From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx116.postini.com [74.125.245.116]) by kanga.kvack.org (Postfix) with SMTP id F10356B005D for ; Tue, 18 Dec 2012 19:00:31 -0500 (EST) Date: Tue, 18 Dec 2012 16:00:30 -0800 From: Andrew Morton Subject: Re: [PATCH] mm: cond_resched in tlb_flush_mmu to fix soft lockups on !CONFIG_PREEMPT Message-Id: <20121218160030.baf723aa.akpm@linux-foundation.org> In-Reply-To: <20121218235042.GA10350@dhcp22.suse.cz> References: <1355847088-1207-1-git-send-email-mhocko@suse.cz> <20121218140219.45867ddd.akpm@linux-foundation.org> <20121218235042.GA10350@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 Wed, 19 Dec 2012 00:50:42 +0100 Michal Hocko wrote: > On Tue 18-12-12 14:02:19, Andrew Morton wrote: > > On Tue, 18 Dec 2012 17:11:28 +0100 > > Michal Hocko wrote: > > > > > Since e303297 (mm: extended batches for generic mmu_gather) we are batching > > > pages to be freed until either tlb_next_batch cannot allocate a new batch or we > > > are done. > > > > > > This works just fine most of the time but we can get in troubles with > > > non-preemptible kernel (CONFIG_PREEMPT_NONE or CONFIG_PREEMPT_VOLUNTARY) on > > > large machines where too aggressive batching might lead to soft lockups during > > > process exit path (exit_mmap) because there are no scheduling points down the > > > free_pages_and_swap_cache path and so the freeing can take long enough to > > > trigger the soft lockup. > > > > > > The lockup is harmless except when the system is setup to panic on > > > softlockup which is not that unusual. > > > > > > The simplest way to work around this issue is to explicitly cond_resched per > > > batch in tlb_flush_mmu (1020 pages on x86_64). > > > > > > ... > > > > > > --- a/mm/memory.c > > > +++ b/mm/memory.c > > > @@ -239,6 +239,7 @@ void tlb_flush_mmu(struct mmu_gather *tlb) > > > for (batch = &tlb->local; batch; batch = batch->next) { > > > free_pages_and_swap_cache(batch->pages, batch->nr); > > > batch->nr = 0; > > > + cond_resched(); > > > } > > > tlb->active = &tlb->local; > > > } > > > > tlb_flush_mmu() has a large number of callsites (or callsites which > > call callers, etc), many in arch code. It's not at all obvious that > > tlb_flush_mmu() is never called from under spinlock? > > free_pages_and_swap_cache calls lru_add_drain which in turn calls > put_cpu (aka preempt_enable) which is a scheduling point for > CONFIG_PREEMPT. No, that inference doesn't work. Because preempt_enable() inside spinlock is OK - it will not call schedule() because current->preempt_count is still elevated (by spin_lock). > There are more down the call chain probably. None of > them for non-preempt kernel. -- 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