From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755291AbeCSG2a (ORCPT ); Mon, 19 Mar 2018 02:28:30 -0400 Received: from forwardcorp1o.cmail.yandex.net ([37.9.109.47]:43679 "EHLO forwardcorp1o.cmail.yandex.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755200AbeCSG22 (ORCPT ); Mon, 19 Mar 2018 02:28:28 -0400 Authentication-Results: smtpcorp1o.mail.yandex.net; dkim=pass header.i=@yandex-team.ru Subject: Re: [PATCH] block, bfq: keep peak_rate estimation within range 1..2^32-1 To: Paolo Valente Cc: linux-block , Jens Axboe , Linux Kernel Mailing List , "'Paolo Valente' via bfq-iosched" References: <152025413365.161046.7757556276366364549.stgit@buzz> From: Konstantin Khlebnikov Message-ID: Date: Mon, 19 Mar 2018 09:28:25 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 19.03.2018 09:03, Paolo Valente wrote: > > >> Il giorno 05 mar 2018, alle ore 04:48, Konstantin Khlebnikov ha scritto: >> >> Rate should never overflow or become zero because it is used as divider. >> This patch accumulates it with saturation. >> >> Signed-off-by: Konstantin Khlebnikov >> --- >> block/bfq-iosched.c | 8 +++++--- >> 1 file changed, 5 insertions(+), 3 deletions(-) >> >> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c >> index aeca22d91101..a236c8d541b5 100644 >> --- a/block/bfq-iosched.c >> +++ b/block/bfq-iosched.c >> @@ -2546,7 +2546,8 @@ static void bfq_reset_rate_computation(struct bfq_data *bfqd, >> >> static void bfq_update_rate_reset(struct bfq_data *bfqd, struct request *rq) >> { >> - u32 rate, weight, divisor; >> + u32 weight, divisor; >> + u64 rate; >> >> /* >> * For the convergence property to hold (see comments on >> @@ -2634,9 +2635,10 @@ static void bfq_update_rate_reset(struct bfq_data *bfqd, struct request *rq) >> */ >> bfqd->peak_rate *= divisor-1; >> bfqd->peak_rate /= divisor; >> - rate /= divisor; /* smoothing constant alpha = 1/divisor */ >> + do_div(rate, divisor); /* smoothing constant alpha = 1/divisor */ >> >> - bfqd->peak_rate += rate; >> + /* rate should never overlow or become zero */ > > It is bfqd->peak_rate that is used as a divider, and bfqd->peak_rate doesn't risk to be zero even if the variable 'rate' is zero here. > > So I guess the reason why you consider the possibility that bfqd->peak_rate becomes zero is because of an overflow when summing 'rate'. But, according to my calculations, this should be impossible with devices with sensible speeds. > > These are the reasons why I decided I could make it with a 32-bit variable, without any additional clamping. Did I make any mistake in my evaluation? According to Murphy's law this is inevitable.. I've seen couple division by zero crashes in bfq_wr_duration. Unfortunately logs weren't recorded. > > Anyway, even if I made some mistake about the maximum possible value of the device rate, and the latter may be too high for bfqd->peak_rate to contain it, then I guess the right solution would not be to clamp the actual rate to U32_MAX, but to move bfqd->peak_rate to 64 bits. Or am I missing something else? >>> + bfqd->peak_rate = clamp_t(u64, rate + bfqd->peak_rate, 1, U32_MAX); 32-bit should be enough and better for division. My patch makes sure it never overflows/underflows. That's cheaper than full 64-bit/64-bit division. Anyway 64-bit speed could overflow too. =) >> update_thr_responsiveness_params(bfqd); >> >> reset_computation: >> >