From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from desiato.infradead.org (desiato.infradead.org [90.155.92.199]) (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 B4BE919342B for ; Thu, 28 Nov 2024 11:27:42 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.92.199 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1732793264; cv=none; b=TxG2A6aNilu4rV9ZgxfyVxXHuADLggipnsfGSFRv2CfX9BwQDZXxbUGZsqHRJk4uumvINfUZ017tnEaMa2yPGMSB0z/PPFJuXK0JIKF+8xnj3Oid2auSE19PHaE7PpcTvQXtKgvCsxdA16uGzNkRjYMeONeJ2pvApeAMKX/qUpA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1732793264; c=relaxed/simple; bh=5Z07kQ6wkZWetlXEqnqulftyq0S5ZK2SEoUbmbHknNo=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=uuuMoil4vcp9jne0lztNBecyZuRLtgYnL8I6PpnXMJ1+c+PcdboTsiI3y8o1QKZqKpAWD6CPnAwnEPAu+sBP16I9p8CDrfFyMagXpXbmoKOLWiNNuqKIY4SILGjrUs5s/Vd9Mi0t8vbwTHZVxs9UFZSzRUjLYBxju59o+GPFayg= 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=JujoEGOP; arc=none smtp.client-ip=90.155.92.199 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="JujoEGOP" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; 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=IOWkx6jQ8cU1Dmq+lOjLLbuN/90D9kA2iRxkCCGaP9k=; b=JujoEGOPrg4hZWPxXFc5b5WoYV NerZG0PvOpgjlFmxkiAuJjoCy1xI3wsf9uKFjcOPwarP0JqhMWqlicJaF8fY6j83iUus8HpDaTFGm I1gpApTsKOWVLLx4kdrGflHmx9uArwOeMqXmSSH+YsYPcbc/kl7lU0lWDlXNE1bur898yT5ubgkI/ vh/qNVl5bZPtv/leiH/y5XCaM/6pZqCUKYfNwKaVw74Te0KJXlAvhYqJ6NTNL18qqa5a3g4rghLVI 5+jEQXf1yU+48GDcgtgjnFgKi+PUbEmhk+mavTpNYMg/u5GJBrFErofo72JDbGv2sv8Ks2WXHHI44 RMpaSNoQ==; Received: from 77-249-17-89.cable.dynamic.v4.ziggo.nl ([77.249.17.89] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.98 #2 (Red Hat Linux)) id 1tGcgI-00000001aqD-2Wnf; Thu, 28 Nov 2024 11:27:34 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id 303C5300271; Thu, 28 Nov 2024 12:27:34 +0100 (CET) Date: Thu, 28 Nov 2024 12:27:34 +0100 From: Peter Zijlstra To: Andrii Nakryiko Cc: Vadim Fedorenko , Borislav Petkov , Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Eduard Zingerman , Thomas Gleixner , Yonghong Song , Vadim Fedorenko , Mykola Lysenko , x86@kernel.org, bpf@vger.kernel.org, Martin KaFai Lau Subject: Re: [PATCH bpf-next v8 0/4] bpf: add cpu cycles kfuncss Message-ID: <20241128112734.GD35539@noisy.programming.kicks-ass.net> References: <20241121000814.3821326-1-vadfed@meta.com> <20241122113409.GV24774@noisy.programming.kicks-ass.net> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Tue, Nov 26, 2024 at 10:12:57AM -0800, Andrii Nakryiko wrote: > On Fri, Nov 22, 2024 at 3:34 AM Peter Zijlstra wrote: > > > > On Wed, Nov 20, 2024 at 04:08:10PM -0800, Vadim Fedorenko wrote: > > > This patchset adds 2 kfuncs to provide a way to precisely measure the > > > time spent running some code. The first patch provides a way to get cpu > > > cycles counter which is used to feed CLOCK_MONOTONIC_RAW. On x86 > > > architecture it is effectively rdtsc_ordered() function while on other > > > architectures it falls back to __arch_get_hw_counter(). The second patch > > > adds a kfunc to convert cpu cycles to nanoseconds using shift/mult > > > constants discovered by kernel. The main use-case for this kfunc is to > > > convert deltas of timestamp counter values into nanoseconds. It is not > > > supposed to get CLOCK_MONOTONIC_RAW values as offset part is skipped. > > > JIT version is done for x86 for now, on other architectures it falls > > > back to slightly simplified version of vdso_calc_ns. > > > > So having now read this. I'm still left wondering why you would want to > > do this. > > > > Is this just debug stuff, for when you're doing a poor man's profile > > run? If it is, why do we care about all the precision or the ns. And why > > aren't you using perf? > > No, it's not debug stuff. It's meant to be used in production for > measuring durations of whatever is needed. Like uprobe entry/exit > duration, or time between scheduling switches, etc. > > Vadim emphasizes benchmarking at scale, but that's a bit misleading. > It's not "benchmarking", it's measuring durations of relevant pairs of > events. In production and at scale, so the unnecessary overhead all > adds up. We'd like to have the minimal possible overhead for this time > passage measurement. And some durations are very brief, You might want to consider leaving out the LFENCE before the RDTSC on some of those, LFENCE isn't exactly cheap. > so precision > matters as well. And given this is meant to be later used to do > aggregation and comparison across large swaths of production hosts, we > have to have comparable units, which is why nanoseconds and not some > abstract "time cycles". > > Does this address your concerns? Well, it's clearly useful for you guys, but I do worry about it. Even on servers DVFS is starting to play a significant role. And the TSC is unaffected by it. Directly comparing these numbers, esp. across different systems makes no sense to me. Yes putting them all in [ns] allows for comparison, but you're still comparing fundamentally different things. How does it make sense to measure uprobe entry/exit in wall-clock when it can vary by at least a factor of 2 depending on DVFS. How does it make sense to compare an x86-64 uprobe entry/exit to an aaargh64 one? Or are you trying to estimate the fraction of overhead spend on instrumentation instead of real work? Like, this machine spends 5% of its wall-time in instrumentation, which is effectively not doing work? The part I'm missing is how using wall-time for these things makes sense. I mean, if all you're doing is saying, hey, we appear to be spending X on this action on this particular system Y doing workload Z (irrespecive of you then having like a million Ys) and this patch reduces X by half given the same Y and Z. So patch must be awesome. Then you don't need the conversion to [ns], and the DVFS angle is more or less mitigated by the whole 'same workload' thing.