From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753968Ab2DQOVo (ORCPT ); Tue, 17 Apr 2012 10:21:44 -0400 Received: from smtp.ctxuk.citrix.com ([62.200.22.115]:39110 "EHLO SMTP.EU.CITRIX.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751319Ab2DQOVn (ORCPT ); Tue, 17 Apr 2012 10:21:43 -0400 X-IronPort-AV: E=Sophos;i="4.75,435,1330905600"; d="scan'208";a="11980571" Date: Tue, 17 Apr 2012 15:26:05 +0100 From: Stefano Stabellini X-X-Sender: sstabellini@kaball-desktop To: Konrad Rzeszutek Wilk CC: Stefano Stabellini , "xen-devel@lists.xensource.com" , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH v3 1/2] xen: enter/exit lazy_mmu_mode around m2p_override calls In-Reply-To: <20120417130340.GA25593@phenom.dumpdata.com> Message-ID: References: <1334075375-25442-1-git-send-email-stefano.stabellini@eu.citrix.com> <20120417130340.GA25593@phenom.dumpdata.com> User-Agent: Alpine 2.00 (DEB 1167 2008-08-23) MIME-Version: 1.0 Content-Type: text/plain; charset="US-ASCII" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 17 Apr 2012, Konrad Rzeszutek Wilk wrote: > On Tue, Apr 10, 2012 at 05:29:34PM +0100, Stefano Stabellini wrote: > > This patch is a significant performance improvement for the > > m2p_override: about 6% using the gntdev device. > > > > Each m2p_add/remove_override call issues a MULTI_grant_table_op and a > > __flush_tlb_single if kmap_op != NULL. Batching all the calls together > > is a great performance benefit because it means issuing one hypercall > > total rather than two hypercall per page. > > If paravirt_lazy_mode is set PARAVIRT_LAZY_MMU, all these calls are > > going to be batched together, otherwise they are issued one at a time. > > > > Adding arch_enter_lazy_mmu_mode/arch_leave_lazy_mmu_mode around the > > m2p_add/remove_override calls forces paravirt_lazy_mode to > > PARAVIRT_LAZY_MMU, therefore makes sure that they are always batched. > > > > > > Changes in v3: > > - do not call arch_enter/leave_lazy_mmu_mode in xen_blkbk_unmap, that > > can be called in interrupt context. > > This is with RHEL5 (somehow the pvops kernels don't trigger this): Do you mean RHEL5 as a guest? Are you using blkback or qdisk? How can I repro the bug? In any case it looks like a similar kind of issue to the one I fixed before: paravirt_enter_lazy_mmu is probably called with paravirt_lazy_mode == PARAVIRT_LAZY_MMU, in this case by gnttab_unmap_refs. But I don't understand how gnttab_unmap_refs can be called when you seem to be using blkback. Anyhow gnttab_unmap_refs can be called by: - mmu_notifier on invalidate_range_start; - vm_operations_struct on close; - file_operations on release; - the unmap gntdev ioctl. I guess the mmu_notifier or vm_operations_struct could be the ones calling gnttab_unmap_refs with lazy_mode set to PARAVIRT_LAZY_MMU. I suspect that something like the following code would fix the issue but it is pretty ugly and I need to test it before a submitting it as a fix: @@ -780,7 +780,7 @@ EXPORT_SYMBOL_GPL(gnttab_map_refs); int gnttab_unmap_refs(struct gnttab_unmap_grant_ref *unmap_ops, struct page **pages, unsigned int count, bool clear_pte) { - int i, ret; + int i, ret, lazy = 0; ret = HYPERVISOR_grant_table_op(GNTTABOP_unmap_grant_ref, unmap_ops, count); if (ret) @@ -789,7 +789,10 @@ int gnttab_unmap_refs(struct gnttab_unmap_grant_ref *unmap_ops, if (xen_feature(XENFEAT_auto_translated_physmap)) return ret; - arch_enter_lazy_mmu_mode(); + if (!in_interrupt() && paravirt_get_lazy_mode() == PARAVIRT_LAZY_NONE) { + arch_enter_lazy_mmu_mode(); + lazy = 1; + } for (i = 0; i < count; i++) { ret = m2p_remove_override(pages[i], clear_pte); @@ -797,7 +800,8 @@ int gnttab_unmap_refs(struct gnttab_unmap_grant_ref *unmap_ops, return ret; } - arch_leave_lazy_mmu_mode(); + if (lazy) + arch_leave_lazy_mmu_mode(); return ret; }