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 CDADA14534B for ; Thu, 16 May 2024 11:25:21 +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=1715858721; cv=none; b=s3mr9ogqtqZUBXri2adwPmN63m6FkQKgnbS9I3F9Hh3Zq3bs7etugvDjsmjKbVH9GYiM4ABkC3ZGp0oeyXEv89evbGoxIm+kFo8jQRP2+VU7eXNiDyXeNuyS++jqq6zXwwYgi+Ps3E8Q6GgylCrZHUqQ8nioirDOTUj2aF73aKA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1715858721; c=relaxed/simple; bh=Z3mpnLIUmt7AHRISjQddMdml+3gLRLCWGiU+qcDJh8g=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=AAHrFa2RnkxlBORl8jVe6S/6alHMsmXWzWe9UvbxlZWr3Cc9Q/LqQy/VxHmbhVsCPHUvVc7i2RPewwbngCs7poZLimV2NonIcVvX5KzDn489fRlEOcrJSslmWhUKPDzUa4hbLYJlwSBmKsHkd9iUtvGIw8wVGWRt08nwd8Vp6m0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=NHESXcPa; 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="NHESXcPa" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 071FFC32782; Thu, 16 May 2024 11:25:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1715858721; bh=Z3mpnLIUmt7AHRISjQddMdml+3gLRLCWGiU+qcDJh8g=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=NHESXcPat6gtqcuZi1lj1aZnOYsoBm2cW7PhCfJuGbmdmZJWkNQceN1sc87j49v3Z GGLKLPmqSPwFXOc5wq7XOeNBDbVwTy/K3IKx26eRKjJx5zffXf3oI/UcjSWJ7EW+QZ cMaASId65loQmqLIp+ImHpQuDsAtFPBAcu3SXyX5PUzxApkF2W88M6oNYx/SJ5REWz uNyaL1bTutiem6s8howHjAnknBBCbm/EDas/gIm9u7fCa7Jb2vHl8wRHoDvgHc12YU 2Yk5n8SxW93BU64LckCwyh94pTKShGUh5jII0/z5N//LjrijG+6iQl2cesE2rIXp1f cqhwGfl+nhf3w== Date: Thu, 16 May 2024 13:25:18 +0200 From: Frederic Weisbecker To: Peter Zijlstra Cc: Yun Levi , Joel Fernandes , Vineeth Pillai , Vincent Guittot , Dietmar Eggemann , anna-maria@linutronix.de, mingo@kernel.org, tglx@linutronix.de, Markus.Elfring@web.de, linux-kernel@vger.kernel.org Subject: Re: [PATCH v4] time/tick-sched: idle load balancing when nohz_full cpu becomes idle. Message-ID: References: <20240508172621.30069-1-ppbuk5246@gmail.com> <20240509092931.35209-2-ppbuk5246@gmail.com> <20240516075628.GC22557@noisy.programming.kicks-ass.net> <20240516084911.GF22557@noisy.programming.kicks-ass.net> 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=us-ascii Content-Disposition: inline In-Reply-To: <20240516084911.GF22557@noisy.programming.kicks-ass.net> On Thu, May 16, 2024 at 10:49:11AM +0200, Peter Zijlstra wrote: > On Thu, May 16, 2024 at 09:20:08AM +0100, Yun Levi wrote: > > > None of that HK nonsense is relevant. The NOHZ_FULL nonsense implies > > > single CPU partitions, and *that* should be avoiding any and all > > > load-balancing. > > > > Do you mean.. tick_nohz_full cpu (non-HK-ticked cpu) shouldn't belong > > to any sched_domain? > > AFAIK NOHZ_FULL still hard relies on the isolcpus garbage, so yeah, it > should be all single cpu partitions, which don't have a domain. > > (this really should migrate to use cpusets partitions) > > > > If there still is, that's a bug, but that's not related to HK goo. > > > > > > As such, I don't think the HK_TYPE_SCHED check in > > > nohz_balance_enter_idle() actually makes sense, the on_null_omain() > > > check a little below that should already take care of things, no? > > > > IIUC, > > currently, whether cpu belongs on domain or null is determined by > > HK_DOMAIN_FLAGS > > No! you can create NULL domains without any of the HK nonsense. Both > isolcpus and cpusets can create single CPU partitions. > > > However, when "nohz_full=" is used, it still on HK_DOMAIN, so it > > belongs to sched_domain > > so, it couldn't be filtered out by on_null_domain(). > > > > unless "isolcpus=domain" or "isolcpus={cpu_list}", it's on null domain. > > with "isolcpus=tick", it participates sched_domain. > > Frederic ?!? You can use nohz_full without isolcpus? That makes no > sense. If you do that you get to keep the pieces. I fear you can yes, even though most users combine it with isolcpus. I know, that interface is terrible but it dates from times when we weren't sure about all the potential usecases of nohz_full. There was a possibility that HPC could just want to reduce ticks without all the hard and costly isolation around. But all the usecases I have witnessed so far in ten years involved wanting 0 noise after all...