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 X-Spam-Level: X-Spam-Status: No, score=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 840B3C433FE for ; Fri, 3 Sep 2021 08:48:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6865860FDA for ; Fri, 3 Sep 2021 08:48:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1348405AbhICItH (ORCPT ); Fri, 3 Sep 2021 04:49:07 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44988 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234817AbhICItD (ORCPT ); Fri, 3 Sep 2021 04:49:03 -0400 Received: from desiato.infradead.org (desiato.infradead.org [IPv6:2001:8b0:10b:1:d65d:64ff:fe57:4e05]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D2764C061575; Fri, 3 Sep 2021 01:48:03 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=rRuSYLlOgO1jvya073Bh7Tpm8VI1vmVBgnv4q9HH8uo=; b=jhTR6SBpkVS6jms5GAwXgbKG5s uuVmf8stn8TaZo6/Q6MKUq7TgbgUrZ/3h6EXaXbPsQnm/O9jog8R1vgjM1G7ObQmeeBXE/TXqZVeD EcufrQD5ZEumO7Gt9Lg3LvalHSHI/y6SqOg8eBYVfQ5jwFuyJNk96DlqJkFrODeE4sQiuKAk/lWUO QzZdwY2eEj2937mp2U5ro7N5vp/PePTEN6IWPBNff/fNALN1zvxw5iLBw5fISkic8LPddrohHaUHw iryjCnRDK1bCHs7HaUKnsugiuF+JxVShxX/ytyZJe5f+Jp8QXLCuxDW9utLIFNsrF9kKbL6tn7Fvu Mkurbs/Q==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1mM4rc-000JaS-0a; Fri, 03 Sep 2021 08:47:56 +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) server-digest SHA256) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 8CF6030024D; Fri, 3 Sep 2021 10:47:55 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 702B828B658E9; Fri, 3 Sep 2021 10:47:55 +0200 (CEST) Date: Fri, 3 Sep 2021 10:47:55 +0200 From: Peter Zijlstra To: Song Liu Cc: bpf@vger.kernel.org, linux-kernel@vger.kernel.org, acme@kernel.org, mingo@redhat.com, kjain@linux.ibm.com, kernel-team@fb.com Subject: Re: [PATCH v5 bpf-next 2/3] bpf: introduce helper bpf_get_branch_snapshot Message-ID: References: <20210902165706.2812867-1-songliubraving@fb.com> <20210902165706.2812867-3-songliubraving@fb.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210902165706.2812867-3-songliubraving@fb.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Sep 02, 2021 at 09:57:05AM -0700, Song Liu wrote: > +BPF_CALL_3(bpf_get_branch_snapshot, void *, buf, u32, size, u64, flags) > +{ > + static const u32 br_entry_size = sizeof(struct perf_branch_entry); > + u32 entry_cnt = size / br_entry_size; > + > + if (unlikely(flags)) > + return -EINVAL; > + > + if (!buf || (size % br_entry_size != 0)) > + return -EINVAL; > + > + entry_cnt = static_call(perf_snapshot_branch_stack)(buf, entry_cnt); That's at least 2, possibly 3 branches just from the sanity checks, plus at least one from starting the BPF prog and one from calling this function, gets you at ~5 branch entries gone before you even do the snapshot thing. Less if you're in branch-stack mode. Can't the validator help with getting rid of the some of that? I suppose you have to have this helper function because the JIT cannot emit static_call()... although in this case one could cheat and simply emit a call to static_call_query() and not bother with dynamic updates (because there aren't any).