From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by sourceware.org (Postfix) with ESMTPS id 5DFA83858C83 for ; Wed, 11 Jan 2023 11:26:17 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.2 sourceware.org 5DFA83858C83 Authentication-Results: sourceware.org; dmarc=pass (p=none dis=none) header.from=redhat.com Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=redhat.com DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1673436376; 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; bh=P26DOOQFzqNeAWOv5m+Y0df/LIVzCBSEjc0esr8v7WM=; b=BiP01qZzk4FrRcQBiSLn6kES1aj1r4qRjR5b1SIsKzzMAjWRGLgvm8uskDS5Z5erbwW7QF Zs95WU/NdJZWVhYs6NDa2Fxe90h5bg6/KSbYpixSj+2vJufETDl+aZWfOLDHZvQZNgzLTT erb9n3fngzECHM/CdTaQwJ1ls8zrZIg= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-495-9j7wO3_EPBmW4mvMbHkoBg-1; Wed, 11 Jan 2023 06:26:11 -0500 X-MC-Unique: 9j7wO3_EPBmW4mvMbHkoBg-1 Received: from smtp.corp.redhat.com (int-mx09.intmail.prod.int.rdu2.redhat.com [10.11.54.9]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 07D08858F09; Wed, 11 Jan 2023 11:26:11 +0000 (UTC) Received: from oldenburg.str.redhat.com (unknown [10.2.16.17]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 3A058492C14; Wed, 11 Jan 2023 11:26:10 +0000 (UTC) From: Florian Weimer To: Mathieu Desnoyers , linux-kernel@vger.kernel.org Cc: Ingo Molnar , Peter Zijlstra , libc-alpha@sourceware.org Subject: rseq CPU ID not correct on 6.0 kernels for pinned threads Date: Wed, 11 Jan 2023 12:26:08 +0100 Message-ID: <87lem9cnxr.fsf@oldenburg.str.redhat.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/28.2 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 3.1 on 10.11.54.9 X-Spam-Status: No, score=-4.6 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_NONE,RCVD_IN_MSPIKE_H2,SPF_HELO_NONE,SPF_NONE,TXREP autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: The glibc test suite contains a test that verifies that sched_getcpu returns the expected CPU number for a thread that is pinned (via sched_setaffinity) to a specific CPU. There are other threads running which attempt to de-schedule the pinned thread from its CPU. I believe the test is correctly doing what it is expected to do; it is invalid only if one believes that it is okay for the kernel to disregard the affinity mask for scheduling decisions. These days, we use the cpu_id rseq field as the return value of sched_getcpu if the kernel has rseq support (which it has in these cases). This test has started failing sporadically for us, some time around kernel 6.0. I see failure occasionally on a Fedora builder, it runs: Linux buildvm-x86-26.iad2.fedoraproject.org 6.0.15-300.fc37.x86_64 #1 SMP PREEMPT_DYNAMIC Wed Dec 21 18:33:23 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux I think I've seen it on the x86-64 builder only, but that might just be an accident. The failing tests log this output: =====FAIL: nptl/tst-thread-affinity-pthread.out===== info: Detected CPU set size (in bits): 64 info: Maximum test CPU: 5 error: Pinned thread 1 ran on impossible cpu 0 error: Pinned thread 0 ran on impossible cpu 0 info: Main thread ran on 4 CPU(s) of 6 available CPU(s) info: Other threads ran on 6 CPU(s) =====FAIL: nptl/tst-thread-affinity-pthread2.out===== info: Detected CPU set size (in bits): 64 info: Maximum test CPU: 5 error: Pinned thread 1 ran on impossible cpu 1 error: Pinned thread 2 ran on impossible cpu 0 error: Pinned thread 3 ran on impossible cpu 3 info: Main thread ran on 5 CPU(s) of 6 available CPU(s) info: Other threads ran on 6 CPU(s) But I also encountered one local failure, but it is rare. Maybe it's load-related. There shouldn't be any CPU unplug or anything like that involved here. I am not entirely sure if something is changing CPU affinities from outside the process (which would be quite wrong, but not a kernel bug). But in the past, our glibc test has detected real rseq cpu_id brokenness, so I'm leaning towards that as the cause this time, too. Thanks, Florian