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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id A0CACC433EF for ; Tue, 23 Nov 2021 07:51:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234518AbhKWHyx (ORCPT ); Tue, 23 Nov 2021 02:54:53 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55250 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233578AbhKWHyu (ORCPT ); Tue, 23 Nov 2021 02:54:50 -0500 Received: from mail-qv1-xf36.google.com (mail-qv1-xf36.google.com [IPv6:2607:f8b0:4864:20::f36]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 37489C061574 for ; Mon, 22 Nov 2021 23:51:43 -0800 (PST) Received: by mail-qv1-xf36.google.com with SMTP id i13so14365517qvm.1 for ; Mon, 22 Nov 2021 23:51:43 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=yvOcIM2j3nZzor18Y5QvSHxXmcg0lQaJBQsgeAYhYDE=; b=X2vIXZM01hoVvANS/dBeyUsKI9ofdstJQdNo443dqZzYdV6jl2LA9Ppc/mLpZfM5oU s1D29/wb86e6HQvFrPIm0MHraXZ97tYRIP9ja5BKDYLb/KvTOnPKbZHa6K4yaW0o89kH jxdsKtKFu3wDWvcq1pVBOawiA6BTjp8SImw1JvTGcz017r3Gfk9b1CuLD6KpsRkn+w41 wVIBYom5kNzWwv0uArfnY76rfA1LPUfe3/W+sJpiVe/tAt7W2pVd2bURJ9XoqCmDXUvz 4AMRObZgzstJJ58jjWvJGPcYLJfWysqQ4jVik7ORgadxxrs9xniUSFz4TsWspZ7+VGHM HjSQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=yvOcIM2j3nZzor18Y5QvSHxXmcg0lQaJBQsgeAYhYDE=; b=cuIE1UPvqOpsdd7fXoYcFG6wbSueOT3xGZGiLxHV/RAUWRxZia2o3LK1y6iSPcNwZV 5GC9VSeuIC1gktufTfst020KiaIxlssXCqDOXtD4lIvhM/Nvcm+s+SQviDKRlqwTtlZm g7uCn9eL2jDd/O0SA0Z/G3FMXrGAU23xqh58Dx1saakr16GQVSQXUvHQyy9T4nfh1Q5A js88IfD48TTf4BokroyRVZN75UcyMeuV3ulc76f8mwP6Ss9epcp/9NhqjxlPWD2+2O7t HAlk19MSdN9u3d+clVccWGNe9qx1n7PvOYrKck8KY0MTW3zZ9+LX2xXHicITbqzH3Mpd xBvQ== X-Gm-Message-State: AOAM531C4FJbcu8c5GhHR5ihZ68TfIHY+0Idjm6Ghi9VNoTvHbyKxZJP tbNlfYiZ1je9fIkDl76UKZc= X-Google-Smtp-Source: ABdhPJxyOwgLpsaDhS4dDED1azy6c4FZd6kIJ75b49VhhDmLxGV1+dnnKDzTPAAftSsY4YCHvi7XfA== X-Received: by 2002:ad4:5f87:: with SMTP id jp7mr3883056qvb.28.1637653902273; Mon, 22 Nov 2021 23:51:42 -0800 (PST) Received: from localhost ([66.216.211.25]) by smtp.gmail.com with ESMTPSA id r26sm5442501qtm.67.2021.11.22.23.51.41 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 22 Nov 2021 23:51:41 -0800 (PST) Date: Mon, 22 Nov 2021 23:51:40 -0800 From: Yury Norov To: Andy Shevchenko Cc: "Vaittinen, Matti" , Matti Vaittinen , Liam Girdwood , Mark Brown , Jiri Kosina , Andrew Morton , Kumar Kartikeya Dwivedi , Rasmus Villemoes , Geert Uytterhoeven , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH 1/4] bitops: Add single_bit_set() Message-ID: <20211123075140.GB1628@lapt> References: <73d5e4286282a47b614d1cc5631eb9ff2a7e2b44.1637330431.git.matti.vaittinen@fi.rohmeurope.com> <2c22b52f-9a1f-06f5-f008-d568096f5c4d@fi.rohmeurope.com> <20211122175414.GA1588@lapt> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Nov 22, 2021 at 09:56:46PM +0200, Andy Shevchenko wrote: > On Mon, Nov 22, 2021 at 09:54:14AM -0800, Yury Norov wrote: > > On Mon, Nov 22, 2021 at 02:57:27PM +0200, Andy Shevchenko wrote: > > > On Mon, Nov 22, 2021 at 12:42:21PM +0000, Vaittinen, Matti wrote: > > > > On 11/22/21 13:28, Andy Shevchenko wrote: > > > > > On Mon, Nov 22, 2021 at 01:03:25PM +0200, Matti Vaittinen wrote: > > > > >> There are cases when it is useful to check a bit-mask has only one bit > > > > >> set. Add a generic helper for it instead of baking own one for each > > > > >> user. > > > > > > > > So, you decided to reinvent hamming weight... > > > > > Please, drop this patch and use corresponding hweight() call. > > > > > > > Thanks Andy. > > > > > > > > There are few differences to hamming weight here. We scan only given > > > > amount of bits - and we will end scanning immediately when we hit second > > > > set bit. Oh, and obviously we only return information whether there is > > > > exactly one bit set. So no, this is not hamming weight(). > > > > > > What do you mean by this? > > > > > > hweight() will return you the number of the non-zero elements in the set. > > > In application to boolean based arrays it means the number of bits that > > > are set. Obviously, the condition `hweight() == 1` is what you are looking > > > for. > > > > Hi Andy, > > > > I think, Matti means earlier return when part of bitmap counts set > > bits to a greater nubmer, and we can skip the rest. Right, Matti? > > > > I agree that for Matti's usecase it's useless because 32-bit int is small, > > and hweight() would count set bits with a single machine instruction. (And > > it should be hweight32(), not bitmap_weight() in this case.) > > > > But in general, it might be useful for long bitmaps. > > > > The more complete way of doing this would be adding a new set of > > functions: bitmap_weight_{eq,neq,gt,le} > > > > I'm looking at how bitmap_weight is used in the kernel and see > > quite a lot of places where this optimization may take place. For > > example otx2_remove_flow() in drivers/net/ethernet/marvell/octeontx2/nic/otx2_flows.c: > > > > if (bitmap_weight(&flow_cfg->dmacflt_bmap, > > flow_cfg->dmacflt_max_flows) == 1) > > otx2_update_rem_pfmac(pfvf, DMAC_ADDR_DEL); > > > > may be replaced with: > > > > if (bitmap_weight_eq(&flow_cfg->dmacflt_bmap, flow_cfg->dmacflt_max_flows, 1) > > otx2_update_rem_pfmac(pfvf, DMAC_ADDR_DEL); > > > > Most of that places are in drivers however, and the length of bitmaps > > there is typically small, so that there's no chance to get any > > measurable performance improvement. > > > > There is always a chance that we have opencoded bitmap_weight_eq() > > et all. If we add these API, it might help people wright better code. > > > > What do you think? > > Before answering this I would like to see how hweight() is currently being used > in the kernel against bitmaps. Like histogram collection hweight() is not used against bitmap. We have bitmap_weight() for it. > comparison number number of usages > variadic X > 1 Y > 2 Z > ... ... I don't think it would be helpful to build this histogram. For the proposed optimization it's (almost) not important what number is compared against bitmap_weight(). The important thing is that some callers in core code can save measurable amount of time if we switch them from bitmap_weight() == XXX to bitmap_weight_eq(..., XXX) Consider cpumask_weight and nodes_weight.