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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] [PATCHv6] x86/xen: allow privcmd hypercalls to be preempted
Hypercalls submitted by user space tools via the privcmd driver can
take a long time (potentially many 10s of seconds) if the hypercall
has many sub-operations.
A fully preemptible kernel may deschedule such as task in any upcall
called from a hypercall continuation.
However, in a kernel with voluntary or no preemption, hypercall
continuations in Xen allow event handlers to be run but the task
issuing the hypercall will not be descheduled until the hypercall is
complete and the ioctl returns to user space. These long running
tasks may also trigger the kernel's soft lockup detection.
Add xen_preemptible_hcall_begin() and xen_preemptible_hcall_end() to
bracket hypercalls that may be preempted. Use these in the privcmd
driver.
When returning from an upcall, call xen_maybe_preempt_hcall() which
adds a schedule point if if the current task was within a preemptible
hypercall.
Since _cond_resched() can move the task to a different CPU, clear and
set xen_in_preemptible_hcall around the call.
Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
---
Changes in v6:
- Add an additional should_resched() check.
Changes in v5:
- Add xen_maybe_preempt_hcall() instead of copy-and-paste of assembly.
Changes in v4:
- Fix 32-bit build.
Changes in v3:
- Export xen_in_preemptible_hcall (to fix modular privcmd driver).
Changes in v2:
- Use per-cpu variable to mark preemptible regions
- Call preempt_schedule_irq() from the correct place in
xen_hypervisor_callback
---
arch/x86/kernel/entry_32.S | 3 +++
arch/x86/kernel/entry_64.S | 3 +++
drivers/xen/Makefile | 2 +-
drivers/xen/preempt.c | 44 ++++++++++++++++++++++++++++++++++++++++++++
drivers/xen/privcmd.c | 2 ++
include/xen/xen-ops.h | 26 ++++++++++++++++++++++++++
6 files changed, 79 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 000d419..31e2d5b 100644
--- a/arch/x86/kernel/entry_32.S
+++ b/arch/x86/kernel/entry_32.S
@@ -982,6 +982,9 @@ ENTRY(xen_hypervisor_callback)
ENTRY(xen_do_upcall)
1: mov %esp, %eax
call xen_evtchn_do_upcall
+#ifndef CONFIG_PREEMPT
+ call xen_maybe_preempt_hcall
+#endif
jmp ret_from_intr
CFI_ENDPROC
ENDPROC(xen_hypervisor_callback)
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index db13655..10074ad 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -1208,6 +1208,9 @@ ENTRY(xen_do_hypervisor_callback) #
do_hypervisor_callback(struct *pt_regs)
popq %rsp
CFI_DEF_CFA_REGISTER rsp
decl PER_CPU_VAR(irq_count)
+#ifndef CONFIG_PREEMPT
+ call xen_maybe_preempt_hcall
+#endif
jmp error_exit
CFI_ENDPROC
END(xen_do_hypervisor_callback)
diff --git a/drivers/xen/Makefile b/drivers/xen/Makefile
index 2140398..2ccd359 100644
--- a/drivers/xen/Makefile
+++ b/drivers/xen/Makefile
@@ -2,7 +2,7 @@ ifeq ($(filter y, $(CONFIG_ARM) $(CONFIG_ARM64)),)
obj-$(CONFIG_HOTPLUG_CPU) += cpu_hotplug.o
endif
obj-$(CONFIG_X86) += fallback.o
-obj-y += grant-table.o features.o balloon.o manage.o
+obj-y += grant-table.o features.o balloon.o manage.o preempt.o
obj-y += events/
obj-y += xenbus/
diff --git a/drivers/xen/preempt.c b/drivers/xen/preempt.c
new file mode 100644
index 0000000..286240b
--- /dev/null
+++ b/drivers/xen/preempt.c
@@ -0,0 +1,44 @@
+/*
+ * Preemptible hypercalls
+ *
+ * Copyright (C) 2014 Citrix Systems R&D ltd.
+ *
+ * This source code is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ */
+
+#include <linux/sched.h>
+#include <xen/xen-ops.h>
+
+#ifndef CONFIG_PREEMPT
+
+/*
+ * Some hypercalls issued by the toolstack can take many 10s of
+ * seconds. Allow tasks running hypercalls via the privcmd driver to
+ * be voluntarily preempted even if full kernel preemption is
+ * disabled.
+ *
+ * Such preemptible hypercalls are bracketed by
+ * xen_preemptible_hcall_begin() and xen_preemptible_hcall_end()
+ * calls.
+ */
+
+DEFINE_PER_CPU(bool, xen_in_preemptible_hcall);
+EXPORT_SYMBOL_GPL(xen_in_preemptible_hcall);
+
+asmlinkage void xen_maybe_preempt_hcall(void)
+{
+ if (unlikely(__this_cpu_read(xen_in_preemptible_hcall)
+ && should_resched())) {
+ /*
+ * Clear flag as we may be rescheduled on a different
+ * cpu.
+ */
+ __this_cpu_write(xen_in_preemptible_hcall, false);
+ _cond_resched();
+ __this_cpu_write(xen_in_preemptible_hcall, true);
+ }
+}
+#endif /* CONFIG_PREEMPT */
diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
index 569a13b..59ac71c 100644
--- a/drivers/xen/privcmd.c
+++ b/drivers/xen/privcmd.c
@@ -56,10 +56,12 @@ static long privcmd_ioctl_hypercall(void __user *udata)
if (copy_from_user(&hypercall, udata, sizeof(hypercall)))
return -EFAULT;
+ xen_preemptible_hcall_begin();
ret = privcmd_call(hypercall.op,
hypercall.arg[0], hypercall.arg[1],
hypercall.arg[2], hypercall.arg[3],
hypercall.arg[4]);
+ xen_preemptible_hcall_end();
return ret;
}
diff --git a/include/xen/xen-ops.h b/include/xen/xen-ops.h
index 7491ee5..8333821 100644
--- a/include/xen/xen-ops.h
+++ b/include/xen/xen-ops.h
@@ -46,4 +46,30 @@ static inline efi_system_table_t __init *xen_efi_probe(void)
}
#endif
+#ifdef CONFIG_PREEMPT
+
+static inline void xen_preemptible_hcall_begin(void)
+{
+}
+
+static inline void xen_preemptible_hcall_end(void)
+{
+}
+
+#else
+
+DECLARE_PER_CPU(bool, xen_in_preemptible_hcall);
+
+static inline void xen_preemptible_hcall_begin(void)
+{
+ __this_cpu_write(xen_in_preemptible_hcall, true);
+}
+
+static inline void xen_preemptible_hcall_end(void)
+{
+ __this_cpu_write(xen_in_preemptible_hcall, false);
+}
+
+#endif /* CONFIG_PREEMPT */
+
#endif /* INCLUDE_XEN_OPS_H */
--
1.7.10.4
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