Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758468AbaLJXwM (ORCPT ); Wed, 10 Dec 2014 18:52:12 -0500 Received: from mail-lb0-f171.google.com ([209.85.217.171]:49926 "EHLO mail-lb0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758444AbaLJXwK (ORCPT ); Wed, 10 Dec 2014 18:52:10 -0500 MIME-Version: 1.0 In-Reply-To: <1418254487-9988-3-git-send-email-mcgrof@do-not-panic.com> References: <1418254487-9988-1-git-send-email-mcgrof@do-not-panic.com> <1418254487-9988-3-git-send-email-mcgrof@do-not-panic.com> From: Andy Lutomirski Date: Wed, 10 Dec 2014 15:51:48 -0800 Message-ID: Subject: Re: [PATCH v2 2/2] x86/xen: allow privcmd hypercalls to be preempted To: "Luis R. Rodriguez" Cc: Ingo Molnar , Peter Zijlstra , Thomas Gleixner , "H. Peter Anvin" , Konrad Rzeszutek Wilk , David Vrabel , Masami Hiramatsu , Steven Rostedt , Jan Beulich , Juergen Gross , bpoirier@suse.de, X86 ML , "xen-devel@lists.xenproject.org" , "linux-kernel@vger.kernel.org" , "Luis R. Rodriguez" , Borislav Petkov Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Dec 10, 2014 at 3:34 PM, Luis R. Rodriguez wrote: > From: "Luis R. Rodriguez" > > Xen has support for splitting heavy work work into a series > of hypercalls, called multicalls, and preempting them through > what Xen calls continuation [0]. Despite this though without > CONFIG_PREEMPT preemption won't happen and while enabling > CONFIG_RT_GROUP_SCHED can at times help its not enough to > make a system usable. Such is the case for example when > creating a > 50 GiB HVM guest, we can get softlockups [1] with:. > > kernel: [ 802.084335] BUG: soft lockup - CPU#1 stuck for 22s! [xend:31351] > > The softlock up triggers on the TASK_UNINTERRUPTIBLE hanger check > (default 120 seconds), on the Xen side in this particular case > this happens when the following Xen hypervisor code is used: > > xc_domain_set_pod_target() --> > do_memory_op() --> > arch_memory_op() --> > p2m_pod_set_mem_target() > -- long delay (real or emulated) -- > > This happens on arch_memory_op() on the XENMEM_set_pod_target memory > op even though arch_memory_op() can handle continuation via > hypercall_create_continuation() for example. > > Machines over 50 GiB of memory are on high demand and hard to come > by so to help replicate this sort of issue long delays on select > hypercalls have been emulated in order to be able to test this on > smaller machines [2]. > > On one hand this issue can be considered as expected given that > CONFIG_PREEMPT=n is used however we have forced voluntary preemption > precedent practices in the kernel even for CONFIG_PREEMPT=n through > the usage of cond_resched() sprinkled in many places. To address > this issue with Xen hypercalls though we need to find a way to aid > to the schedular in the middle of hypercalls. We are motivated to > address this issue on CONFIG_PREEMPT=n as otherwise the system becomes > rather unresponsive for long periods of time; in the worst case, at least > only currently by emulating long delays on select io disk bound > hypercalls, this can lead to filesystem corruption if the delay happens > for example on SCHEDOP_remote_shutdown (when we call 'xl shutdown'). > > We can address this problem by trying to check if we should schedule > on the xen timer in the middle of a hypercall on the return from the > timer interrupt. We want to be careful to not always force voluntary > preemption though so to do this we only selectively enable preemption > on very specific xen hypercalls. > > This enables hypercall preemption by selectively forcing checks for > voluntary preempting only on ioctl initiated private hypercalls > where we know some folks have run into reported issues [1]. > > [0] http://xenbits.xen.org/gitweb/?p=xen.git;a=commitdiff;h=42217cbc5b3e84b8c145d8cfb62dd5de0134b9e8;hp=3a0b9c57d5c9e82c55dd967c84dd06cb43c49ee9 > [1] https://bugzilla.novell.com/show_bug.cgi?id=861093 > [2] http://ftp.suse.com/pub/people/mcgrof/xen/emulate-long-xen-hypercalls.patch > > Based on original work by: David Vrabel > Cc: Borislav Petkov > Cc: David Vrabel > Cc: Thomas Gleixner > Cc: Ingo Molnar > Cc: "H. Peter Anvin" > Cc: x86@kernel.org > Cc: Andy Lutomirski > Cc: Steven Rostedt > Cc: Masami Hiramatsu > Cc: Jan Beulich > Cc: linux-kernel@vger.kernel.org > Signed-off-by: Luis R. Rodriguez > --- > arch/x86/kernel/entry_32.S | 21 +++++++++++++++++++++ > arch/x86/kernel/entry_64.S | 17 +++++++++++++++++ > drivers/xen/Makefile | 2 +- > drivers/xen/preempt.c | 17 +++++++++++++++++ > drivers/xen/privcmd.c | 2 ++ > include/xen/xen-ops.h | 26 ++++++++++++++++++++++++++ > 6 files changed, 84 insertions(+), 1 deletion(-) > create mode 100644 drivers/xen/preempt.c > > diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S > index 344b63f..40b5c0c 100644 > --- a/arch/x86/kernel/entry_32.S > +++ b/arch/x86/kernel/entry_32.S > @@ -982,7 +982,28 @@ ENTRY(xen_hypervisor_callback) > ENTRY(xen_do_upcall) > 1: mov %esp, %eax > call xen_evtchn_do_upcall > +#ifdef CONFIG_PREEMPT > jmp ret_from_intr > +#else > + GET_THREAD_INFO(%ebp) > +#ifdef CONFIG_VM86 > + movl PT_EFLAGS(%esp), %eax # mix EFLAGS and CS > + movb PT_CS(%esp), %al > + andl $(X86_EFLAGS_VM | SEGMENT_RPL_MASK), %eax > +#else > + movl PT_CS(%esp), %eax > + andl $SEGMENT_RPL_MASK, %eax > +#endif > + cmpl $USER_RPL, %eax > + jae resume_userspace # returning to v8086 or userspace > + DISABLE_INTERRUPTS(CLBR_ANY) > + cmpb $0,PER_CPU_VAR(xen_in_preemptible_hcall) > + jz resume_kernel > + movb $0,PER_CPU_VAR(xen_in_preemptible_hcall) > + call cond_resched_irq > + movb $1,PER_CPU_VAR(xen_in_preemptible_hcall) > + jmp resume_kernel > +#endif /* CONFIG_PREEMPT */ > CFI_ENDPROC > ENDPROC(xen_hypervisor_callback) > > diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S > index c0226ab..0ccdd06 100644 > --- a/arch/x86/kernel/entry_64.S > +++ b/arch/x86/kernel/entry_64.S > @@ -1170,7 +1170,23 @@ ENTRY(xen_do_hypervisor_callback) # do_hypervisor_callback(struct *pt_regs) > popq %rsp > CFI_DEF_CFA_REGISTER rsp > decl PER_CPU_VAR(irq_count) > +#ifdef CONFIG_PREEMPT > jmp error_exit > +#else > + movl %ebx, %eax > + RESTORE_REST > + DISABLE_INTERRUPTS(CLBR_NONE) > + TRACE_IRQS_OFF > + GET_THREAD_INFO(%rcx) > + testl %eax, %eax > + je error_exit_user > + cmpb $0,PER_CPU_VAR(xen_in_preemptible_hcall) > + jz retint_kernel I think I understand this part. > + movb $0,PER_CPU_VAR(xen_in_preemptible_hcall) Why? Is the issue that, if preemptible hypercalls nest, you don't want to preempt again? > + call cond_resched_irq On !CONFIG_PREEMPT, there's no preempt_disable, right? So how do you guarantee that you don't preempt something you shouldn't? Is the idea that these events will only fire nested *directly* inside a preemptible hypercall? Also, should you check that IRQs were on when the event fired? (Are they on in pt_regs?) > + movb $1,PER_CPU_VAR(xen_in_preemptible_hcall) > + jmp retint_kernel > +#endif /* CONFIG_PREEMPT */ > CFI_ENDPROC All that being said, this is IMO a bit gross. You've added a bunch of asm that's kind of like a parallel error_exit, and the error entry and exit code is hairy enough that this scares me. Can you do this mostly in C instead? This would look a nicer if it could be: call xen_evtchn_do_upcall popq %rsp CFI_DEF_CFA_REGISTER rsp decl PER_CPU_VAR(irq_count) + call xen_end_upcall jmp error_exit Where xen_end_upcall would be witten in C, nokprobes and notrace (if needed) and would check pt_regs and whatever else and just call schedule if needed? --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/