From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752571AbeB1Mk6 (ORCPT ); Wed, 28 Feb 2018 07:40:58 -0500 Received: from mail-lf0-f67.google.com ([209.85.215.67]:40648 "EHLO mail-lf0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752339AbeB1Mk5 (ORCPT ); Wed, 28 Feb 2018 07:40:57 -0500 X-Google-Smtp-Source: AG47ELsZzk/67/qHvhViJ+CqcCxRVXAUXui4DK5C0aGkCCF1irAqfsGlgcDDoOS1XxQ8fuVd8e4Eaw== From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Wed, 28 Feb 2018 13:40:47 +0100 To: Matthew Wilcox Cc: "Uladzislau Rezki (Sony)" , linux-mm@kvack.org, LKML , Ingo Molnar , Thomas Garnier , Oleksiy Avramchenko , Andrew Morton , "Kirill A . Shutemov" , Steven Rostedt , Thomas Gleixner Subject: Re: [RFC v1] mm: add the preempt check into alloc_vmap_area() Message-ID: <20180228124047.cnrcaqkvlngsz6ln@pc636> References: <20180227102259.4629-1-urezki@gmail.com> <20180227130643.GA12781@bombadil.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180227130643.GA12781@bombadil.infradead.org> User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Feb 27, 2018 at 05:06:43AM -0800, Matthew Wilcox wrote: > On Tue, Feb 27, 2018 at 11:22:59AM +0100, Uladzislau Rezki (Sony) wrote: > > During finding a suitable hole in the vmap_area_list > > there is an explicit rescheduling check for latency reduction. > > We do it, since there are workloads which are sensitive for > > long (more than 1 millisecond) preemption off scenario. > > I understand your problem, but this is a horrid solution. If it takes > us a millisecond to find a suitable chunk of free address space, something > is terribly wrong. On a 3GHz CPU, that's 3 million clock ticks! > Some background. I spent some time analyzing an issue regarding audio drops/glitches during playing hires audio on our mobile device. It is ARM A53 with 4 CPUs on one socket. When it comes to frequency and test case, the system is most likely idle and operation is done on ~576 MHz. I found out that the reason was in vmalloc due to it can take time to find a suitable chunk of memory and it is done in non-preemptible context. As a result the other audio thread is not run on CPU in time despite need_resched is set. > > I think our real problem is that we have no data structure that stores > free VA space. We have the vmap_area which stores allocated space, but no > data structure to store free space. > > My initial proposal would be to reuse the vmap_area structure and store > the freed ones in a second rb_tree sorted by the size (ie va_end - va_start). > When freeing, we might need to merge forwards and backwards. Allocating > would be a matter of finding an area preferably of the exact right size; > otherwise split a larger free area into a free area and an allocated area > (there's a lot of literature on how exactly to choose which larger area > to split; memory allocators are pretty well-studied). > Thank you for your comments and proposal. -- Vlad Rezki