From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754937AbcIVBPD (ORCPT ); Wed, 21 Sep 2016 21:15:03 -0400 Received: from sender153-mail.zoho.com ([74.201.84.153]:25352 "EHLO sender153-mail.zoho.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754412AbcIVBPC (ORCPT ); Wed, 21 Sep 2016 21:15:02 -0400 DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws; s=zapps768; d=zoho.com; h=subject:to:references:cc:from:message-id:date:user-agent:mime-version:in-reply-to:content-type; b=NkkZLbkExG4GssAVnVpon+q5Kpd8JrrdzgMtCUibEg5d1a1wRX0FOsc3GWYnU0iAxEJSwMzUb9m/ 5n8mX4gvabA5tZ5IGFlgtjm3LwAW2zT/trBdym/LLFuVkUUKQOit Subject: Re: [PATCH 3/5] mm/vmalloc.c: correct lazy_max_pages() return value To: David Rientjes References: <57E20C49.8010304@zoho.com> <3ef46c24-769d-701a-938b-826f4249bf0b@zoho.com> Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, zijun_hu@htc.com, Andrew Morton , tj@kernel.org, mingo@kernel.org, iamjoonsoo.kim@lge.com, mgorman@techsingularity.net From: zijun_hu Message-ID: <57E3304E.4060401@zoho.com> Date: Thu, 22 Sep 2016 09:13:50 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09/22/2016 08:35 AM, David Rientjes wrote: > On Thu, 22 Sep 2016, zijun_hu wrote: > >> On 2016/9/22 5:21, David Rientjes wrote: >>> On Wed, 21 Sep 2016, zijun_hu wrote: >>> >>>> From: zijun_hu >>>> >>>> correct lazy_max_pages() return value if the number of online >>>> CPUs is power of 2 >>>> >>>> Signed-off-by: zijun_hu >>>> --- >>>> mm/vmalloc.c | 4 +++- >>>> 1 file changed, 3 insertions(+), 1 deletion(-) >>>> >>>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c >>>> index a125ae8..2804224 100644 >>>> --- a/mm/vmalloc.c >>>> +++ b/mm/vmalloc.c >>>> @@ -594,7 +594,9 @@ static unsigned long lazy_max_pages(void) >>>> { >>>> unsigned int log; >>>> >>>> - log = fls(num_online_cpus()); >>>> + log = num_online_cpus(); >>>> + if (log > 1) >>>> + log = (unsigned int)get_count_order(log); >>>> >>>> return log * (32UL * 1024 * 1024 / PAGE_SIZE); >>>> } >>> >>> The implementation of lazy_max_pages() is somewhat arbitrarily defined, >>> the existing approximation has been around for eight years and >>> num_online_cpus() isn't intended to be rounded up to the next power of 2. >>> I'd be inclined to just leave it as it is. >>> >> do i understand the intent in current code logic as below ? >> [8, 15) roundup to 16? >> [32, 63) roundup to 64? >> > > The intent is as it is implemented; with your change, lazy_max_pages() is > potentially increased depending on the number of online cpus. This is > only a heuristic, changing it would need justification on why the new > value is better. It is opposite to what the comment says: "to be > conservative and not introduce a big latency on huge systems, so go with > a less aggressive log scale." NACK to the patch. > my change potentially make lazy_max_pages() decreased not increased, i seems conform with the comment if the number of online CPUs is not power of 2, both have no any difference otherwise, my change remain power of 2 value, and the original code rounds up to next power of 2 value, for instance my change : (32, 64] -> 64 32 -> 32, 64 -> 64 the original code: [32, 63) -> 64 32 -> 64, 64 -> 128