From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752033AbaJFQVP (ORCPT ); Mon, 6 Oct 2014 12:21:15 -0400 Received: from bombadil.infradead.org ([198.137.202.9]:42308 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750888AbaJFQVO (ORCPT ); Mon, 6 Oct 2014 12:21:14 -0400 Date: Mon, 6 Oct 2014 18:20:56 +0200 From: Peter Zijlstra To: Alexander Shishkin Cc: Ingo Molnar , linux-kernel@vger.kernel.org, Robert Richter , Frederic Weisbecker , Mike Galbraith , Paul Mackerras , Stephane Eranian , Andi Kleen , kan.liang@intel.com Subject: Re: [PATCH v4 02/22] perf: Add AUX area to ring buffer for raw data streams Message-ID: <20141006162056.GD6758@twins.programming.kicks-ass.net> References: <1408538179-792-1-git-send-email-alexander.shishkin@linux.intel.com> <1408538179-792-3-git-send-email-alexander.shishkin@linux.intel.com> <20140908070242.GS19379@twins.programming.kicks-ass.net> <87lhpuuztj.fsf@ashishki-desk.ger.corp.intel.com> <20140908113422.GD6758@twins.programming.kicks-ass.net> <87a96auv9s.fsf@ashishki-desk.ger.corp.intel.com> <20140908131230.GE6758@twins.programming.kicks-ass.net> <87k34do9a4.fsf@ashishki-desk.ger.corp.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87k34do9a4.fsf@ashishki-desk.ger.corp.intel.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Oct 06, 2014 at 12:08:19PM +0300, Alexander Shishkin wrote: > Peter Zijlstra writes: > > > On Mon, Sep 08, 2014 at 03:55:11PM +0300, Alexander Shishkin wrote: > > > >> Fair enough. Then I'd like to disable the ACTIVE ones before freeing AUX > >> stuff and then re-enabling them since perf_event_{en,dis}able() already > >> provide the convenient cross-cpu calls, which would also avoid > >> concurrency between pmu::{add,del} callbacks and this unmap path. Makes > >> sense? > > > > But why? The buffer stuff is RCU freed, so if the hardware observes > > pages and does get_page_unless_zero() on them its good. The memory will > > not be freed from underneath the hardware writer because of the > > get_page(). > > > > Then when the buffer is full and we 'swap', we'll find there is no next > > buffer. At that point we can not provide a new buffer, effectively > > stopping the hardware writes and release the old buffer, freeing the > > memory. > > There are several problems with this. Firstly, aux buffers can be quite > large, which means that we have to do get_page() on thousands of pages > on every pmu::add, which is a hot path and free_page() again in > pmu::del. > > Secondly, all the sg bookkeeping that the driver keeps in aux_priv needs > to be refcounted. Right now, in the mmap_close path we just free > everything. But if we want to free the aux_pages in pmu::del, we need to > keep a list of these pages still around after mmap_close() and same goes > for the actual sg tables. I can see a way of doing that on the ring > buffer side (as opposed to the driver side), but are you quite sure we > should go down this road? No, and I think I realized this after sending that email, but at the time I figured there was another way to do it. Of course, now that several weeks have passed I cannot for the life of me remember what it was. Lemme go over these patches again to refresh my mind and maybe I'll remember.