From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mgamail.intel.com (mgamail.intel.com [192.198.163.8]) (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 DF44C14A639 for ; Fri, 12 Apr 2024 16:47:12 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=192.198.163.8 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1712940434; cv=none; b=bLZDAIaqEcG8lowIm0aKWZ2PSrPVHY4uTW+pMANmkP7lL23/o6JJL4e8StXjISVi+vj+wgh1/xi6mR9AMFRnnwG3YMaESXH++3MnwQp2ykJT4ZI9WqQuFAMLZAdI/d2ocTCU4gJWlebvA9O4Vs3KUN11yWuADFL1lhnBvtrs7g8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1712940434; c=relaxed/simple; bh=723TyUM84tFd/tNvuYS/SabsJNTtPpFWjxVSgSpvVNc=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=u4sGDo35K/dlLSeZyYKjIDyWU/NJ/m5VEjBpktMsHD4n5lQ+3BmJf7AlsYEV0gecXIcnFtAhHlAmLZyevuU7c39u4aF1kZTzrZq/SgpuSMDO1KeFUOMql4soVvNvenPWE18eYzwWG6tLGd1QvJXhXeVWXM54q4rSRX9zEJMS3Lo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=intel.com; spf=pass smtp.mailfrom=intel.com; dkim=pass (2048-bit key) header.d=intel.com header.i=@intel.com header.b=KehtsQ3f; arc=none smtp.client-ip=192.198.163.8 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=intel.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=intel.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=intel.com header.i=@intel.com header.b="KehtsQ3f" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1712940433; x=1744476433; h=message-id:date:mime-version:subject:to:cc:references: from:in-reply-to:content-transfer-encoding; bh=723TyUM84tFd/tNvuYS/SabsJNTtPpFWjxVSgSpvVNc=; b=KehtsQ3fBxKd7ipGr8EizLilC5CjJgCaGj2u2n6Vnoy2xvZ2tKIWcxqy Bp7OLOCamV6ZvotpQGK54i6ZRCnAeTfUavXDoLrT+9TfwEKrlRYo7HVNm RY60pkbd2lH7rbbXMklOIZNFQd1O2jG8wfGaySnaUoARYiHnB1gMCAV4F /xmrp95J9Gtda0u4uZk0BGaTmr8NzP78DaKW0O0xARICBT6/4uH7pz8Vt cjy6U/NRO5s0/M7psF800NScRCJXBky8UhT5WbjWgnSq0qJGZ+/v6lnOC dTu2pQJaBRXOzjXNlBxupKBcJti0uqOITllDywJ4h/n37jtgPNSzq0T9G Q==; X-CSE-ConnectionGUID: FTy8B/dRTIaiZE4zhGuppA== X-CSE-MsgGUID: HhwdqmDnRUyK1DsH6WQbQQ== X-IronPort-AV: E=McAfee;i="6600,9927,11042"; a="25917094" X-IronPort-AV: E=Sophos;i="6.07,196,1708416000"; d="scan'208";a="25917094" Received: from fmviesa004.fm.intel.com ([10.60.135.144]) by fmvoesa102.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 12 Apr 2024 09:47:12 -0700 X-CSE-ConnectionGUID: 7yd/4pDaQ4C+y4gF/pL9zA== X-CSE-MsgGUID: eLGOAeRyQAaD7zcJ8ydvAw== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.07,196,1708416000"; d="scan'208";a="25928693" Received: from soc-cp83kr3.jf.intel.com (HELO [10.24.10.40]) ([10.24.10.40]) by fmviesa004-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 12 Apr 2024 09:47:12 -0700 Message-ID: <0d366f20-e332-45a9-8545-4513fdce6e21@intel.com> Date: Fri, 12 Apr 2024 09:47:12 -0700 Precedence: bulk X-Mailing-List: linux-kselftest@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] selftests/rseq: take large C-state exit latency into consideration To: Sean Christopherson Cc: linux-kselftest@vger.kernel.org, pbonzini@redhat.com References: <20240322163351.150673-1-zide.chen@intel.com> Content-Language: en-US From: "Chen, Zide" In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 4/5/2024 4:01 PM, Sean Christopherson wrote: > On Fri, Mar 22, 2024, Zide Chen wrote: >> Currently, the migration worker delays 1-10 us, assuming that one >> KVM_RUN iteration only takes a few microseconds. But if C-state exit >> latencies are large enough, for example, hundreds or even thousands >> of microseconds on server CPUs, it may happen that it's not able to >> bring the target CPU out of C-state before the migration worker starts >> to migrate it to the next CPU. >> >> If the system workload is light, most CPUs could be at a certain level >> of C-state, and the vCPU thread may waste milliseconds before it can >> actually migrate to a new CPU. > > Well fudge. That's definitely not on my bingo sheet. > >> Thus, the tests may be inefficient in such systems, and in some cases >> it may fail the migration/KVM_RUN ratio sanity check. >> >> Since we are not able to turn off the cpuidle sub-system in run time, >> this patch creates an idle thread on every CPU to prevent them from >> entering C-states. > > First off, huge thanks for debugging this! That must have been quite the task > (no pun intended). > > While spinning up threads on every CPU is a clever way to ensure they don't go > into a deep sleep state, I'm not exactly excited about the idea of putting every > reachable CPU into a busy loop. And while this doesn't add _that_ much complexity, > I'm not sure the benefit (preserving the assert for all systems) is worth it. I > also don't want to arbitrarily prevent idle task (as in, the kernel's idle task) > interactions. E.g. it's highly (highly) unlikely, but not impossible for there > to be a bug that's unique to idle tasks, or C-states, or other edge case. > > Are there any metrics/stats that can be (easily) checked to grant an exception > to the sanity check? That's a very hand-wavy question, as I'm not even sure what > type of stat we'd want to look at. Actual runtime of a task, maybe? > > If that's not easy, what if we add an off-by-default command line option to skip > the sanity check? I was resistant to simply deleting the assert in the past, but > that was mainly because I didn't want to delete it without understanding what was > causing problems. That would allow CI environments to opt-out as needed, while > still keeping the sanity check alive for enough systems to make it useful. Sorry for not replying earlier. I overlooked your email from my inbox. :) Alternative to the busy loop, how about using the /dev/cpu_dma_latency interface to disable c-states (I wish I had learned this before writing the initial patch)? The good thing is it can do automatic cleanup when it closes the fd. The reason why I still think of disabling c-states is, even in the low c-states exit latency setup, it can still increase the chances of successful migration. Otherwise, I can implement a command line option to skip the sanity check, as I was not able to find out a metrics/stats that is easy and reliable to indicate that the scheduler is not able to wake up the target CPU before the task is scheduled to another CPU.