From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) (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 0256534E743; Tue, 21 Oct 2025 17:19:06 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.50.34 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761067148; cv=none; b=ltXiFKW1iJ+Itk54v1eIGtYsVEq16c32x7w87ddfQBbEOvNT0iIJT+EgFHikzoBXaqBTLveyZDnAryAeQ3Elhv/+N/Orl3A4aL2NhKp8rr7hT6GLGdkwTIuO5AVaJWMLzzD0YKXeh6hdv7DFzdOvJwo5A8qI3l4g4BfWSQv22pk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761067148; c=relaxed/simple; bh=2YDkoOJDBp9Nbjj2vW2HkcQGmRFOiFoNvp3AjR0IkaY=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=hFlvlsh6t+7tVDMO+zy6NlRA9fIU1W93V1ospTrQyOGs+hJ7DMJTe9vTbPQoLwxgqCQpAcE1Jpm/J7CyW+OiZMEveRZiZXEsJysMuXSbNNACZpc62tX6kLLSPkCfmvMa3WC9PPE+DR8DSmBhECNM9dWMf/15P2fCGascyue0oEE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=OjISmd9r; arc=none smtp.client-ip=90.155.50.34 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="OjISmd9r" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; 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=gHFywIhXem6Wo6k3BOTsD13pvTGelkbwQt/Im/rNtDY=; b=OjISmd9r25aLSPCGLiW3hqoolQ JBoz3xniO5N9k4GGhDuHiY6HF/Q5HZmpdcJLKJ/QyoLs+rq/OMEjwdAI1B/aX6IpFXdVcsXupvQx1 OOygZtWMsE4EkPbI6ncy14zLzJ0xfd7ty7crnY8ZRjSvv3+Qecbo1RN3FaD4AxKypsv29ke2KwIOS qpGtgEBL5EUVgfsivtaFYZfTCWnvWaOVJL6d+hODrScePjweqcVIY/We4OInxr+AejkZZSpzP36N+ mA8k+D6fOdLyZspaXYwPwK5r4GAS4/or+2mNAvCKqmdEH/1WOpo/gQIOvsi/l4I1f2QNsr0cSK34g 650ac66g==; Received: from 77-249-17-252.cable.dynamic.v4.ziggo.nl ([77.249.17.252] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1vBG0c-0000000DsWZ-1B8U; Tue, 21 Oct 2025 17:18:55 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id 89D023030D0; Mon, 20 Oct 2025 13:03:03 +0200 (CEST) Date: Mon, 20 Oct 2025 13:03:03 +0200 From: Peter Zijlstra To: Tao Chen Cc: mingo@redhat.com, acme@kernel.org, namhyung@kernel.org, mark.rutland@arm.com, alexander.shishkin@linux.intel.com, jolsa@kernel.org, irogers@google.com, adrian.hunter@intel.com, kan.liang@linux.intel.com, song@kernel.org, ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org, martin.lau@linux.dev, eddyz87@gmail.com, yonghong.song@linux.dev, john.fastabend@gmail.com, kpsingh@kernel.org, sdf@fomichev.me, haoluo@google.com, linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org, bpf@vger.kernel.org Subject: Re: [PATCH bpf-next v3 2/2] bpf: Use per-cpu BPF callchain entry to save callchain Message-ID: <20251020110303.GS3419281@noisy.programming.kicks-ass.net> References: <20251019170118.2955346-1-chen.dylane@linux.dev> <20251019170118.2955346-3-chen.dylane@linux.dev> 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: <20251019170118.2955346-3-chen.dylane@linux.dev> On Mon, Oct 20, 2025 at 01:01:18AM +0800, Tao Chen wrote: > As Alexei noted, get_perf_callchain() return values may be reused > if a task is preempted after the BPF program enters migrate disable > mode. Drawing on the per-cpu design of bpf_bprintf_buffers, > per-cpu BPF callchain entry is used here. And now you can only unwind 3 tasks, and then start failing. This is acceptable, why? > - if (may_fault) > - rcu_read_lock(); /* need RCU for perf's callchain below */ > - I know you propose to remove this code; but how was that correct? The perf callchain code hard relies on non-preemptible context, RCU does not imply such a thing. > if (trace_in) > trace = trace_in; > - else if (kernel && task) > trace = get_callchain_entry_for_task(task, max_depth); > - else > - trace = get_perf_callchain(regs, NULL, kernel, user, max_depth, > - crosstask, false);