From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx206.postini.com [74.125.245.206]) by kanga.kvack.org (Postfix) with SMTP id A17A96B0070 for ; Thu, 20 Dec 2012 15:27:48 -0500 (EST) Date: Thu, 20 Dec 2012 12:27:46 -0800 From: Andrew Morton Subject: Re: [PATCH v2] mm: limit mmu_gather batching to fix soft lockups on !CONFIG_PREEMPT Message-Id: <20121220122746.72d889fd.akpm@linux-foundation.org> In-Reply-To: <20121220124710.GA31912@dhcp22.suse.cz> References: <1355847088-1207-1-git-send-email-mhocko@suse.cz> <20121218140219.45867ddd.akpm@linux-foundation.org> <20121218235042.GA10350@dhcp22.suse.cz> <20121218160030.baf723aa.akpm@linux-foundation.org> <20121219150423.GA12888@dhcp22.suse.cz> <20121219131316.7d13fcb1.akpm@linux-foundation.org> <20121220124710.GA31912@dhcp22.suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Mel Gorman , Rik van Riel , Peter Zijlstra On Thu, 20 Dec 2012 13:47:10 +0100 Michal Hocko wrote: > > > + */ > > > +#if defined(CONFIG_PREEMPT_COUNT) > > > +#define MAX_GATHER_BATCH_COUNT (UINT_MAX) > > > +#else > > > +#define MAX_GATHER_BATCH_COUNT (((1UL<<(30-PAGE_SHIFT))/MAX_GATHER_BATCH)) > > > > Geeze. I spent waaaaay too long staring at that expression trying to > > work out "how many pages is in a batch" and gave up. > > > > Realistically, I don't think we need to worry about CONFIG_PREEMPT here > > - if we just limit the thing to, say, 64k pages per batch then that > > will be OK for preemptible and non-preemptible kernels. > > I wanted the fix to be as non-intrusive as possible so I didn't want to > touch PREEMPT (which is default in many configs) at all. I am OK to a > single limit of course. non-intrusive is nice, but best-implementation is nicer. > > The performance difference between "64k" and "infinite" will be > > miniscule and unmeasurable. > > > > Also, the batch count should be independent of PAGE_SIZE. Because > > PAGE_SIZE can vary by a factor of 16 and you don't want to fix the > > problem on 4k page size but leave it broken on 64k page size. > > MAX_GATHER_BATCH depends on the page size so I didn't want to differ > without a good reason. There's a good reason! PAGE_SIZE can vary by a factor of 16, and if this results in the unpreemptible-CPU-effort varying by a factor of 16 then that's bad, and we should change things so the unpreemptible-CPU-effort is independent of PAGE_SIZE. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org