From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 68C1DC433EF for ; Sat, 6 Nov 2021 00:47:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 29B2D61139 for ; Sat, 6 Nov 2021 00:47:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231553AbhKFAuV (ORCPT ); Fri, 5 Nov 2021 20:50:21 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52966 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229504AbhKFAuU (ORCPT ); Fri, 5 Nov 2021 20:50:20 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 95BB0C061570; Fri, 5 Nov 2021 17:47:40 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Transfer-Encoding: Content-Type:MIME-Version:References:Message-ID:Subject:Cc:To:From:Date: Sender:Reply-To:Content-ID:Content-Description; bh=dYeKck/iXvULKvcfsrGrG7rdH6xeTDD5bHWpXPPzQV4=; b=NQ8SeqqGoxXShKBofpd+svv9d7 dLDGbHxmiQSNCbA4OYwXPUg7Aqj1vqmdECPQkwH9FWDRLYHbPLv6BYpjXfSJCazbzH6xPFKpyZhSU 64hrX5YFaU7CAzjqBkNZoeV9f+iQMDyuPZ6Liy2KQlArIR4Fsd17Hm9EG+YBHjwoiLdXVEJUe11i6 shCOoNWDUpNDB0ZXx680UkmsHSaG32aZS7OI7ZnbEd83GJBBbGZSy1NDJRFUKkuSLJZmYpl03+5a9 gHTzIV7C9m7cTH8NpshyUIbn6p6AfqcOTASh0j/9Xs0iXvXfdPyNhsXmh5MIFgMo0zvGu9pmkD7z7 i/dDMXVA==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1mj9qI-006xhR-86; Sat, 06 Nov 2021 00:46:39 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 97E6330031A; Sat, 6 Nov 2021 01:45:57 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 7E2DC2C8F4FD4; Sat, 6 Nov 2021 01:45:57 +0100 (CET) Date: Sat, 6 Nov 2021 01:45:57 +0100 From: Peter Zijlstra To: Nadav Amit Cc: kan.liang@linux.intel.com, LKML , linux-perf-users@vger.kernel.org Subject: Re: Using perf_event_open() to sample multiple events of a process Message-ID: References: <92645262-D319-4068-9C44-2409EF44888E@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <92645262-D319-4068-9C44-2409EF44888E@gmail.com> Precedence: bulk List-ID: X-Mailing-List: linux-perf-users@vger.kernel.org On Thu, Nov 04, 2021 at 10:57:50PM -0700, Nadav Amit wrote: > Hello Ken, Peter, > > I would appreciate some help regarding the use of perf_event_open() > to have multiple samples getting into the same mmap’d memory when they > are both attached to the same process. > > I am doing so (using both PERF_FLAG_FD_NO_GROUP and PERF_FLAG_FD_OUTPUT), > but it results in -EINVAL. Debugging the code shows that > perf_event_set_output() fails due to the following check: > > /* > * If its not a per-cpu rb, it must be the same task. > */ > if (output_event->cpu == -1 && output_event->ctx != event->ctx) > goto out; > > However, it appears that at this point, event->ctx is not initialized > (it is null) so the test fails and the whole perf_event_open() syscall > fails. > > Am I missing something? If not, I am unsure, unfortunately, what the > proper way to fix it is… > > I include a small test that fails on my system. The second > perf_event_open fails due to the check in perf_event_set_output(): > Works when you use the SET_OUTPUT ioctl()... I think something went sideways in the syscall path and things went out of order :/ I'll try and have a look. --- #define _GNU_SOURCE 1 #include #include #include #include #include #include #include #include long perf_event_open(struct perf_event_attr* event_attr, pid_t pid, int cpu, int group_fd, unsigned long flags) { return syscall(__NR_perf_event_open, event_attr, pid, cpu, group_fd, flags); } int main(void) { pid_t pid = getpid(); int group_fd, fd, err; void *p; struct perf_event_attr pe = { .type = 4, .size = sizeof(struct perf_event_attr), .config = 0x11d0, .sample_type = 0x8, .sample_period = 1000, .precise_ip = 2, }; group_fd = perf_event_open(&pe, pid, -1, -1, PERF_FLAG_FD_CLOEXEC | PERF_FLAG_FD_NO_GROUP | PERF_FLAG_FD_OUTPUT); if (group_fd < 0) { perror("first perf_event_open"); exit(-1); } p = mmap(NULL, 3 * 4096, PROT_READ|PROT_WRITE, MAP_SHARED, group_fd, 0); if (p == MAP_FAILED) { perror("MAP_FAILED"); exit(-1); } pe.config = 0x12d0; fd = perf_event_open(&pe, pid, -1, -1, PERF_FLAG_FD_CLOEXEC | PERF_FLAG_FD_NO_GROUP | PERF_FLAG_FD_OUTPUT); if (fd < 0) { perror("second perf_event_open"); exit(-1); } err = ioctl(fd, PERF_EVENT_IOC_SET_OUTPUT, group_fd); if (err < 0) { perror("ioctl"); exit(-1); } printf("success\n"); }