock_nanosleep()/ sleep from userland - those are scheduled accurate). It also does some time keeping and accounting. TICK =3D nohz makes it possible to skip ticks if the CPU is idle. A notebook CPU which has nothing to do and goes idle could stay longer idle because the HZ interrupt will not wake it up on the next tick. So it can stay longer idle and safe energy. If the CPU does not go idle the timer tick fires every tick. TICK =3D no_hz_full you can disable the tick if the CPU remains in userland for longer period of time. Remains in userland means it does not call into the kernel. State tracking is higher with nohz, higher with no_hz_full. Most RT applications are periodic. Cyclictest should show you lower numbers with periodic than with nohz. You can monitor the timer interrupts in /proc/interrupts and compare an idle CPU vs one that runs an application (which does a busy loop). > Marc Sebastian